1999 Acura EL or Civic EX sedan - dropping Cd from 0.33 to below 0,20
 

Welcome to my 21 years car improvement adventure,

I am starting this thread to report on the evolution and advancement of the modifications brought to my 1999 Acura EL. The Acura EL was built with the same engine and basically the same mechanical components as the Honda Civic EX Sedan in North America. The EL was, on the outside, a replica of the Honda Domani in Japan but with a North American power plant. The EL was only marketed in Canada.

I am the first owner of the car purchased in July 1999.

Cd = 0,32 1996-2000 Honda Civic sedan

Cd=0,34 estimated by automobile-catalog https://www.automobile-catalog.com/c...ura_1_6el.html

Engine D16Y8 stock, 5 gears manual

Tires: 195-55R15 (wider than the 14 inches tires on most Civic of the time)

Tire pressure : 32 psi all around

Weight with only the driver : 2550 lbs

Mileage Canada Energy - Same as EPA

Highway 7,5 L/100km, City 9,7 L/100 Km, Combined 9,7 L/100 km
Highway 24 mi/US Galon, City 31 mi/US Gal, Combined 27 mi/US gal

Year 2019, Spring to Fall, not used in winter, combined mileage 6,9 L/100 Km

Actual odometer reading in excess of 232 000 Km, 144 000 miles

Even compressions, no oil burn, LRR General Tire RT43, new brakes all around

YEAR 2020 GOAL :
Bring combined mileage down from 6,9 L/100 Km to 5,4 L/100: from 34.1 mpg to 43,6 mpg US or better.

Since 1999, the car went through different phases of modifications.

(Continued on next entry)

Welcome to my 21 years car improvement adventure,

Real

Continued : 1999 Acura EL or Civic EX sedan - dropping Cd from 0.33 to below 0,20
 

The car improvements went through 5 phases

1. Finding out the car was actually eco-friendly
2. Searching for means to repeat the best mileage reported
3. Mods inspired by products claims and performance magazines and forums
4. Mods inspired by solid advice : EricTheCarGuy and Autospeed.com (J.Edgar)
5. Aerodynamics mods supported by trials and monitoring data

Phase 1. Finding out the car was actually eco-friendly

First, for all my cars since 1977, I have a log book and record every fill and distance on fill. This meticulous logging and mileage computation alerts me to maintenance needed. For this particular car, I believe that only about 20 Litres (5 gal) have not been recorded in almost 21 years of use.

I purchased the car on Acura claims of better mileage than the Civic sedan and IIHS good results for crash tests. I shopped a lot on the web at the time.

During summer 2000, a highway trip of more than 600 km resulted in a 5 L/100 Km mileage (47 mpg). My driving was agressive, speeding at times at more than 150 Km/h (95 mph), using the air conditioning at times. The car was loaded with my second other and our luggage for a whole week. The car was still under warranty and going to the dealer for mandatory inspections. After the next inspection, the mileage dropped and I suspect the dealer mechanic did something to set the car to standard parameters.

Still, when one was enlightened once, one wants to get back in grace! I was driving the car mainly on highways and started searching for means of improving the mileage. It took years to get as good a mileage as that warm day of summer 2000. I entered Phase 2. Searching for means to repeat the best mileage reported

(Continued)

Real

Continued phase 2: 1999 Acura EL or Civic EX sedan - dropping Cd from 0.33 to below 0,20
 

Phase 2. Searching mods to get steadily the best mileage ever

Internet and Google are my friends for a long time. I scoured the web for years when I could afford the time with a super busy professional schedule.

First mod was a K&N air box filter, supposed to let more air flow through the engine (probably in 2000 or 2001): little improvement on mileage, less air restriction?

Then, I went from regular 5w30 oil to synthetic oil same grade, sure improvement on mileage and better car performance (since 2002).

Then, hotter spark plugs to promote more complete gas combustion, little improvement on mileage (around 2005 at first and through 2012)

I did not have enough time to myself to search and find solid evidence to support proposed improvements. From 2000 to 2012, the available money was scarce and I rather learned to control the "nut behind the wheel" and got fairly good mileage.

After retiring in 2012, I got into phase 3

(Continued)

Phase 3: 1999 Acura EL and Civic EX sedan
 

Phase 3. Mods inspired by products claims and performance magazines and forums

After retiring in the end of 2011, I decided on the pass time of working on this Acura EL aging car. The car had always had a good handling. It was fun for sporty high speed driving. It is even the only car I did not fall asleep in! And it needed love.

I started to read the forums on mods like turbo and cold air intakes and lighter wheels and low rolling tires and whatever could make more power. This "grocery getter" was not peppy enough on hilly roads and that bugged me. Also the car was less silent than in its first years. Noisy is not comfortable.

So, I started with the sound insulation stuff and spent most of 2012 season. The sound proofing allowed to determine the performance summer tires were the main culprit to my ears discomfort. I decided the car was having some rust and having a second car the Acura could rest during the winter.

2013-2014 money scarce in retirement. Still, I procured and installed a short cold air intake instead of the air box and got some mileage improvement. I later discovered in my readings that the short cold air intake is rather a warm air intake allowing less dense air to enter the cylinders. It probably also helped me realize that more power requires both more air and more gas. Thus, pursuing the power development, I was abandoning my old goal of better mileage.

2015 a new complete body repair and paint job, lighter alloy wheels and low rolling tires. Wow, the little Acura is getting peppier! Less weight and low rolling tires and the car accelerates better and mileage improve again.

2016-2017 The winter last almost 6 months in Quebec City Canada. It is a long time to read and decide on maintenance and checks and also decide if the car is worth continuing on improvements.

Also, during the fall of 2016, I went to work on my brother in-law pickup and got back into self confidence that I can maintain and most of the time fix my cars myself. I worked on his clogged plastic engine air intake manifold, a complete engine tune-up, complete rear drum brakes job. I guided myself with Youtube and the likes of Chrisfix, EricTheCarGuy and others.

