CivicTail project: ginormous aerodynamic tail on '07 Civic sedan

[aerohead]

Originally Posted by MetroMPG

Thanks, Phil! It should be easy to sell. It really is a nice car.

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Not worth 30 gallons of gas for a cardboard + coroplast boat tail that won't fit your car properly! DIY, baby.

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I posted the fancy test graphs and raw numbers in post #4 (which I had 'reserved' for that purpose when I started the thread).

You'll see in the first graph something that we haven't talked much about: the Kammback section on its own apparently made no difference vs. a bare back end. Very strange, this Civic. (More likely: not optimal, my handiwork.)

I caught the comment but wanted to wait 'n see how things shook out.
At least one member has shared an example of a notchback with lower drag than a hatchback/Kamm style car.
Chrysler ran into something like this with the Daytona.At the time,they considered a 12-degree roofline to be a proper 'fastback'.(Kamm had tossed out a 10-degree rule of thumb for HOT ROD Magazine,but didn't follow his own rule)

With the Dodge,we see that an 18-degree slope notchback comes in at lower drag than the 'fastback.' The flow on a notchback is governed by a delicate dance between the backlight,C-pillars,boot,and body sides.
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The template is based upon a body of revolution where the sides and roof begin their curvature at the same point on the body,and the sides are indistinguishable from the roof as far as contour goes.

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Looking at the views of the CIVIC I notice that the roof hits its max. camber point between the door handles as we see it,but the beltline,at the base of the greenhouse is still rising all the way to the taillight lens,putting its max camber point there.
This would suggest that the tail for the main body section would have to start from zero down-slope at the rear transom,with no slope at all,but with whatever plan-taper Honda already had.Just like I had to do with the T-100.
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The CIVIC's roof would be like Jaray's combination form cars,with a 'faster' greenhouse than the main body.
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The tricky part is that without a rolled edge over the taillight void (as I did with foam and Plexiglas with the T-100) we're asking the air to camber downwards and inwards at this 'edge' without a structure to guide the transition.
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I've made a bunch of images of the CIVIC to toss over in my head and my thoughts go to a 'slower' tail on the main body,using the template at the top of the taillight as a guide,then let the 'stinger' from the greenhouse die into it,blending,as on the VW XL1 car.
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Without a plan-view it's
hard to tell,but it seem like the 'stinger' could be a little more aggressive in plan-view,it's essentially perfect in side elevation.
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Seems like we were very 'close' and I have no idea how the engine behaves.
AeroStealth found that between say, 55-70-mph,the F-150 gets better mpg with the AC on than without,after Brett's modifications were added.Then above 70-mph,it's better to lose the AC as far as mpg goes.Which suggests that the 'sweet-spot' for BSFC IS affected by the streamlining.
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My ignorant suspicion is that you lowered drag more than the mpg would suggest,and that the bugaboo is with the engine mapping/gearing/load.
If you were commuting between Austin,TX and San Antonio on the 85-MPH Texasbahn toll road,no other CIVIC would be able to get anywhere near your mpg.

[2000mc]

Metro - at the speed you tested at, do you know what you see for vacuum, load, throttle%, rpm?

[California98Civic]

Phil, I think this is the plan view you are looking for. Though it is the type R, it is a 2007 sedan civic. I don't think the plan view is different, judging from pics online


And for easy reference...

[ECONORAM]

Originally Posted by MetroMPG

Near perfect conditions yesterday (mild, dry, light to no wind), so I got out to do testing on my usual flat test route, with no other traffic to interfere.

And I have to say: I am super disappointed with the numbers! I've never seen a tail have such a small impact:

2% improvement (1 mpg US) at 90 km/h / 56 mph.

I'll post the full details later on.

First off, wish I'd found this thread sooner! Second, my thoughts exactly (disappointed) when I saw the graphs you posted previously. Based on what I'd seen from other Kamm/Tail combos, this didn't compute...
[edit] I just experienced similar disappointment when I (temporarily) removed my truck's topper and fabled a quick corroplast bed cover. Negligible mpg difference; so I'm confused.

Originally Posted by aerohead

We'll have to except the numbers for what they are.It is what it is.
All the time,energy,and trouble to do the testing will be valuable just the same.
Sir James Dyson said he never learned a thing from his successes.It was the 'failures' which were the most valuable.
Carl Breer's team, Dr.Gino Sovran,and Hucho have all warned about limited results from drag reduction if the engine could not be maintained at a constant load to maintain BSFC.
I don't know that this is what's going on,but since it's been emphasized in the literature,it's something obvious that we must consider.
If the load is reduced,you're at even less part-load.The throttle body may be nearly shut as,unlike a Honda carbureted CIVIC,with three throttle bodies,the '07 is taking all air in through a single barn door.Barely cracked open.(horrible volumetric efficiency).
Without another gear to load the engine back up,the BSFC may have fallen off its island of peak efficiency.
Darin,we're indebted to you for going through the whole process and all on your own nickel.Lets see what we can figure out.
Since the US dumped the 55-mph speed limit,the CIVIC may be calibrated to run at higher posted speeds.If we had a baseline at 75-mph,we might see a completely different trend.
Thanks for all you do!

That is an interesting thought. My truck is also a single blade throttle; it causes me some difficulty when dawdling along on base (25 mph limits typically). I'm thinking a warm air intake will help this?

I like SH130; 80-85 mph is nice (except when the vehicle guzzles fuel at 15-16 mpg)

[MetroMPG]

Don't forget that a 1 mpg difference in a 15 mpg truck shouldn't be as 'disappointing' as a 1 mpg difference in a 50 mpg compact car when you calculate the volume of fuel saved for each. Just a thought.

