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Old 01-19-2010, 05:45 PM   This thread is in the EcoModder Project Library | #1 (permalink)
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The Daq Civic aero project continues . . . slowly

Awhile back I started my 1st thread with pics and descriptions of my aero mods--mostly an undertray and front tire deflectors. I've gone a bit further with it over the last few months, adding pieces and refining what was there before.

This is a side-view schematic of my car, a 92 Civic 4D DX 5MT:

It's not a bad design for its age. The main problem areas as I see it:
  • the steep angle of the rear window
  • the rough underbody, with open engine bay and parachute rear bumper void
  • the front bumper, which does not extend low enough, so that the stagnation point is relatively high and forces too much air underneath the car rather than over
  • the rear body, which tapers gradually to prevent clean flow separation
  • the fenders (front and rear), which are too large and do not fully cover the front profile of the tires.


Another thread posted this pic of tuft testing the front. The front of my car is identical in shape to the one pictured.



So far I have mainly focused on improving underbody flow, since it has more room for improvement and it's easier to attach things there. I would greatly appreciate any analysis and suggestions!

This is the front undertray as it was when I posted the first time.



It is essentially the same now, but I have since covered the strip in the center where you can see the exhaust tube, and also significantly closed up the wheel wells.


This was my first attempt at an aerodynamically shaped front tire deflector (as opposed to the flat deflectors seen on production vehicles).

I was not happy with these at all. They are far too small, and I have since learned 2 critical points: it is just as important to trip flow past the low-pressure void of the wheel wells as it is to deflect air around the front face of the tires, and on most cars the airflow approaches this area at an outward-facing direction, so the deflectors need to be placed and angled inward.

I just installed these new prototype deflectors:





I'm reasonably happy with the angles, however I'd like to expand them further inward to "cover" the entire exposed wheel well.


This shows the mid underbody untouched. It's not the worst, but far from optimal.



I've since extended the undertray all the way back. These pics show the new section from the front doors forward.



The opening is wider in the view here since this is where I want to extract air from the engine bay. It narrows greatly further back. Unfortunately this zone is too far forward. It is so close to the wheel wells that for several inches the undertray reduces to 2 narrow strips in between, meaning that there is little attached flow at this point. In the future I may narrow the opening at the widest point.


Here is a newer pic showing where I have covered up the place where the exhaust downtube was exposed, and also the lip I added on the leading edges of the opening at the exhaust/transmission tunnel.


Another pic of the lip:



A view from the rear wheels forward.

Unfortunately the exhaust hangs down below the undertray by a couple inches at the point where cuts over to the passenger side. This will break up the attached flow on the passenger side. I may try to use metal flashing to cover it, trying to make it as flat as possible. I have left a small section of the gas tank uncovered as it is already quite flat.


A bit further back:



A closer view around the rear passenger-side wheel:

The trailing arms forward of the wheels take up quite a bit of space, and since they pivot down at full suspension travel I had to cut out the undertray underneath them. Further down is a pic showing a cover I made for the trailing arms/lower control arms.


The rear of the undertray at the passenger-side corner:

I got as close to the muffler and tailpipe as comfortable. The coroplast doesn't show any signs of melting or warping. I did my best to preserve smooth flow around the muffler area considering that it hangs rather low.


The full width of the rear undetray:

You can see that the trailing edge extends past the bumper, by about 4 inches. This keeps flow attached a bit longer and allows it to break off sharply and smoothly. The angle of the undertray from the rear wheels back is about 5 degrees, well within the range necessary to keep flow attached, but tapering up so that the wake height is reduced. It could be angled more, but there isn't an easy way to do it without cutting up the rear bumper cover.


Recently added are rear tire deflectors (just to have something there; they're from a failed front deflector design) and rear trailing arm/control arm covers, shown here:

I'm going to redo both; these are just trial versions.


And my grill-to-radiator duct:





It's very tight in there, so it looks like a hack job, but I think they're effective enough that the same amount of air is getting to the radiator as stock even with a 2/3 grill block.


I've also been thinking about how to reduce the drag created by the side mirror (there's only one on my car). While I can't really reduce the frontal area, I would like to reduce the degree to which the mirror housing breaks up airflow along the side of the body. Looking at newer cars with more sophisticated aero design, it seems that compared to earlier designs they are (1) reducing the frontal area of the arm that attaches the mirror to the body, (2) making the inner edge of the housing more parallel with the body, and (3) making the entire width of the front of the housing angle away from car instead of having the inner half angle toward the car. I used a few scraps to test how easy it is to approximate the latter 2 aspects:



When I redo this I will do before/after tuft testing.


Finally, here are the places I've tried to seal up the gaps in the hood:







Planned mods to add soon:
  • revised tire deflectors
  • plastic strips on the front edges of the fenders to trip flow past the wheels (in lieu of full covers--too much work)
  • revised smooth grill block cover
  • Prius-style tapered extensions behind the rear wheels
  • rear trunk lid extension (to lower reduce angle from the top of the rear window so flow is less detached and turbulent as it reaches the wake)

Apology: The more committed among you will be wondering where my coast-down test results are. I don't have them 'cause I haven't done any. I'm reasonably sure that the mods I've done have reduced aero drag measurably, though not nearly to the degree of someone like Basjoos. First, my HWY-only FE is significantly greater (>15-20%) than before the mods (yes, at the same speed and using hypermiling techniques). Further, there are several downhill freeway sections I frequent on which I notice that I can coast down and loose almost no speed, whereas before I would loose quite a bit. I use markers and constant speeds, but I haven't written results down. Alas, I love FE and aero but I'm just not that scientifically minded (and I don't typically have several-hour chunks of free time).