So, I entered phase 4. Mods inspired by solid advice

(continued)

Phase 4: 1999 Acura EL and Civic EX sedan
 

4. Mods inspired by solid advice : EricTheCarGuy and Autospeed.com (J.Edgar)

Maintenance
First 4 wheels brake job in this Acura life (221 341 Km)

Reduce lost power and fuel wasted
Polishing activities are borrowed from the performance phase

- Throttle cable readjusted
- Air conditioner compressor and fan control: switch off under heavy load
- Air conditioner cabin temperature controlled by thermostat
- Fuel injectors checked and cleaned
- Ground cables and fixing points checked for continuity and cleaned
- Original fuel filter replaced (221 341 Km)
- Throttle body cleaned and polished (new gasket)
- Intake manifold mouth and part of runners polished (new gasket)
- Knock sensor replaced (broke when intake manifold removed)
- Intake air control valve replaced (from junk yard)
- Air intake mouth relocated to bottom front grille (highest pressure zone)


Aerodynamics
- Upper grille blocked, lower grille partly blocked
- Air dam (lawn edging) first part of season
- Homemade front bumper lip and spoiler replacing air dam
- Home made side skirts (house vinyl siding board)
- Rear trunk lid low height spoiler from junkyard Civic 2006
- Splash guards removed on 4 wheels

Unfortunately no partner to perform tuft tests, just observations and mileage logging

Observations
Engine withstands lower revs cruising in all gears
Acura gear change recommended RPM is 3000 passing 1st to 5th gear
Now gear change as low as 2200 RPM, cruise as low as 1600 RPM

Car is far more stable in cross wind or being passed by a large vehicle (both ways)

Engine coolant temperature is affected by the upper grille block but is maintained between 87 and 95 Celsius on the highway. I remember from reading Julian Edgar in autospeed.com that engine coolant temperature between 90 and 95 Celsius is optimum for combustion thus for power and mileage.

Measures and data logging
Android phone using “Torque Pro” app to improve driving skills and check on engine operating parameters like coolant temperature, GPS speed, RPM

Mileage from fill logging, highway speed around 100 Km/h – 61 mph, summer driving

Best mileage highway with air dam : 4,48 L/100 Km, 499 Km ride (52 mpg US, 310 miles)

Best mileage with front bumper spoiler 4,74 L/100 Km, 523 km ride (50 mpg, 325 mi)

Season mileage combined: 6,9 L/100 Km, 8240 Km (34,1 mpg US, 5120 miles)

Comparison
Mileage Canada Energy - Same as EPA

Highway 7,5 L/100km, City 9,7 L/100 Km, Combined 9,7 L/100 km
Highway 24 mi/US Galon, City 31 mi/US Gal, Combined 27 mi/US gal

6,9 L/100 Km combined vs EPA estimate 9,7 L/100 Km, I am winning!

Planned modifications not performed

Replace 195-55R15 tires by 195-65R15 tires:higher, narrower, larger diameter for lower revs at same speed

Catalytic converter and muffler not replaced to accommodate 2.25 inches headers and tail pipe to reduce exhaust restriction - improve gas flow

Continued with phase 5. Aero dynamics improvements

Phase 5: 1999 Acura EL and Civic EX sedan
 

5. Aerodynamics mods supported by trials and monitoring data

Finally, I am getting there. Driving behavior and mechanical mods brought some good results. Time to add some improvement in the aerodynamics field.

For starter and since the beginning of 2019, I read a lot of scientific papers on the web. What works. What works better? What is the most recent knowledge about a particular road car aero topic?

I also purchased Julian Edgar, 2018, Modifying the aerodynamics of your road car, SpeedPro Series, Veloce Publishing, England : Step-by-Step instructions to improve the aerodynamics of road cars.

Secondly, modifications monitoring methods and tools

Fuel log: from the first fill of this car in 1999, I log every fuel fill and distance covered to compute mileage and check on car maintenance need.

Digital dashboard: standard car dashboard is so weak on information that I use an ELM 327 Bluetooth OBD2 adapter and Android phone or tablet app TorquePro or RealDash: both apps allow real time engine monitoring and datalogging of parameters, direct from car ECU (Engine Control Unit), like coolant temperature, gps speed, RPM, throttle % opening, intake air temperature, instant and long time mileage and else: mileage can be calibrated. OBD2 adapter cost less than 20 US$, each app pro version costed below 10 US$ (free version of apps log less parameters but are still usefull).

Digital differential manometer to measure air pressures on the car body in various points: allowed to relocate the air intake mouth in the front “dead zone” of higher air pressure: also useful to evaluate the air intake tract components, from mouth to throttle body, to reduce restriction and improve the flow of air to engine.

Tufts test: assistant required for pictures or small movies using a smartphone: observations of tuft movements and positions frame by frame with some movie viewing app or movie editing app better done on larger computer screen. I use an Apple MacBook and Quicktime player to review frame by frame and extract interesting frames with a screen capture for further examination.

Dust test: to better see actual vortex deposited dust areas on car panels and choose correction, e.g. separation edges

“Throttle stop” method to evaluate modifications according to Julian Edgar on Youtube 2020 : https://www.youtube.com/watch?v=tVAokIdaXm0

Drag improvement speed difference computation proposed by Julian Edgar on Youtube in 2020: https://www.youtube.com/watch?v=Wi-TemRJ7CM&t=353s

Fuel tank filled to the brim, every fill in 2020: no two fuel pumps are the same and will stop at the same filling level thus introducing errors in mileage. From this 2020 spring, I fill the fuel tank to the mouth. I stop when I see fuel about to pour out. That way I am sure the difference between fills is minimum and get accurate mileage measurements.

Test facing low or no headwind on flat straight road when possible
. It seems that testing with tail wind and gusts "steals" air from the engine thus results are not as good as when facing low wind speeds. I will have to check on that with the manometer.

Now, modification, testing, measuring, data logging, approving or disproving modification, next modification and loop…

(Continued with some actual results)

Phase 5: Aerodynamics results: 1999 Acura EL and Civic EX sedan
 

Aerodynamics modification results

But for the first modification package (following) every mod is evaluated against the “throttle stop” results of the previous modification.

