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I plan on making a full bumper to bumper belly pan on my 08 Fit, but I have a few questions.

I plan on using both coroplast and aluminum. It will be about 80% coroplast, and the aluminum will be for the areas covering the exhaust.

#1- How close can the coroplast get to the exhaust? I am going to try and keep it about 8-12" away.

#2- I plan on drilling some holes in the aluminum where it will be covering the cat (and maybe a few where it covers the exhaust. Will this help the cooling of these components?

#3- I plan on leaving a small area uncovered, so that access to the drain plug and the oil filter is unobstructed. Will this also help keep the under hood temperatures from getting too high? I am sure the vacuum effect of the air under the car pulls heat from the engine bay, will this uncovered area be sufficient and not cause the car to overheat?

#4- For those with belly pans at least to the firewall; what type of temp increases have you seen?

#5- What are some realistic FE gains I may see?

Thanks for all your insight:D

#1 Minimum coroplast/exhaust clearance of 3" on my car.
#2 You can add the holes if you want (I don't have them on mine). On my car, I've set it up so there is an airflow running from the engine compartment through the exhaust tunnel with no problems in 2+ years/100,000 miles of driving.
#3 I have a hatch in the belly pan to access the oil drain plug and filter. I exhaust most of the air from the engine compartment out through the front wheel wells and through the exhaust tunnel.
#4 My car is too old for a Scangauge, but adding the belly pan had no effect on the temp shown by the radiator gauge.
#5 2 to 4 mpg depending on what speeds you drive, also less road noise.

#1 Minimum coroplast/exhaust clearance of 3" on my car.
#2 You can add the holes if you want (I don't have them on mine). On my car, I've set it up so there is an airflow running from the engine compartment through the exhaust tunnel with no problems in 2+ years/100,000 miles of driving.
#3 I have a hatch in the belly pan to access the oil drain plug and filter. I exhaust most of the air from the engine compartment out through the front wheel wells and through the exhaust tunnel.
#4 My car is too old for a Scangauge, but adding the belly pan had no effect on the temp shown by the radiator gauge.
#5 2 to 4 mpg depending on what speeds you drive, also less road noise.

Thank you VERY much for your response, the way I see it you are the aero master:D.

I will post up step by step pictures when I do the project in a few weeks.

The faster you drive, the more impact aero will have for MPG. Depends on your car, but a threshold speed I hear tossed around a bit is between 45 and 50mph as the point where drag starts taking a toll. :)

Eh... Go for Alu all over.
Coroplast I hear tends to rip.

I am about to put Alum and Zip ties together.

Coroplast is very tough and difficult to rip, but you need to use a washers on the screw head when fastening it down. It does break down after a few years of exposure to sunlight (use paint to protect). Aluminum is good for the permanent version of the mod, except for underside items like wheel spoilers and side skirts that need to be able to flex when they make contact with an uneven road surface (aluminum takes on a permanent crunch when it makes contact with these). I will usually make the prototype in coroplast because it is cheaper and easier to work with and then rebuild with aluminum when I am satisfied with the design. Coroplast has about a 2 year lifespan when used in the most exposed locations.

Thanks for all the replies guys!

I have two more questions;

#1- I plan on putting deflectors in front of the front and rear wheels to limit the amount of on coming air that hits the tread portion.

How far down should the deflector go (i.e. how much of the tire should be blocked)?


#2- I know the prius and some sports cars have front deflectors, though I can't say I have seen ones in the back.

Would putting deflectors in front of the rear tires help as well?

#1 The more of the tire you can block the better, but if you go too far down then it abrades on the road when you hit a bump. I initially made simple coroplast deflectors that reached 2/3 of the distance from the chassis to the ground and then trimmed it back until it didn't scrape the road on bumps, then I made a permanent version of the deflector. It ended up reaching half the distance to the ground.

#2 I have deflectors on both front and rear wheels and a mini-boattail behind the rear wheels. Double side skirts between the front and rear wheels completes my aero treatment of the wheels.