[aerohead]

Originally Posted by ECONORAM

First off, wish I'd found this thread sooner! Second, my thoughts exactly (disappointed) when I saw the graphs you posted previously. Based on what I'd seen from other Kamm/Tail combos, this didn't compute...
[edit] I just experienced similar disappointment when I (temporarily) removed my truck's topper and fabled a quick corroplast bed cover. Negligible mpg difference; so I'm confused.



That is an interesting thought. My truck is also a single blade throttle; it causes me some difficulty when dawdling along on base (25 mph limits typically). I'm thinking a warm air intake will help this?

I like SH130; 80-85 mph is nice (except when the vehicle guzzles fuel at 15-16 mpg)

I'm not much of an engine guy.On my Honda, air temp is thermostatically controlled at about 100 F and fed to the carb.
On an EFI engine,if you were going to do this I'd think you'd want it upstream of the mass airflow sensor,or else the computer couldn't properly control the injectors.
But horsepower for horsepower,the engine would need the same mass of air,and you'd still be stuck with the inlet restriction of the barely-cracked throttle plate.
Mitsubishi and Toyota have GDI engines which are throttle-less,just like a Diesel,with 15% higher thermal efficiency because they're always at WOT.
I've heard of gasoline engines with dual throttles,one for low speed,then transitioning to the secondary for higher power settings.I've never seen one,but it sounds like a good compromise.

[aerohead]

I looked back over the CRX project and here are some numbers to consider:
*At 30 mph,Aero hp = 1.21 / Friction and tire losses = 1.76 hp,for 2.97 hp.
*At 50 mph,Aero hp = 5.61 / Friction and tire losses = 2.94 hp,for 8.55 hp.
*At 70 mph,Aero hp = 15.3 / Friction and tire losses = 4.12 hp,for 19.50 hp.
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*At 50 mph,the road load was 14.25% of rated power.
*The engine would be developing 9.28 bhp (15.46% of rated power).
*With a 3-barrel carburetor,the reduced load might have allowed WOT idle circuit throttle,plus WOT secondary throttle,and zero throttle at the 3rd barrel.
*This would allow a very good volumetric efficiency as far as pumping losses.
*The streamlining allowed the CRX to achieve an mpg @ 70 mph,which she used to see at 55 mph.(52 mpg).
*Top speed went from 93 mph,to 100 mph.
*It wasn't uncommon to see 60+ mpg at the old double-nickel speed limit (the speed the car was designed for).(highest ever single tank= 82 mpg).
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*For an EFI engine,with a single throttle,a reduced load would mean a throttle position trending towards closure,extremely high drag across the throttle plate,high pumping losses,and volumetric efficiency all shot to hell.
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*Taller gearing would 'load' the engine,force a more open throttle,improve the volumetric efficiency,and tend to move the engine back to it's optimized island of BSFC.
*I don't know the horsepower for the CIVIC but I suspect that it's much higher than the CRX and my opinion is that maybe even a smaller throttle body might help the situation.I'm not an engine guy.
The upper two images show the stock car and mods as far as I got.The drag coefficient is estimated from top speed,mpg, and coastdowns @ Cd 0.235.

[Vman455]

Originally Posted by aerohead

*For an EFI engine,with a single throttle,a reduced load would mean a throttle position trending towards closure,extremely high drag across the throttle plate,high pumping losses,and volumetric efficiency all shot to hell.

I'm curious what we are to make of the efficiency increase with the initial round of aeromods, before the kammback and tail were added. 15% improvement with wheel covers, grill blocks, mirror delete, etc; then the tail did hardly anything. Is it possible Metro just happened to hit a plateau in engine efficiency?

[aerohead]

Originally Posted by Vman455

I'm curious what we are to make of the efficiency increase with the initial round of aeromods, before the kammback and tail were added. 15% improvement with wheel covers, grill blocks, mirror delete, etc; then the tail did hardly anything. Is it possible Metro just happened to hit a plateau in engine efficiency?

We're kinda at a point where we'd need a laboratory and an engine map for Metro's car.
We can speculate for ever.The science says we're looking at a BSFC problem.This dates at least to 1934,with the Chrysler DeSoto aerodynamic test mule based upon the Airflow.They are the first to report on the problem.Dr. Gino Sovran of GM Labs and SAE published a paper reflecting on the problem.Ditto Wolf Heinrich Hucho.
Unless the engine can be shifted back to it's original BSFC it doesn't matter how much we reduce rolling resistance or aerodynamic drag.We're gonna be facing this demon."Gear-matching" has historically been the solution to the problem.
Whatever rpm occurs at the original top speed,gear the streamlined car to be at exactly the same rpm at it's new higher top speed.

[ECONORAM]

Originally Posted by aerohead

I'm not much of an engine guy.On my Honda, air temp is thermostatically controlled at about 100 F and fed to the carb.
On an EFI engine,if you were going to do this I'd think you'd want it upstream of the mass airflow sensor,or else the computer couldn't properly control the injectors.
But horsepower for horsepower,the engine would need the same mass of air,and you'd still be stuck with the inlet restriction of the barely-cracked throttle plate.
Mitsubishi and Toyota have GDI engines which are throttle-less,just like a Diesel,with 15% higher thermal efficiency because they're always at WOT.
I've heard of gasoline engines with dual throttles,one for low speed,then transitioning to the secondary for higher power settings.I've never seen one,but it sounds like a good compromise.

Copy all.

Since my RAM is a MAP reading vehicle, I pull warm air through the radiator near the hot water inlet, and duct it into the air box. Couldn't tell you how much throttle blade angle difference it makes, but it does make the tranny downshift earlier. PCM is intake air temp sensitive; a real drag when climbing hills.

My 1984 Jetta had a two blade throttle; the smaller opened at low power settings and linkage made the second open after the first one was wide open... At that time people were complaining of the on-off feel of mono-throttle blades.