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Old 01-19-2010, 04:45 PM   This thread is in the EcoModder Project Library | #1 (permalink)
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Join Date: Nov 2008
Location: Portland, OR
Posts: 53

Daq Civic - '92 Honda Civic DX sedan
90 day: 42.54 mpg (US)

The Hardbody - '91 Nissan D21 (pickup)

The Hardbody - '91 Nissan D21 (pickup)
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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 12:07 PM..
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