1. Under belly, upper grille block, 45 psi tires, side skirts, 4 wheels deflectors: throttle stop speed increase, 15,2 %, drop in Cd estimated 32,6 %

2. Folded external mirrors: throttle stop speed increase, 2,6 %, drop in Cd estimated 4 %

3. Tires overinflated from 32 PSI to 45 PSI: throttle stop speed increase, greater than 2 %, drop in Cd estimated 2,5%

4. Wheel covers: throttle stop speed increase, greater than 4 %, drop in Cd estimated 8,1 %

5. Vortex generators installed: throttle stop speed increase, 2 %, drop in Cd estimated 2,5%

6. Rear wheels “boat tails”: throttle stop speed increase, 1-2 %, drop in Cd estimated 2,5 %

If computing against the estimated Cd=0.34

Adding all the estimated reductions per modification comes to 52,2 %

May be or may be not a real total reduction, if “may be” then means a new Cd= 0.17

“Strange” result obtained with the first set of modifications (under belly and else), with filling the gas tank to the brim before and after the trip, with rear wind, the highway mileage computes to 2,15 L/100 Km (about 110 US mpg) OMG, Holy something! What went on this 234 km ride? Some of that fuel went to a 20 km country road at 70 km/h speed and I got stopped by a patrol car on the highway for part of the rear underbelly dropping to the pavement: stop on highway means I had to resort to heavy acceleration to merge in traffic and get to cruise speed of about 100 Kph.

The next best mileage result for a 231 Km ride is 4,0 L/100 km or 59 US mpg

On the mechanical front, 0w20 synthetic motor oil is in use for 2020. Tires to higher, narrower less dragging tires have not been replaced. Catalytic converter and muffler have not been replaced yet. Will wait till more discretionary money available or worn out parts. Sigh!

As soon as possible, modifications description will be presented with pictures and cost estimates.

Regards,

Real

Subscribed. There is a lot to digest here but I am impressed. And I have had my 98 Civic since June 2001, so I am only a little behind your 21 years. Love Acuras. I'll be back to see more.

Acura EL 1999 picture as out of storage in May 2020
 

This is a profile view of the car just out of storage before a "dust test". The car is showing the front spoiler that acts as an air dam for the front wheels; unseen is the belly pan, front part from spoiler to wheel shafts and direction links in aluminium, middle part in choroplast, rear part is a 6 degrees diffuser also in chloroplast. The car is also equipped with vertical deflectors in front of the rear wheels and OEM side skirts are extended down by 2 1/2 inches with painted vinyl gutter split in halves (see picture, costed 5 $ at hardware store).

The front spoiler is only 120 mm (about 5 inches) above ground. The middle part of the car is about 150 mm (6 inches) and as flat as possible: the exhaust from headers to rear is not covered. From the rear suspension links, a "diffuser" sweeps up by 6 degrees attached at the front with "zipties" and affixed to the rear plastic bumper cover with self taping screws : no more brake parachute effect from the rear bumper cover! Total height from ground under the rear bumper cover is 30 cm (12 inches). I guess the car "rake" and the "venturi effect" are improved.

Front upper invisible grille block
 

This is my second attempt at an upper grille block as I am working under aesthetical constraint from my 42 years partner: "the car has to look good or you can't do it or you will have to remove the ugly modification".

The bottom spoiler is tolerated on promise that I will remove the fabric tape and finish it shiny when the body filler goes on special.

So, instead of my first work at an external grille block that hid the grille and the Acura crest (picture), I went around and blocked the grille from behind with black choroplast held by some "big monkey" fabric tape (picture).

I suppose the aerodynamics effect of having the grille blocked from the back rather than the front is not that much. I suppose a boundary layer builds up at the surface of the grille features and serves the same purpose as attaching some solid material in front of the grille to block it. So air flows up the grille area "sliding" on the boundary layer.

A mask on the top wide slot under the upper grille where the licence plate should sit adds to the air blockage. Invisible to the left of this blocage is the air intake mouth that collects air for the engine (more on that later).

These grille blockages give pretty much the same results for the engine coolant temperature as the previous in front of the grille blockages. The previous and actual grille blocks cover the same areas. So the coolant temperature on highway gets from 82 to 95 Celsius when outside temperature ranges from 16 to 36 Celsius (180 to 203 F when outside temp from 60 to 97 F). The cooling fan kicks in at 97 Celsius (208 F). Going up a hill or with an heavy load at speeds under 70 km/h makes the coolant temperature go up to turn the fan on. Generally on highway the coolant temperature is maintained between 90 and 95 Celsius (194 to 203 F) which I read somewhere is the ideal temperature window for complete combustion.

As the belly pan works with the grille block to maintain a certain temperature in the engine compartment, the front of belly pan stops about the oil pan and in front of the wheels shafts and direction links. Before the second part of the belly pan, a belly wide opening is left for the air warmed in the engine compartment to exit under the car as it was initially engineered. The outsides of the middle pans are attached up to OEM side skirts with self taping screws and the front and inner sides are attached with zipties wherever there is a solid attachment point allowing.

Coolant temperature data is read from the car computer (ECU) through an OBD2 Bluetooth adapter and the data is logged on an Android phone or tablet. Coolant grade is good for up to 115 Celsius (240 F) and engine oil is synthetic 0W20.

On the aerodynamics side, the grille block was not evaluated separately from the belly pan, the in front of the rear wheels deflectors and the 45 PSI inflated tires. The preliminary combined result of drag reduction for the all these mods is 32,6%. The 45 PSI inflated tires were later compared to the Honda recommended 32 PSI inflated tires to yield a 12% drag reduction with the latter.

2019 Aerodynamics modifications fantasies
 

Fantasy as the required data to assess the benefit of the modification was not recorded.

Part of my work on the car is looking at newer cars style and devices to compare it with scientific literature and ecomodders experiences, looking for benefitting opportunities. Thus 6 modifications were fabricated in 2019 with little performance assessment on the aerodynamics benefit.

These are
1. Upper grille block with an attempt to streamline
2. Behind rear wheels under belly
3. Air dam
4. Rear trunk lid spoiler
5. Lower side skirts
6. Front spoiler

Some mods are not easy to mock-up with cheap materials like cardboard and tape. In retirement, the available money is scarce. Also, aesthetical constraints and life partner regard on the work done are limits.

Upper grille block with an attempt to streamline

Some aluminum left over from building renovation was modeled to completely block the upper front grille. It was then painted black to match the car color. It was attached at its bottom with two screws in the licence plate holes. At the top, under the hood lid, it was just taped with the "big monkey" fabric tape.

The engine coolant temperature went up from the 70s and lower 80s Celsius to the high 80s and 90s with was intended as an added bonus to the aerodynamics improvement. It seems that engine coolant temperature in the range of 90-95 Celsius is the best for the most complete combustion of the fuel load.

