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Old 07-04-2009, 07:34 PM   #1 (permalink)
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Underbody improvements- C5 Corvette

Hi all,

this is my first post after I read through the archives for some months to get some inspiration.
After a brief trial with cardboard bellypan and rear undertray, I settled this weekend for a more permanent solution using coroplast.
The Corvette is my daily driver, so I'm interested in some improvements in FE. My current FE is not bad, about 20 mpg with mixed driving and about 29-30 mpg on the highway. The engine has about 100hp more than stock and I have a higher stall torque converter which is mainly the reason for some losses of mpg in city driving.
Anyways, I wanted to post some pics but the function doesn't seem to work.
I'll report on the results and maybe I'll figure out the attachment button in the meantime.


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Old 07-04-2009, 08:14 PM   #2 (permalink)
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Alex99 -

Welcome to EM! It looks really clean. Is the coroplast on the rear touching the muffler exhaust piping?

I'm guessing that you used the same measurements as the cardboard, so that configuration would have been "heat tested" already. But, I have to ask.


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Old 07-06-2009, 10:34 AM   #3 (permalink)
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Actually, the coroplast extends a bit further out than the cardboard. It was touching the exhaust pipe, so I snipped off the edges so they clear the tubing. Thanks for the reminder. I'm sure it would have melted.
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Old 07-07-2009, 04:33 PM   #4 (permalink)
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Alex,I don't know if GM used under car turbulence to help keep the rear-end lube from cooking.The exhaust passes so close to the housing,I would be a bit skittish about really sealing that area up.Do you have a way to monitor the gear oil temp from the cabin? If not,would it be a good idea to epoxy a thermo-couple from a remote digital thermometer to the housing and keep an eye on that? I like what your up to,but it would really suck if the DeDion went south on you.
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Old 07-07-2009, 06:23 PM   #5 (permalink)
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Looks like it's NACA duct time! Or some reflective shield around the diff?

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Old 07-07-2009, 08:35 PM   #6 (permalink)
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I think thats a a transverse style rear suspension no de dion tube to my knowledge just a lot of cast aluminum so I tend to agree with you aerohead. He needs cooling back there. all that fibreglass wont like the added heat either.

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Old 07-07-2009, 09:10 PM   #7 (permalink)
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It's a wishbone suspension (or similar multilink) but the transmission is back there. I wonder how it cools.
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Old 07-07-2009, 10:48 PM   #8 (permalink)
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Drilling a simple hole in the diff cover would allow a thermal sensor (like a water temp gauge sending unit) to be bolted into the rear diff and sealed. It would have to be in the bottom, where the oil is likely to pool around it.

To help keep temps down, change the fluids out for lighter synthetics, unless you're already using SynchroMesh in the tranny. If you are, LEAVE IT IN THERE.

Rear diff can use something lighter than the GL90 that's likely to be in there.

You won't lose much by opening up the gearbox and diff housing to get some air, but you'll lose alot if they need to be cooled and don't get the air they need. If you're unsure, open them back up partially, and add some heat sink-like structures to them. Even epoxying some simple aluminum fins to your diff housing and the bottom of your gearbox can seriously affect lube temps.

You could also leave them closed up and (if you're feeling really frisky) duct your cabin air out through that area, instead of where it normally goes. The drawback, obviously, is that you must now always have the vents or windows open.
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Old 07-08-2009, 01:45 AM   #9 (permalink)
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I'd try heat shielding first, and leaving a bit of directed airflow around the diff (or exhaust), but that's just me.
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Old 07-08-2009, 08:32 AM   #10 (permalink)
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Aerohead, very good point that you brought up. I have thought about it also before putting the cover on.
The differential housing is heavily finned with the fins above the subframe structure. It can't be seen in the ictures but the air can still reach this area undisturbed and flow through them. The exit path for this air is now slightly moved from the rear to the sides. Probably a little worse than stock but uncritical in my opinion. Especially since I don't do half-hour long WOT runs @ 180mph. These are usually the circumstances where the rearend gets overwhelmed by too much heat. Corvette drivers in germany experience these problems on the autobahn.
I have synthetic lube for added safety in the diff.
As vtec-e suggested I might put some small intake and exhaust ducts into the coroplast to promote airflow along the fins.

As a sidenote, my first results for highway mpg were very successful. I'm now able to cruise about 1.5 - 2.5 mpg better than before! I even hit 40mpg on a straight stretchof the road. I have never seen that number before @ 65mph.

Thanks for the thoughts.

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