re: Turbo heat shield
Agree with below 100%. Form a thermodynamic point of view, we should wrap the whole exhaust.
The problems have nothing to do with above but with some very heat sensitive parts on the turbo. It is the turbo bearing or the center section. Both the compressor and turbine sides can take a lot of heat. It is the bearing and the oil in it that gets cooked (literally). Once it does, the bearing needs replacement. It is $1000. The problem with heat soak in the bearings was the major barrier to tubo adoption in passenger cars. Shut off the engine and the bearing heat soaks and dies. The OEM is in for a $1000 warranty repair part... The solution is to water cool the bearing. That is why all OEM and most aftermarket turbos have a water feed.
If you wrap the turbo. The heat will stay in the turbo. It is made of (mostly) iron. It conducts heat well and will heat soak the bearing. The water cooling so cleverly provided may not be enough! That is why there is no OEM wrapped turbo. They use metal shields but not to keep the heat in but to keep heat away from other parts i.e. channeling its escape.
I wrapped my turbo's once. Blew the bearings during the first 1 hour race and never tried again.
Wrapping headers and exhaust pipes is a totally different story. It is a good idea. Given that an OEM or cheap part is made from cast iron or 309 or 409 stainless (originally made for kitchen-ware i.e. knife and forks but works on exhaust parts in a pinch
), it will start to scale and oxidize with all that extra heat, so much faster. This is a given. Read the material spec sheet. Iron will start looking ugly, stainless will look like regular iron. Flakes of iron oxide will come out of your exhaust pipe, if the car is not run for a week. Therefore, it will look ugly under the wrap. It will wear out and need replacement faster (depends on how thick the pipe wall was in the first place.) I do not think, this is all that bad. Just beware! caveat emptor!
Take a look at Inconel header that was wrapped. It changed color (chromium oxide is green). It would probably polish up. There is no scale! Take a look at the collector. It is 321 stainless (from Burns). It is starting to look grey and scaly. 321 was designed for WWII aircraft exhaust. 309 and 409 is much worse. Here is a comparison all in one pic. The whole exhaust was wrapped and driven hard i.e warmed up to cherry red.
http://www.lotustalk.com/forums/f163/elise-exige-racing-wide-body-81258/index5.html
Anton
Using Thermodynamic theory you want the gases to stay as hot as you can for as long as you can. Does it make a difference, probably not enough to measure. What wrapping DOES do is limit the amount of radiated heat to nearby structures and helps to lower the temps in the engine bay. That is a good thing. The bad thing is the parts you wrap get hotter and stay hotter. They are made to handle the heat but it probably does shorten the lives of those parts. On the other hand it helps to prevent them from cooling quickly and that is a good thing. It also reduces noise. It does make servicing harder and does not "add lightness". IMHO it is probably better to wrap than not. I would at the very least wrap the pipe going to the turbo so there is the most amount of heat (read energy) as possible getting to the turbo.
David Teitelbaum