Behind rear wheels under belly

Having some thin aluminum left overs, a sort of rear diffuser was built before the car went out of winter storage. I am no manual worker and certainly not a good craftsman. Pieces were joined together. The whole pan was curved and baling down without enough support. The diffuser angle was in the 1-2 degrees slope at most. The rear car tow hook was hidden and lost. Bad work.

Air dam

As many ecomodders, a 4 ½ inches lawn edging was used to build a front air dam. As on other cars, it was recessed from the front of the bumper cover and almost invisible. It probably brought some mileage improvement that was not seriously assessed. It was replaced by the front spoiler discussed further. Total cost below 10 Cdn$ including self taping screws and sales taxes.

Rear trunk lid spoiler

Looking at 8th generation Civics and Civic hybrid, adding a trunk lid spoiler seemed like a good idea. At first, it was a mock-up. Three layers of 4.5 inches lawn edging were piled up with the next coat rounded edge exceeding the previous, thus building up height but also a rear end slant. Each lawn edging coat was taped with the “big monkey” fabric tape. For a cheap mock-up, it was looking fairly good. It did not pass the aesthetical assessment.

The mocked-up trunk lid spoiler was replaced by a real OEM one from the junk yard. It was taken from a black 8th generation Civic (2006-2010). It remains on the car even though it has not been assessed.

The junkyard cost is 15 $, some two faces 3M automotive tape for moldings and one tube of black silicone caulking were used. About 40 $ and some leftovers.

Lower side skirts

Low budget right material is not easy to come by. Building vinyl siding board for 10 $ was acquired from the hardware store. Strips 3 inches wide, retaining some cornering material, were cut and joined to the OEM existing side skirts with self-tapping screws. It was also painted black before installation to match the car color.

Unfortunately, the siding vinyl board is not a sturdy material. Whether in making the strips or when screwing through to the side skirt some tearing resulted. The material being soft was easily bent to follow the side skirt contour but was also prone to tearing. These side skirts addition lasted the season but one was so badly damaged that both were replaced for 2020.

Preliminary assessment – 1st third of season

Rear section belly pan, air dam, lower side skirts, and deck lid spoiler were combined and resulted in a combined mileage of 7.3 L/100 Km for 2517 Km (32 mpg US for 1564 miles).

The real interesting result is in the whole lot better car stability to side winds and being passed by a truck and box trailer on highway. The car stays the course or requires much less correction than previously.

Front spoiler

Newer cars like Prius, Corolla, Civic, Sentra, Volt and many others have a front spoiler as the lower part of the plastic bumper cover. It may be just a designer fantasy that was copied by competitor professionals at other cars manufacturers or it may have some aerodynamics value. So, I set to fabricate one with cheap building materials available at the hardware store. Pictures attached represent the fabrication process.

The front lawn edging air dam was replaced by this fabricated spoiler at the beginning of the second third of the season.

The styrofoam 2 x 8 x 2 in. thick insulation board was selected for the core. It was than cut in some resembling spoiler shape. Some ½ inch thick window plastic material was used on top and bottom to add strength to the bumper cover attachment. Each coat of material was recessed from the previous bottom one in order to create steps that would allow for a slanted surface when filled with polyurethane foam. The core and strengthening parts were then installed on the car and joined to the bumper cover with 3 ½ inches wood screws.

Polyurethane foam filling the steps and knife sculpting the spoiler to its final shape were the next steps. Not knowing if the spoiler would stay or hold on, the “big monkey” black tape was applied to make it look as good as possible. It seems that air flowing does not care about a very smooth or somewhat rough surface. To this day, by the end of June 2020, the spoiler holds on and awaits the final fiberglass and body filler that will give it the kind of look that will be acceptable.

The cost for this mod is about 25 $ for the styrofoam 2 inch thick core, 60 $ for the white PVC finish for window, 3 polyurethane spray cans for 20 $ and 21 $ for the largest 2 in. wide roll of the "big monkey" black fabric tape. This is all in Canadian dollars, sales tax included.

2nd assessment

But for the replaced air dam, all other mods remain. The car was driven a total of 5723 Km and yielded 6,73 L/100 km in mileage over that period (35,2 mpg US for 3556 miles). Driving conditions and driving style were the same, most of the time with the same driver.

The mileage improvement over the 1st third is 0,6 L/100 km combined (3 mpg US). The second third of the season started with the highest outside temperatures that require the use of the air conditioner. The 2019 mods although not assessed seriously were effective and are to remain or be improved for 2020.

2020 improvements on 2019 mods

Such improvement includes better more rigid side skirts and complete better built belly pan. Also, the front spoiler scrapes badly and gets damaged frequently. If possible the lower white reinforcements coat will be removed. Also the front "air curtains" found on many newer cars seem to have enough merit to be tried on this car and the spoiler will have to be modified to accommodate for that.

Dust test on rear end of 1999 Acura EL - 1999 Civic
 

It seems that separation edges are important in terms of drag and car noise in the passenger compartment. All newer cars and SUVs have separation edges at the rear end. So maybe I could fix the lack of such edges on my 1999 Acura EL and reduce this car drag somewhat.

Folowing indications from Julian Edgar book, I drove many runs on a dusty road. I am at a loss to read the dust deposition and identify the appropriate position of the separation edges. So please, have a look at my pictures and give me your opinion on the position of such edges. HELP! HELP! HELP!

https://ecomodder.com/forum/attachme...p;d=1593550318

I like the build method for your front splitter/airdam. I think it is likely to produce some downforce with that forward and scooping design. It might also lower the press peak point and separation point at the nose? Generally, I think that is considered good. But you would want to try testing to be sure.

That dusty road technique is old, and I think it can reveal useful things. I did some a couple times on the El Mirage Dry Lake LSR site in maybe 2013 and 2019. I find too that morning mist and due settled on my car will blow off in ways that are suggestive, reminiscent of tuft testing results and dust results.

Your results in the pic about suggest a trip line or other separation device might help. You might consider a box cavity type spoiler with side plates like I tested last year. Thee is a very good study on the box cavity that I and others have posted about repeatedly.

Your rear bumper cover seems mostly clear of dust in the middle. Is that correct?

EDIT: I will add this photo from my dust test last summer:
https://ecomodder.com/forum/attachme...1&d=1563813420

Ridiculous how similar the shape of our cars are. In this shot you can see that without the rear spoiler, dust settled heavily on my bumper cover. When I added the decklid spoiler mockup, this was the result.
https://ecomodder.com/forum/attachme...1&d=1563813553

2 Dust tests - mods and different results
 

California98Civic - Your observation of the bumper cover is right for this test conducted in May 2020. (first picture attached is from the same series) I missed it. Thank you. There was little dust on the center of the bumper cover. The picture showing the dust on the corner of the bumper cover confirms that. Also, it seems that this dust pile up is directly related to the rear wheel located just ahead.

