Just wondering. I have searched thru the threads and most talk about different sized exhaust sections, but is a 3" exhaust for the twin turbo too much? In Corky Bells book, his exhaust chart show 3" is necessary for 500-600+hp. To stay in the 300-400 range, requires only 2.5"-2.75".Too big slows down exhaust gas velocity. So, where did 3" come from? I know most of our cars do not put out 500-600hp at least the ones w/ stock turbos. I see all the aftermarket exhausts(not including tips) are 3". Again, why, if you want to keep w/ what Mr Bell states. He did state that a good measure is 10% bigger than the turbine outlet. Ok, anyone know what the outlet size is? I ask, because I am going to have my own exhaust constructed, but It will start after my 3" DP. Just want to contruct the optimum exhaust.
And my exhaust will keep w/ one of Mr Bells rules, Low backpressure.
Please respond if you have good information. Like I said, I dont think this specific topic has really been covered here. I know there are some very knowledgable people of this topic.

Thanks,
Steve

 

Alright, I measured my N1 duals and the inside diameter is 2 5/8". I think my petit single tip on the car is the same. i believe my resonated MP is 2 3/4". I did not measure my DP, so is it actually 3"? That just might be the outside diameter, so inside is about 2 5/8-2 3/4. So, after all this, I guess a 3" diameter exhaust is right, due to it being the outside measurement that is 3" and the inside diameter is ~2 5/8", just like Mr Bell suggested.
Still if anyone has info, chime in. But I have figured out my own question. All I needed to do was type it out.
I love it when a plan comes together.

Steve

 

So I am posting again
I was just looking at the exhaust size graph per Hp in the maximum boost book. The exhaust measurements; 2 5/8" is still too big. graphing it out for 400bhp requires ~2.34" exhaust and 500bhp would require 2.5". So maxed out stock twin turbo FD would max put out ~ 420-430 bhp max. That would not require even a 2.5" exhaust. So, even what is avail now, seems to be too much for twin turbo'd FD's. For single they still might be too big, but closer to what one would need.
So, I am thinking an inside diameter of 2.5" is what I would want.
Now, is this for a straight exhaust? Do curves and bends add resistance, that would slow velocity(ya, I know they would) but, is this resistance made up by having a larger diameter pipe than what is required to equal things out? So, do I actually need the 2 5/8" exhaust, to keep the velocity constant at 250 ft/sec(to account for the bends in the piping)?

Damn, I never thought so much about this car before. Sheesh.

Steve

 

Is he talking about rotaries or piston engines? There may be a difference. What works well on a piston engine does not always work so well on a rotary... I know the V8 guys love flowmasters, but put one of those on your rotary car and see what happens!