Is this a good idea? i have a 3' downpipe and i can get the 2.5 ss exhaust piping for cheap, but i dont know if it will restrict my car too much with turbo. Should i just go 3' instead?? if so does anyone know a shop that carries SS exhaust piping and will do a custom exhaust for not tooooo expensive? the prices ive been hearing scares me.

 

A single 3" downpipe split into a dual 2.5" will be able to hold up to 500 rwhp or more. This is due to the experiences of the Supra owners. I have a 3" downpipe split into a dual 2.25" and I have no restricition at all. Mine has around 330-340 rwhp. But if you run a single 2.5" all the way out, I'm thinking your're limited to around 300 rwhp. For turbo, the bigger exhaust, the better. But for only a short distance of the downpipe of 2.5", it might not hurt that much, given if it's smooth bend and nearly straight.

 

no this is a dual 2.5' from the downpipe to mufflers, that sounds good for 500hp?

 

A pipe will flow ~115CFM for every square inch in cross-sectional area. Standard rule of thumb is that 2.0-2.2 CFM of flow is required for every horsepower being made at the crank to prevent unnecessary back pressure.

Given these conventions, the following numbers can be extrapolated:
2" = ~115hp
2.25" = ~200hp
2.5" = ~250hp
3" = ~360hp
4" = ~650hp
5" = ~1040hp

That is the approximate horsepower of flow capacitated by the given dimension of pipe, assuming that the pipe diameter is kept constant throughout (ie. mandrel bending).

A dual 2.5" system will flow 500hp. Dual 2.25" pipes will more closely match the flow rate of the single 3" pipe that you have already.

Turbo cars are different animals, and using those exhaust conventions as biblically as the na/ and supercharged guys do would be misleading, since there is inherent back pressure in the exhaust system from the turbocharger itself. How those numbers would be altered for turbo conventions is difficult to say, and I have seen no rule of thumb. In general, it is best to oversize the exhaust slightly, if anything, in a turbo set up due to the back pressure of the turbo itself.

That being said, Eric from TTC Performance made 891whp on a 3" exhaust system from downpipe to muffler. Additional power can definitely be made in the face of back pressure. But, back pressure is certainly costing you horses in every case.

Regardless, I highly doubt you need a dual 3" system if you are only utilizing a 3" downpipe. To keep the system most equilibrated with a dual exhaust, 2.25" is the way to go. If your build is intended to exceed 400hp, I think 2.5" duals would be the appropriate choice.

 

personally, my advice would be dont waste the extra money on stainless steel... there's no real benefit... a good mild steel system with mandrel bends will do the job plenty good enough

 

I agree with Blizzy.

I see the only advantage for stainless steel for the downpipe is it won't rust if your car sits for awhile. Mild steel does. But if you keep boosting your car like crazy..LOL, it'll heat up the pipe and will form more carbon in the material itself, and it'll prevent the rust better. You can notice that if any exhaust pipe that's been used and heated up for a long time and is red-like color (being cool), it's not likely to be rusted as the new pipe. Its surface also gets smoother than the new pipe.

 

Something else to consider is the larger and straighter the exhaust, the less restriction, the harder it can be in some cases to control boost. There are those that have actually went smaller diameter exhaust to give restriction because there was bad boost creep. I think 2.5" duals would be good for your setup, yield a nice sound and not to excessive.

 

ok what is this dual setup mean? is my stock exhaust a 2.25 dual or 2.25 single? im starting to get confused lol

 

Dual just means two pipes/mufflers. Single means one pipe/muffler. No SC came from the factory with a single exhaust.

 

great! this would mean 2.5 is good enough thanks man!

as for the SS thing, we need it here in Washington because it rains and snows and all that.... not that ill EVER drive it in the snow but the rain sucks.