Second picture from a dust test today July, 2, shows that the situation is changed. Now, the dust piles up on the bumper cover center and much less behind the rear wheels. Modifications realized since the first "novice" dust test are the vortex generators, my take on the rear wheel boat tails, some side skirt deflectors just ahead of the rear wheels and the exhaust extension side oriented (assuming it energizes the air coming up from the diffuser into the wake).

Rear wheel both tails are made from the vinyl gutter leftover originally purchased for the side skirts (5,50 cdn$ for 10 feet length). Funny that I chose the open box design for these boat tails! Vortex generators at the center are the Amazon shark fin type, and the external ones are simple vinyl 90 degrees corner 3/4 x 4 inches while awaiting for more black shark fins ordered.

Green tapes on both sides show the actual locations of flow separation "read" from the dust deposition.

More on the mods later!

Thanks for your useful comments,

Regards,

Real (Acel)

Interesting and hard to tell what the changes indicate since there have been a few mods. Do you have covers over the rear wheel wells?

Covers on rear wheel wells to affect dust deposition
 

No covers on rear wheel wells and no wheel covers either (pics). Not yet.

My rear wheel boat tails deflect the rear wheel flow to the middle of the car. So the result of having dust piling up on the center of the bumper cover is expected.


I am looking at the Honda Clarity to build at least the same type of rear wheel well covers.

I will probably have some wheel covers too (pic). I tested some "Tesla" type wheel covers built with pizza pans. (black lettering vinyl bonded to pizza pan) Joining to the wheel was not stable and conduced to wheel balancing problems. I had to remove the covers and think about some other way to secure it to the wheels. Coming soon!

Real

https://ecomodder.com/forum/attachme...8&d=1593827911

This is a very clever wheel cover design. There should be some way to get them right.

Wheel cover securing to wheels
 

First attempt was zipties

Second attempt was with long nylon toggle bolts

In both cases the wheel covers were moving slightly causing a wheel imbalance. Also the pizza pan edge was damaging the wheel finish.

Next try will be longer wheel lug nuts pierced at center and a nut bonded with epoxy glue, then a machine screw. Wheel covers should not move anymore.

Any other idea?

Real

2nd dust test - looking for Rear separation edges
 

Pics are not commented and results from a dust test to determine where to install separation edges.

All comments and suggestions are welcome,

Acel

Separation edges V.01
 

After a 2nd dust test, not using water this time, some separation lines appeared.

Some separation edges were fabricated and tested in a 3rd dust test (see pics).

3 firsts pics: On the driver side, there is dust accumulating on the side of the bumper cover behind the rear wheel: this suggests that the air flow is already detached so the dust can stick. There is also some but less dust deposition on the metal panel above the driver side leg of the bumper cover. The wheel and/or the well need some treatment!
Also there is more dust on the side leg of the bumper cover behind the "separation edge" than immediately after.

4th and 5th pics : show dust deposited on the upper flat surface of the bumper cover. Note that there is less deposition near the separation edges: also, these area are right above the rear wheels "boat tails" which deflect some air coming from the wheel directed to the middle under the car.

6th pic shows some dust deposition on the C pillar resulting from a separation around the rear door window.

7th pic shows the passenger side separating edges and some dust deposition.

Did I get these separation edges right? Is the analysis correct?

All comments and questions are welcome. I want to do those separation edges right.

Real

3rd Test: Separation edges V.02

Separation edges location from the side panel above the tail light and on the tail light seem right: little dust deposition before the edges and no dust deposition after the edges.

Looking at the bumper cover dust deposition on the V.01 separation edges got me thinking. If dust deposits before the trialed separation edge, the edge is either forming a ridge that creates a “no flow zone” or is located too far back and flow separates before to get to the “edge”.

Then, I removed the bumper cover questionable edges and as I had to drive for errands, decided to look at the dust deposits remaining after to locate the V.02 separation edges.

From the remaining dust deposit, the bumper cover separation edges were relocated and some fabric tape was added to make a gradual transition of the flow from the side panel to the edge.

There is a few dry days to wait after some heavy rainy days to get the dust moving again to test this V.02 mod.

Thanks for all the pictures and thinking about what you are doing. I looked at them all and your notes. To remind: I am not expert and I have no formal training. I just own a car that I have modding and testing on the basis of research and my own and others testing.

I think your photos confirm for your specific car configuration what we broadly know for these three box sedan and coupe car bodies. A large turbulent wake develops behind the car, especially behind the trunk (and the rear window, though dust won't show it). The wheel wells and the underside in general are major contributors to such turbulent separation.Your dust shows that the angle of the curves on the body panels at the rear, such as at the driver and passenger sides of the rear bumper cover, become too sharp, too fast, and very quickly. At first, there is perhaps no dust because there is relatively little turbulence, and then the dust deposits show up as the turbulence grows.

Better separation edges should help, but despite the dust, these cars were pretty well optimized from the factory. So, testing experience here with 6th and 7th gen Civics (similar to your Acura) has shown it is hard to improve at the tail with small changes. A rear flat spoiler, a box cavity, wheel well covers, side skirts, grill blocking, an airdam, or more ambitiously a full undertray with a carefully tuned rear difuser angle. I think it is important to study placement and shape of the mods and to test them as best you can. Go bigger than a small separation edge; try a box cavity of some sort on combination with wheel well skirts and a diffuser.

I am planning to test a flat rear decklid spoiler again this summer. But I have a ton of work and family responsibility to wade through before that it really possible.

Wheel cover trials
 

Before to fabricate and install the "definitive" wheel covers, two mock-up were tested. First was a open perimeter wheel cover. Second was an almost full wheel cover with the wheel center open (pics). Tape mock-up was installed on all four wheels to get aerodynamics information.

The goals are :
1. Find the most appropriate wheel cover for this 1999 Acura EL car in terms of actual fuel economy and aerodynamics behavior.
2. Determine which wheel cover will allow brakes heat to escape.
3. Aesthetic constraint: the final wheel covers have to look good on the car and not disfigure the good looking wheels (wifey requirement)

To measure the parameters to satisfy goal 1, Torque Pro App was used to log : throttle opening (%), acceleration (x), mileage instant, RPM, Speed GPS. The data series were logged every half second.

