sperry -- you're a little bit mistaken on some of those points. Most importantly, just the
huge and extremely dramatic drop in underhood and intercooler temperatures a WRX sees with a thermal coated or heat wrapped downpipe vs. a bare one. There is a heck of a lot of exposed downpipe metal that is still inside of the engine bay and there is a lot of surface area for it to radiate heat from. It does, of course, get very hot. Your
turbo heat shield does cover an inch of the downpipe, but that doesn't prevent the other ~ 18 inches under the hood from radiating heat like crazy and, as we know, heat tends to go upwards and upwards is where you keep your intercooler. You'll notice that when you come to a stop, you can literally see hot air shooting out of your hood scoop. That hot air is going up through your intercooler and out your scoop, and a whole lot of that is thanks to your downpipe. While a heat shield does an okay at blocking direct radiant heat, it doesn't block conducted heat or hot air flow very well at all and once that heat shield gets hot, which it does!, it starts radiating its own heat into the air as well. However, if you coat the downpipe or wrap the downpipe, you're actually preventing heat from leaving it in the first place... radiant or conductive. In fact, because we coat outside AND inside, we're preventing the exhaust heat from soaking into the metal to begin with.
I think you'd be shocked at the difference. Without even measuring, it is just painfully obvious when you open the hood after a drive. You wont feel a rush of hot air on your face. You'll be able to press your hand on the intercooler to find it at ambient temperature. It takes a lot longer to see hot air coming out of your hood scoop and the intercooler stays cool to the touch much, much longer after you come to a stop. In fact, we see a DROP in underhood temperatures and intercooler temps with our uppipe and downpipe vs the OEM parts with all of their heat shields. That drop isn't huge, but it's big enough that many many customers have mentioned it in their reviews and some even took the time to verify with measurements. The difference between coated and a bare aftermarket pipe.... MASSIVE.
If you're running a short ram intake, you get direct HP benefits from sucking in colder air thanks to your cooler engine bay. If you're running a TMIC, you get benefits of a higher knock threshold thanks to the lower intercooler temps and you're much less likely to knock if you drive hard immediately after coming to a stop for a little while (like after sitting at a traffic light or staging at the track before you get to race), as the rate at which the intercooler heat soaks is slowed way, way, way down.
While it's very easy to assume that if you're moving, the air flow is going to outweigh the heat coming off of the downpipe, it's typically not true. First of all, there aren't many situations where you never stop
... BUT... there simply is not enough airflow and pressure through the intercooler and the grill (and, btw, after the air goes through the radiator it's not cool when it comes out the other side) to push all of that heat out the back of the engine bay and under the car. There is a difference in underhood temps while driving. Less than the difference after you come to a stop, yes, because of that air flow, but it is NOT sufficient to make up for all of that scorching hot, bare metal under the hood.
....we do a lot of custom coatings for people, btw. It's typically $85 for a shorty downpipe and $105 for a full-length downpipe... It's worth the price, unquestionably. If you don't want to spend that sort of $$$, spend the $50 and heat wrap it (which, although it also works very well in the end result of dropping underhood temps and we do have a good account with DEI, is not our preference because it tends to cause metal fatigue and the wrap is very tough on whatever is underneath it... eating into the metal and causing pretty nasty surface rust).
For what it's worth, the WRC Subaru ceramic coats its ENTIRE exhaust, in-and-out, from cylinder heads to muffler tip.
Jeremy