We're all forgetting Boehr's law for gases...basic chem right? (Although I didn't spell his name right, but I only got a B, so sue me) As gases cool, they expand in volume. To go from 2.5" to 3" is okay, that is how they build scavenger headers, they step them up as they go back because as the gas cools, it expands in volume, therefore to maintain exhaust gas velocity you should step up the pipe otherwise you will create pressure in the pipe. This is why personally I don't think you would see a significant hp increase by going with a 3" downpipe, the gases are so hot coming out of the turbo that they are thin and travel very fast, Corky Bell talks a bit about this, but this all really makes more of a difference on N/A cars where low end torque is an issue. On a turbo car, you shouldn't lose low end torque, because you are decreasing spool time and allowing some boost creep. You won't notice it at all. I have the CS S/J which is 2.4" ID, and it made a huge difference. I actually gained torque on the low end, but I run a cat, which many test have proven that you can make more torque with goo high flow cat than without. A lot of the Hot Rod/ Car Craft mags do these comparisons all the time 2.5 vs 3, cat vs no cat etc, and so far everything I've read from those tests have been right in line with what Corky rates hp to pipe diameter.
However, that all being said, I do think a diverged wastegate path is useful to decrease the turbulence of an internal wastegate. What would be interesting to do (and relatively easy) is to measure the temps along the GHL before the cat to see how far away from the turbo the gas starts cooling, then you can calculate volume, then in turn mach velocity. The GHL is diverged, so the main pipe is about 2.5" and the Wastegate pipe is probably 1.5" (which no backpressure at all on the WG is best - hence the reason the WRC cars just dump the waste gates about two feet from the turbo) and then the GHL becomes 3" about maybe 10" from the turbo flange, this is probably too close to step up to 3" anyways, it should run about 18" before the step (of course depends on temps) because of the different flow characteristics of the downpipes/exhausts, the temp step down/volume increase will happen at different points.
If I had the money, I'd go with the GHL downpipe but extend the WG path back to right before the cat and then dump it back in, and mate the GHL with the Vibrant without the resonator, but with the cat and muffler.
this is my plan, but I'm gonna have a hard time cutting up the GHL downpipe since it's so stupidly expensive. But oh well...
Britt
In fact, I'm so bored, I will measure the temps on my CS exhaust tonight just for curiosities sake, see where it starts to cool. Maybe I'll even plumb in my pressure gauge into the exaust to measure the backpressure.