To satisfy goal 2, simple observation and "touch feeling" were used

Goal 3 is satisfied when "wifey" says it looks good. She did. See pics.

Testing of the mock-ups were conducted "back-to-back" on the same morning and road segment. The previous day, on the same road and similar conditions a series of runs without any cover were done in order to obtain a baseline data for comparison.

The metrics for comparison were the top speed achieved at a constant throttle opening according to the "throttle stop" method proposed by Julian Edgar on Youtube. The instant mileage averages over the few seconds that the speed remained constant at the said throttle opening were also compared.

Tests were conducted in the following order:
0. Baseline test, no covers (32 Celsius)
1. Open perimeter mock-up (17 to 20 Celsius)
2. No perimeter mock-up (17 to 20 Celsius)
3. Final wheel cover installed

Results and observations:

1. Open perimeter mock-up : allowed a top speed increase of 1 to 3 % over the base line even though the air temperature was lower by more than 10 degrees Celsius.
2. No perimeter mock-up : did not allow an increase in top speed as the open perimeter did. On the contrary, it decreased the top speed obtained by the open perimeter mock-up by at least 13 %. Also, the tape on the wheel perimeter felt hot to the touch on one front (disc) and one rear (drum) wheels. For mileage, the open perimeter mock-up used at least 7 % less fuel than the full perimeter blocked wheels. The final wheel covers will be with an open perimeter like the Tesla "aero" wheel covers and the design will have to match the underlying wheel.

3. Final wheel covers (17 Celsius : same road): Projected wheel covers fabricated with aluminum pizza pans and decorated with back sticking lettering vinyl was okay’ed for aesthetic.

These final wheel covers did not allow for a significative increase in top speed for a given throttle opening over the mock-up open perimeter wheel covers. On the other hand, fuel economy was improved by 2 to 5 % over the said mock-up. No particular heat accumulation was felt on the perimeter of the aluminum pizza pans.

Conclusions

Cheap modification mock-up is a good way to test various alternatives to orient the search for the appropriate modification.

Definitive wheel covers design retained are appropriate to the 1999 Acura EL and are proved to be a worthy modification that saves fuel.

Addition

Final wheel covers had to be removed from wheels because the attachment method was wrong and the covers moved causing wheel balance problems.

First attachment method was simply with zipties attached to the “spokes”

Second method was with toggle bolts: removing and reinstalling the covers made the toggle part move between the spokes thus promoting cover movement and wheel imbalance.

Next wheel cover attachment method will be to use longer lug nuts, drill their center, stick a nut on hit with epoxy glue and use fine pitch machinery screws to secure the cover to the wheel. (this attachment method is proposed on ecomodder.com for a Tesla owner).

Happy and best success in EcoModding !

Acel

I like the decorated Pizza pan approach, too. Make sure your vinyl is the kind that can take the sun and the heat without pealing or fading. Also, the same for the paint. Drilling the longer lug nuts is a good, tired and true method. Good stuff!

Vortex generators trials and choice (post 1 of 5)
 

Vortex generators trials and final installation

Two vortex generators disposition were trialed from scientific literature results. Five locations were trialed until the appropriate one was determined by rear window tuft test observation. Tufts movements were filmed and a frame by frame examination allowed to get the story about geometry and location.

As for many car models from the end of the 1990’s, the rear window showed a center bubble flow recirculation thus making vortex generators an appropriate modification to reduce the drag induced.

1rst trial : geometry location proposed by

Gopal P. and Senthilkumar T., 2013, Influence of Wake Characteristics of a Representative Car Model by Delaying Boundary Layer Separation, Journal of Applied Science and Engineering, Vol. 16, No. 4, pp. 363_374 (2013)

1rst Position : Rear of VGs lined at 100 mm (4 inches) from the car rear window top edge.

The vortex generators are of the “airplane” type and are rectangular 4 inch long by ¾ inch high. Rear window was tufted so tufts movements could be observed and photographed from inside the car.

Vortex generators trials and choice (post 2 of 5)
 

2nd Position : Rear of VGs were lined along the roof edge (rearward by about 100 mm – 4 inches

VGsTufTest 2nd

Some improvement in the tufts movements. One center tuft is pointing forward and probably sticking to the masking tape. The bottom of window tufts are still very excited. The tufts along the window side are also moving a lot. This first geometry is not promising so, for the next trial, the VGs are put in the second geometry starting along the roof end.

Vortex generators trials and choice (post 3 of 5)
 

3rd Position, 2nd geometry along the roof end

2nd geometry location proposed by

Dheeraj Sagar et Al, 2010, Aerodynamic Effects of Rear Spoiler and Vortex Generators on Passenger Cars, Proceedings of ICTACEM 2010 , International Conference on Theoretical, Applied, Computational and Experimental Mechanics, December 27-29, 2010, IIT Kharagpur, India

This geometry was adopted for remaining trials.

Some improvement in the tufts movements along the window side edges. The tufts on the window center and bottom still move some.

Vortex generators trials and choice (post 4 of 5)
 

4th Position, 2nd geometry, 2nd VG type

Since previous trials demonstrated some improvement, shark type VGs which will be the definitive ones installed are trialed along the roof edge. The number of available shark VGs is only 8 and 12 are required for a complete row from the car left to right.

The window center tufts stabilize. The bottom ones still move. The side edges ones move pretty much.

Vortex generators trials and choice (post 5 of 5)
 

5th Position, 2nd geometry, VGs type mix
As experienced and reported by Julian Edgar in his book on a 2007 Honda Legend, vortex generators are positioned on the rear window glass. In my case, along the top edge. I am not tall enough to have my rear sight hindered. The “airplane” type VGs are installed on each sides to replace the missing shark type ones to make a complete row from left to right.

Ah! Finally! The window center, bottom and side edges tufts all stabilize. No picture needed! I proceed to a “clean” definitive install. Some “Dollar Store” two sides transparent acrylic gel tape was used to secure the “airplane” type VGs to the glass. The commercial shark fin type VGs have their own two side tape. The “airplane” type VGs will be replaced when the shark fin ones ordered come in in August. The acrylic gel tape glue is easily removed when soaked with WD-40, like may other glues.

