Dave, no argument from me on more holes = more airflow, whatever the variety. For me though the pain of cutting my car and therefore eliminating the option to ever return to stock combined with the added moisture entry points, encouraged me find other ways to reduce the heat without changing to hi-flow cats. I had listened to a few Vipers with hi-flows and didn’t like the acoustics.
Anyway, years ago I posted several threads on my quest for stopping sill corrosion and lower sill temps. Since the links are all broken I’ll repost here – Note that none of what I’ve done permanently changed the car – I used a donor exhaust and kept my original.
Warning #1: A lot more detail than anyone wants but at least you’ll understand what I base my opinion on.
Warning #2 – I’m a marketing guy not an engineer, so all of my opinions are based on my observations and my logic, not science.
First I charted what I had. It confirmed that there was significant airflow through the sills as speed increased which lowered sill temps despite the higher RPM’s/exhaust flow. In addition, max sill temp wasn’t while idling but after engine shutdown.
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I decided that to lower temps further, I need more airflow during idle and shutdown. To do that I plumbed a couple of ceramic tubes into the sills and used the radiator fan / ramair to force air in. Did it help? Not that I could tell but total cost was $20.00 and added weight was negligible so I left them in.
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Next project was the corrosion. By this time I noticed a bubble on the sill and pulled them apart to try to figure out the root cause.
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As you can see below the inner sill insulation is unwrapped and therefore a sponge when exposed to water. Start it up and you have a 10 cylinder steamer.
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Having had one sill replaced under warranty, I observed that the new insulation was updated by being metal wrapped cat side and sill side, not just cat side. I manually did the same on my other sill by coating the sill side insulation with an adhesive foil coated insulation wrap and then heavy duty aluminum foil. Not a permanent fix but would by me some time.
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Looking closely you can see the difference
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OK with that done, back to temps – I then decided to have the exhaust coated to keep the heat in the exhaust gas itself and therefore (theoretically) forcing the heat out the back in the exhaust stream itself. A friend had done a similar exhaust coating and his exhaust gas temp at the tips were 100+ degrees hotter than an uncoated Viper parked next to him. All credited to the coating? At the time, yes.
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Anyway, after the entire exhaust system was coated with a combination of stuff, including zirconium, I installed only one side in order to get an A/B comparison on the same car under the same conditions. Results:
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I will admit that the cast iron manifold was still on the uncoated side so perhaps that had something to do with it, but it couldn’t have been significant. My guess is that the other coated exhaust Viper was running a different air/fuel ratio (it was lightly modded, w/ non OEM cats) which caused more of the increase vs. the coating.
Well, I’m this deep in so the coated exhaust is going on. At least I’ll get the 4 unclaimed horsepower with the shorty late GENII headers! Nope, the car dyno’d 3 HP less with the stainless shorty’s versus the OEM cast iron manifolds. Probably dyno variance but was tested on the same dyno under almost identical weather conditions. At least I saved 20 lbs….
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So here are the internal temp measurements for those curious enough to care (I had ports cut and welded to the front and back of the cats and each exhaust tip):
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And external temps in direct contact with the part (warmed up & idling w/ sill off):
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My final modification was suggested by Craig, of power steering vent tube fame. To ensure airflow around the cat, he suggested adding a cat wrap of 3 dimensional screen over-wrapped with 3M Nextel ceramic cloth, good to 2012 degrees. It’s apparently a popular NASCAR insulation. The idea was to provide additional cat insulation while ensuring airflow around the cat itself both under and over the Nextel wrap:
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Did it help? Yup, sill temps dropped as did peak temps post shutdown. Unfortunately I can’t find that spreadsheet but for the two people that are still reading it was an improvement on the order of 30 degrees as I remember.
. Don't think so.
.The car is rear engined and a lot of heat goes on in there.I'm thinking of using this material to attach on the inside of the sills and see what happens.I think the main problem is that the OEM cats are too large in diameter and long,nestling against the insulation blocking airflow.I thought using the OEM insulation only on top as a tunnel above the cats.Use vents on the underside of the sills as you did,plus adding the reflective material inside the sills.
Alternatively, just paint your car yellow. It is slower, less heat, cooler temps.
