What size do your kits come with?

 

I have a 3" and make well over 600whp. When I go to a bigger turbo than the 76mm I will go 4" dp to a 4" cutout, but keep my 3" cat back.

 

The closer you are to the manifold and high EGT, you can get away on the smaller stuff. Further you go back and it cools off the big stuff helps.

Mine is actually closer to 2.5-.2.75 because at the time Kyles jig was off and my downpipe mounted to close to the frame. It's self clearance to this point now.

 

Actually, you have that backward The closer to the motor, the higher EGT, the MORE volume of exhaust gas you have. (hot gas is less dense, cooler gas is more dense) The further away from the motor you get, it cools and the LESS volume you have, there for the less exhaust size you need.

 

I would like you to do a dyno test comparing down pipe size. Only change the pipe and make tuning corrections too see if it helps enough to worry about at your power level. Would be interesting. Even though the outlet on the turbo is only 3" there are like 4 45* bends on the first couple feet that has to be restricting some.

 

Agree to disagree?

The amount or volume of gas is the same from the point it leaves the headers/turbo to the point it leaves the tail pipes, it doesn't go anywhere but thru the pipes.

The only difference is temperature, as it cool off it becomes more dense and hints, flows better in larger piping.

I've read dyno results from a turbo builder (forcedperformance.com) where they swapped a 2.5" 02 housing to a 3" 02 housing on 600HP build and the dyno results supported no gains. This is about a 10" pipe coming directly off the turbo. The reason stated was due to the high EGT thus resulting in less density at that point in the system, there was no advantage to larger piping.

They sell both products and the 3" cost an extra $150 bucks, but they provide the data there is no reason for it unless you need to be able to say, "yeah it's 3" off the turbo".

 

Thats why i couldnt ever figure out wheather to go 3 or 3.5" on the DP..
I chose 3.5 for the heck of it.

 

The volume exiting the tail pipes is not the same as the volume leaving the engine. There are differences in temperature, pressure, and velocity, but the mass is constant. Assuming no change in pressure, the volume at the exhaust tip is lower than that leaving the engine due to the lower temperature/density. Getting into the nitty gritty of the ideal pipe diameter is beyond the expertise of anyone here including myself. One would need pressure and temperature readings at various points in the exhaust to make an educated calculation on pipe diameter. And then velocity adds another dimension to the problem as some clever things can be done by having faster/slower flows.

Long story short, just go with a downpipe and exhaust that is large enough so you know you won't be maxing it out. Hence my original question. Sounds like a 3.5" DP will suffice for my desirements.

 

I wanna see some of those fancy calculations