Aerodynamics result

The recirculation flow bubble on the car rear window causes drag. So, using the “throttle stop” method (Julian Edgar, 2019, Youtube) against the last modification results data to date, the maximum speed allowed at a given throttle opening is increased by 2 %. The fuel economy measured at 2,5 %.

In the end, adding up small economies make up some big economy!

Flat trunk lid spoiler mock-up sparking my interest
 

EDIT: I will add this photo from my dust test last summer:
https://ecomodder.com/forum/attachme...1&d=1563813420

Ridiculous how similar the shape of our cars are. In this shot you can see that without the rear spoiler, dust settled heavily on my bumper cover. When I added the decklid spoiler mockup, this was the result.
https://ecomodder.com/forum/attachme...1&d=1563813553[/QUOTE]

California98Civic your trunk lid flat spoiler mock-up is clearly reducing the wake size as the "dead water zone" of little air movement is diminished thus allowing less dust deposition on the bumper cover. The "flat spoiler" seems to move away further rear the "dead water zone". Also, some air probably coming from under the car is moving up along the bumper cover.

The flat spoiler seems like a good modification. Fuel economy figures or "throttle stop" speed increase figures would be informative and very interesting if coming from your next flat spoiler trial.

I look forward to see your modifications.

Regards

Rear wheels boat tails
 

As seen on “Aerocivic” and some other ecomodded cars, rear wheel boat tails may help in car fuel economy. In the end, small economies a make large economy. So, I decided to try some.

I had some vinyl corners that were used as airplane type vortex generators and I had some vinyl gutter leftover from the side skirts fabrication.

Attachment 28708

Attachment 28710

The vinyl corners were first attached to the rear part of the belly pan with self tapping screws.

Attachment 28709

The boat tail is made. Note that the front of the boat tail is angled so that when installed air moved by wheel is deflected under the car. Also, note that the rear part of the boat tail will be opened like the no bottom box concept used on car rear ends.

Attachment 28711

Attachment 28712

Attachment 28713

The boat tail is attached to the corners support with self tapping screws on both sides. Et voilà!

Attachment 28714

Results

For constant throttle opening of 17,65 %, the average top speed increase is 4,3 Km/h from 110,7 Km/h. A 3 % average top speed increase at 18,43 % to reach about 116 Km/h.

Now fuel economy. That is what we are here for! The Boat tails behind the rear wheels seem to reduce fuel consumption by about 1 L/100 km or a 20 % mileage improvement at speeds between 100 and 120 Km/h. That is what the data says!

Sure improvement measured by road mileage
 

Every year we do a road trip across a hilly country area. When I compare the mileage of the recent 2020 edition to the 2019 drive in similar conditions, I get 6,13 L/100 km in 2020 compared to 6,69 L/100 Km with a major difference.

The 2020 gas tank fill up, before and after the road trip, was till gas almost flowed out. In the 2019, the before and after fill up were to the 3rd gas pump stop leaving some play because I cannot ascertain the gas pump was the same before and after.

Still, a potential gain of 0,56 L/100 Km is interesting. That is 3,18 MPG US and 3,89 MPG Imperial. That is a 8,4 % less fuel consumed with the modifications since the 2019 trip. If I blindly apply the following :

"“The Effect of Aerodynamic Drag on Fuel Economy”
Aerodynamic drag is proportional to the square of velocity, and hence the power needed to overcome drag is proportional to the cube of velocity.
if you make a 10% reduction in aerodynamic drag your highway fuel economy will improve by approximately 5%, and your city fuel economy by approximately 2%."

Then applying this the reverse way, with a 8,4 % increase in fuel economy, I may claim at least a 16% drag reduction. That means from a Cd=0,34 to Cd=0,28. If Cd is the same as the 1999 Civic EX at Cd=0.32 than my actual Cd is 0,27 on this 1999 Acura EL.

Anyway, I am not running after Cd record figures, but after fuel economy. So any improvement in fuel economy is welcomed. The rest is for around campfire boasting stories.

Congrats on a great result! And congrats for campfires and campfire boasting stories, too. What are the new mods on the car this year, beside the wheel covers?

Modifications to date in 2020 on 1999 Acura EL
 

The actual aerodynamics modifications that are on the car right now:

Tires overinflated to 45 psi over the 32 psi,
Front air deflector - splitter,
Complete belly pan or under tray if you prefer,
Front wipers deflector,
Upper grille block,
Front and rear wheels deflectors,
Side skirts,
Vortex generators,
Rear wheels boat tails,
Air flow separation edges

The wheel covers are not on the car yet.

Drag coefficient evaluation
 

I read somewhere :

“The Effect of Aerodynamic Drag on Fuel Economy”
Aerodynamic drag is proportional to the square of velocity, and hence the power needed to overcome drag is proportional to the cube of velocity.
if you make a 10% reduction in aerodynamic drag your highway fuel economy will improve by approximately 5%, and your city fuel economy by approximately 2%.

If I take this upside-down, it would read like:

If your highway fuel economy improves by 5%, that is because you made a modification that brought a 10% reduction in aerodynamic drag.

So that means sort of one can compute the drag coefficient inferring from the fuel economy improvement like you get a percentage in drag reduction that is double to the percentage in fuel economy.

So if I go creasy about this hypothesis:

Right now, I get 27,3% fuel economy over EPA, then did I get a 54,6% reduction in Cd? From the original 1999 Acura EL Cd=0,34 or Cd=0,32, the actual Cd would be around 0,15? That seems very optimistic.

I revised the data, mod by mod, and the results are in the attached pic file. The drag reduction computed would rather be 36% in average in the speed range of 116-125 km/h for an actual Cd in the range of 0,20-0,22.

Boy, it seems that car had an aerodynamics improvement potential from the start. There are some other mods I want to try, before I am done:

- Front wheel air curtains,
- Front wheel "aero-corners" - separation edges,
- Reduce front wheel arches,
- Rear wheel skirts

With some luck and budget, I will go back to the mechanical side of the Force and replace the 195-55R15 tires by some 195-65R15 with will allow to go from about 2700 RPM at 100 km/h to 2550 RPM. Then, to improve the exhaust flow, I may replace the catalytic converter and muffler. I do not want a noisier car but a better breathing engine, better torque and better fuel economy.

After all, a better fuel economy, a quieter car that looks good is all that I work for.

