Exhaust

Quick question for anyone who may have this set up. I have very minor mods on my SC at this time. It's an '89 and thus far, I have added a '95 supercharger and 1.73 roller rockers. Before I open up the intake, I want to make sure my exhaust is ready for it. I want to put long headers in and have found them from Coy Miller (the stage II long headers) and am planning on getting a custom dual 2-1/2s from the downtubes (also from Coy Miller w/ the high flow cats). I realize that this set up will reduce my low end tork. Not too woried about it because I'll get it back with future mods. But I am wondering about the overall effect of that exhaust. I have been told that a certain amount of back pressure is needed to keep any car operating correctly. I don't want to open the exhaust up and then overall performance will reduce. To me it seems that at higher RPMs I should notice an increase in performance. But I don't understand the relationship between the exhaust and back pressure and why it is needed. So if any one can shed a little light on this for me it would be much appreciated. I have read the report on the SC exhaust written by Charles Warner and that sheds some light, but with as few mods as I have right now, I just want to make sure that I am not going to be taking 3 steps back until I get my intake opened up. Thanks.

Hey Sam, what's going on. I think I can relate to this. I have 2.5" true duals with high flow cats. Currently I still have the stock manifolds, but will be porting those shortly. Anyway, I understand you concern for torque/power/performance. When I first did my exhuast I had nothing more done then removed air silencer and a K&N filter. I didn't notice a whole lot of low end torque, but did notice some nice improvements up at 4K and higher. I wouldn't be afraid of going that big since you do plan to further mod your car. Mine ran and performed just fine with little mods to fully modded. At this point I'm having pretty decent traction problems. You will grow into your exhuast system, and it should work out for you. As far as the theory and detials on back pressure, I can't help out too much. It's been a while since I took physics. Charles Warner's article is a very good start. If you have any more questions, let me know.

Thomas

From what I understand, it isn't back pressure that you are looking for. Back pressure, by it's definition is a restriction. That you don't want.

What you do want (and not quite as badly on a SC as on a naturally aspirated engine) is some sort of scavenging. That is when the exhaust gast pulses are redirected in a way that clears the combustion chamber more completly on the exhaust stroke.

The good news is that improvement in scavenging is one of the features of a well designed long tube header.

The concern can be that people make the exhaust too open. Using a overly large diameter exhaust pipe can allow the sides of the pipe to be cooler than the gasses passing through it, thus cooling the gasses as they pass through it. This cooling can dramatically impact the velocity of the exhaust through the pipes and create flow issues.

If you are purchasing these parts from Coy, feel free to ask this questions of him or his people. They have tons of experience with these SC's and can give you very specific information about how to setup your exhaust for your SC.