how big can my exhaust go with a good powerband?
 

Hey all, I have an 84 with a 13b cosmo 4 port with a racing beat intake, weber 45mm carb, racing beat header, and 2.5" exhaust. I was just wondering how big I can get my exhaust and keep the powerband within my range; 5-8000 rpms. I'm looking to gain the most attainable power.

Trying to maintain the most power in your powerband and most attainable power are a little differnt. What you want is the most average horsepower in that range that you can get. The first thing you should do is to fab up a long primary exhaust system that collects at that rear of the car and exits through a single muffler. A 2.5" is really all that you need. A 3" at the very max but the system may get a little peaky. When I had my long primary and 2.5" muffler, I was very happy with it. When I changed to a 3" muffler, I found that it lost some low end and still wanted to rev higher. While nice, my powerband isn't that high. Alot of it depends on your porting though and collector design. It would be somewhere between 2.5" and 3". Anything else is too large.

Got two 2.5' pipes on mine, I'm happy RX4/twin turbo. I don''t understand why peaple get hung up on back perssuer?

Well it's not backpressure just the fact that if I gain 20hp from 8-1000rpms, that does me no good. So have each header get a 2.5" that collect into one muffler?

two 2.5" all the way back

If you have 2 turbos and 2- 2.5" pipes going to the back of the car then that is fine. The best exhaust after a turbo is no exhaust what so ever. If you run 2-2.5" pipes all the way back on a naturally aspirated engine, you are losing a ton of power. The exhaust ports aren't even equal to a 2" pipe in terms of area so there really isn't much point trying to decrease its velocity by 50% in the name of flow. Increased flow through a larger pipe doesn't do much good if the port itself can't flow anywhere near that much. Use 2-2" pipes from the engine (same size the header already has), and run them seperately to the rear of the car. Then collect them to a 2.5" single pipe and run through a muffler. That will make a whole lot more power over the entire powerband than the seup on the car now. 2-2.5" primaries would almost guarantee less power than the car has right now. You can't use turbo exhaust logic on a nonturbo car.

I like to see some dyno stats