I am familiar with the rule of thumb about fuel economy and aero drag. I think aerohead has posted it up a couple times. I do not know what it is based upon. Nonetheless, 0.15 is certainly not the Cd of your car, though I can easily believe that 0.27 figure, plausibly. Factors like ambient temp, humidity, wind, tire wear condition, and drivetrain and charging system conditions can all affect our results, so as you onow we take results as provisional possibility only unless the testing is very rigorous. I like you project a lot!

Here is a muffler/exhaust thread and summary I created years ago to distil into one place what seemed the best thinking on this site about such mods. I think it will help you, maybe:
https://ecomodder.com/forum/showthre...tml#post289125

Proper "mileage to drag relationship" citation
 

“The Effect of Aerodynamic Drag on Fuel Economy”

"For passenger cars this means that aerodynamics is responsible for a much higher proportion of the fuel used in the highway cycle than the city cycle: 50% for highway; versus 20% for city. This means that if you make a 10% reduction in aerodynamic drag your highway fuel economy will improve by approximately 5%, and your city fuel economy by approximately 2%."

This is copied from url:

The Effect of Aerodynamic Drag on Fuel Economy | ARC

The company that claims the above seems very serious, rigorous and knowledgeable:

"Auto Research Center is a specialized research facility, which sells time in its various test rigs (such as our wind tunnel and seven post rig) and provides design and consultancy services to government/military, commercial vehicle, automotive and racing clientele. ARC prides itself on remaining a competent tier one supplier to multiple OEMs throughout these industries. We also offer scale model development, vehicle aerodynamics and vehicle dynamics capabilities and expertise, computer aided engineering software, vehicle dynamics software and computer modeling programs for the purpose of trackside simulation. ARC is a worldwide company that is committed to developing innovative solutions in the ever-changing field of transportation."

With private clients and OEM intellectual property jealous safe keeping, this company and its personnel do not write scientific papers freely or low cost accessible. They may publish on SAE with permission from the client. Reading these articles is not for free.

Anyway, with such credentials claims, I thing the above general rule they publish on their web site is credible. They offer some white papers under the "media" site thumb that are well supported by a solid bibliography.

The site offers some history notes :

"ARC was opened in 1998, as the North American headquarters for Reynard Motorsports. Reynard Motorsports was based at Reynard Park, Brackley, England and at that time was the largest race car designer and manufacture in the world, producing cars for both open wheel and sportscar racing. In the USA, the primary effort was on open wheel in Indycar/CART."

Yes, I think these guys know what they are talking about.

Yes, I think it is credible too. One can easily find graphs of speed/FE relationship online to suggest further corroboration. Too, we know the drag force magnitude increases faster than velocity until it becomes a major factor in determining a car's top speed. I'm using laypeople lingo for it, but you get the principle.

I am done toying with the Cd drag coefficient
 

In a number of posts on this thread, I have posted Cd figures attained according to my way of doing the throttle stop before and after modification comparison and Cd difference computation.

I can nor say that I was wrong or right. I can only say as in engineering teaching (https://www.engr.colostate.edu/~alla.../page8f.html):

"How much power does it take to go 60 mph? 80 mph? The weight of the car and its speed all are factors that affect the amount of power required. The force balance of a vehicle is

Drag force= Fd = Cr .m.g + 1/2 Cd. Ro. A V^2

where

Cr = coefficient of rolling resistance
Cd = drag coefficient
m = mass of vehicle [kg]
A = frontal surface area [m2]
g = 9.8 m/s
Ro = density of air, 1.2 kg/m3 @STP

First, to really compare before and after modification Cd, one has to take into account the Ro air density parameter. Air density varies with temperature and atmospheric pressure.

Then the above Ro definition relates to STP : Standard Temperature and Pressure

"Since 1982, STP is defined as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 10E5 Pa (100 kPa, 1 bar)" (https://en.wikipedia.org/wiki/Standa...e_and_pressure)

Even in advanced education there can be mistaken information. I doubt that 0 °C, 32 °F, is a standard temperature in a wind tunnel.

So, from the same wikipedia article, there is a more realistic "standard conditions" definition that would suit wind tunnel drag measures :

"NIST uses a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 1 atm (14.696 psi, 101.325 kPa). This standard is also called normal temperature and pressure (abbreviated as NTP)."

So, to precisely compare drag force, thus Cd, before and after modification to the OEM published Cd, one would have to trial mods in the same "normal temperature and pressure (abbreviated as NTP)" as the car manufacturer. This would hold true only if the manufacturer used this exact set of conditions. Which one can also doubt as these are not published data. Marketing claims sell cars better than real technical and scientific data.

On the other hand, EPA and other countries government agencies that require the car manufacturers to provide the information for the car window tag that you see when visiting the car dealership, do not measure the drag force or evaluate the Cd. Since 2007, the EPA, Canada and some others use the 5 cycles fuel economy tests. (https://www.fueleconomy.gov/feg/fe_test_schedules.shtml). For the highway speed test, the lab temperature may be anywhere between 20 and 35 Celsius (68 to 86 F). So, no reliable and accurate data to compare.

Finally, from my old days in engineering training, the car drag force coefficients analogically relate to the "safety coefficient". Before 1980s, this parameter was introduced in constraints computation by engineers to size posts and beams in buildings, machine components and whatever to take all unknown and extra loads into account. So the Cr, rolling resistance coefficient, and the Cd, the drag coefficient are just some compound value of various "unknowns" that the measuring conditions are kept from us car drivers mere mortals.

In the end, the only true measure of all car eco-modifications is the actual fuel economy realized.

However, I will keep on measuring modification with the throttle stop procedure to decide if I stop the improvement path experiment with a satisfactory result or should I discard the mod and trial something else until I get satisfied.

The path to enlightenment is a tortuous road frequently giving the impression of the necessity to begin the journey all over again. Wow! did I just write that?

I would guess you have already seen this EM wiki, but just in case you have not seen it... it has a number of 1990s era Acuras on it: https://ecomodder.com/wiki/Vehicle_C...t_of_Drag_List

EM Thread on Cd
 

California98Civic,

Yes, I have seen this thread. Since Acura EL has been only built and offered in Canada, its Cd is probably the same as Civic sedan or in between the latter and Civic SI same year at 0,34.

Unfortunately, there is no better data than Cd = 0,34 for Honda Domani estimated by car-catalog https://www.automobile-catalog.com/c...automatic.html

Cheers!