Last edited by daqcivic; 01-22-2010 at 01:07 PM..
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Old 01-19-2010, 05:52 PM   #2 (permalink)
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Looks good!

Undertrays are not as easy as some people think.
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Old 01-19-2010, 07:02 PM   #3 (permalink)
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I admire your attention to detail.


The smaller your radiator opening, the smaller the opening at the rear of the engine bay has to be.

How many repair/maintanence jobs will require you to remove the engine bay tray? How many fasteners will have to be removed?

My engine bay tray was torn off at >100mph airspeed, and it was held on by two bolts up front, two at the back.
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Old 01-19-2010, 07:36 PM   #4 (permalink)
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did you makes a hole to drain the oils?
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Old 01-19-2010, 07:57 PM   #5 (permalink)
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nice work on the pans, im interested in your tire deflector design as this and skirts are my next mods,, i noticed that some factory tire spates are angled to defect air under the car, backing up your theory
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Old 01-19-2010, 07:58 PM   #6 (permalink)
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I have small holes in the undertray for the oil drain plug and the front jack points. I have access to the oil filter and some other rear-engine components through the exhaust tunnel opening.

Unfortunately the front edge of the undertray is fastened by a long row of screws along the edge of the bumper. I have thought that if I have to get more access underneath I will simply cut out a section and reattach it later. I couldn't come up with a good way to make the undertray more modular because shaping and attaching it over quite uneven surfaces required very exacting measurement and the ability of the undertray to bend smoothly over some contours. If I have to remove it, I will probably use it as a template for cutting a new one, as I have a few areas that are patched up.

On another note, I have been extremely happy with the durability of the undertray. My suspension is extremely soft and I live in a city with lots of uneven pavement and steep driveways, so I have scraped bottom many times (I drive kind of hard over bumps too, so as not to lose speed). All the undertray has to show for this punishment is a couple small scrapes and punctures, far too small to make a difference to aero. It's at least as solid as stock.
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Old 01-19-2010, 08:24 PM   #7 (permalink)
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Tire deflectors are something I've been studying even since I noticed how many cars have them. The are almost always positioned inward of the tire centerline, and for a long time I wondered why. I believe it was here I read in some discussion that the front underside is a high pressure zone, which means the air wants to divert outward along the sides of the car. That was a "duh" moment for me. There is some talk about the wheel wells being a major source of drag and a complex problem in this post of an interview with a BMW aerodynamicist. For more of my thoughts on deflectors and wheel wells, see this thread.

I've paid particular attention to BMW and Mercedes designs. The deflectors often wrap around and curve inward along the arc of the wheel well. Usually the height of the deflectors also tapers as you move inward. A good example is the E-class, which in coupe form has a cD of .24.




My understanding is that they do this to both divert air around the front tire surface and across the void of the wheel well. The large difference in pressure fore/aft of the deflector also makes the wheel well a prime location to extract hot air from the engine bay. This makes more sense than extracting it at the center, where the slow exiting air breaks up the clean and fast flow underneath the otherwise smooth underbody. The airflow near the sides of the car is already somewhat dirty, especially rear of the front wheels, so extracting air there doesn't hurt anything and may even help by equalizing pressure.
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Old 01-19-2010, 08:34 PM   #8 (permalink)
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im also going to do side skirts on back maybe the front if i can muster the courage to drive such an odd looking car, if the front wheels are skirted, like basjoos with rollers for tire contact do these front skirts change the tire deflector design, if so how?
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Old 01-20-2010, 02:25 PM   #9 (permalink)
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I'd lower it about 1-1/2".
I have the same car a year newer with a B20B. The lowering on AGX's and sport springs made it handle better, and cut some drag. I see 33mpg in mixed driving with the 2.0L, where I was lucky to see 31mpg with the stock engine/stock car.
The lowering has increased negative camber which wears the tires on the inside a little, but I feel this reduced contact patch is helping mpg. When you throw the car into a turn, tire roll pitches in and you get full contact patch and grip when you need it. I've been running Nitto NT450's on 15" rims in the 195-50-15 size. They have a soft carcass and during cruise I'm on the inside edges, but in heavy braking and cornering the "squirm" makes them hook up well so the car still performs. I get about 30k miles out of a set, but have to have them swapped to the other side or the inner tread is done alot sooner.
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Old 01-20-2010, 05:58 PM   #10 (permalink)
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I used to have a 91 SI with Eibach Sports and AGXs and performance tires. It was stolen, and I really miss driving it. This 92 DX doesn't handle worth a darn owing to its soft springs and old limp struts, but it's what I got. Yes, new suspension would improve aero and handling, but it's a really old car and I just want to get the most out of its remaining life for as little money as possible. The aero mods I've done so far are extremely cheap and reversible, which is why I've done them.

BTW, I plan to have newer pics up tomorrow, and I'm keen to get feedback on the prototype front deflectors.

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