I've been wondering about this for a while. I plan on getting a custom 3 inch exhaust made, but would I get even more hp if I went to 3 1/4 inch or 3 1/2 inch piping?
Past what size piping does it stop making more hp?

 

the size of the piping is going to depend on engine compression, the size of the engine and the size of the turbo.

from how i understand it, N/A engines need some backpressure so that they are less peaky when it comes to street-torque. turbo engines don't need it, because the exhaust that goes out your tailpipe had to go through the turbo first, so all that matters is what the turbo's exhaust is limited to (A/R, and its flow capabilities). and the size of the pipe will not really "make" power, rather it will release it.

in your case, i'm assuming you're running a 13B with some sort of turbo, and 3-inches is probably enough for a "streetable turbo".

 

The biggest misconception is the bigger means more hp and a loss of low end torque. That is usually inaccurate. You want to get a pipe that is just big enough to flow the required exhaust for your particular set up (pobably 3" on a mid-powered turbo rx-7 like diabolical1 said), but you don't want to go any larger. The faster you can get it the exhaust out the better. Therefore it is VERY important to pay much attention to details such as utilizing mandrel bends, and designing the system without sharp bends, and a nice flowing style.

Get mandrel bends for your system. Your local muffler shop won't have them, but you can order them for about $15-20 per U-bend from Burns Stainless, Summit Racing, Magnaflow, and a couple others.

I won't recommend a piping diameter yet since I don't know your set up. Please post the details on your car/engine. Thanks!

 

Sorry, I guess shouldn't have been so general on what I wanted. I own a '88 Turbo II AE, and right now I have a series 4 13BT, but I'm going to swap a series 5 13BT in soon. I'm planning on making at the very least 320rwhp, while still smog legal (if it's possible). I'm going to use the car for autocross, road course events, and the occasional grudge match at the local dragstrip. Thats all I can think of right now, let me know what other info you might need on my car.

 

well if that's what you're planning then that means about 360 horses at the flywheel ... in my opinion, a 3-inch system would be fine, regardless of turbo.

since you autocross and go on road courses, it might be worth trying to get a bigger (maybe 3.25 inches that tapers back down to 3-in, but don't go larger than 3.50 inches) downpipe that will bolt into your system, so you can try to manipulate spool-up on the turbos, but that's only if you have the money or experimentation. otherwise, like i said, 3-inches should be fine.

 

OK, I think I'll go with the tapering down thing. Thanks for the help.

 

just to clarify what i meant ... the piece that tapers down is something that i was suggesting for track use (where you would bolt it in at the track and then put the straight 3 inch piece back after your race), i mean, i don't see it as presenting a physical problem for your engine, but i was thinking it might help the turbo spool up quicker in agressive throttle applications (autocross and road courses) ... however, it "might" make less agressive street driving a little soft.

i just want to make sure you understood me ...

good luck

 

Oh, ok.... I thought you meant leave it on there all the time. LOL