Re: Proof of larger DP/ Fluid Dynamics
 

I'll answer yours and Jakey's response since it's the same answer.

You're both forgetting that the turbo is a restriction to the exhaust pulses, and if the compressor side of the turbo didn't feed back into the intake as a feedback loop, you'd have a plain old restriction. It's a little different that the turbo creates positive pressure going into the combustion chamber, therefore creates higher cylinder pressures. The more pressure the turbo makes, the more of a "restriction" the turbine side of the turbo is. I just can't justify calling it a restriction though since the turbo itself creates more cylinder pressure and exhaust pressure than a NA engine would. I guess it's all semantics and maybe the engineering world would disagree. But I don't see something that adds more than it takes away to technically be a restriction overall.

And please, a GT35 or SCM61? Neither are capable of much more than 600 whp, and besides Shane and Cher, I haven't seen anyone here over that amount anyways. Just because half a dozen people have those turbos, doesn't mean they're all maxing out their 3" exhaust. You're looking for people at that power range or higher who have also done back to back comparisons with either timeslips or by measuring exhaust backpressure after changing exhausts. Do you really think you're going to get multiple good answers here?

Re: Proof of larger DP/ Fluid Dynamics
 

Think velocity. For one thing you would need a smooth transition to maximize gains. For another, you are allowing the gases to expand to fill a 4" diameter pipe and then "compressing" them back down into a 3" pipe. Regardless of how you look at it that will take energy, and I think you would agree that it would be a waste of energy. So, you MAY see gains from an ass-backwards setup like that, but they will be very inefficient gains.

I actually had a 2.5" downpipe on the stock catback. This setup was basically to keep the car quiet on the street. The catback was to be removed at the track. Did I see gains over running the stock DP? Well, yes. Not significant gains though, where I would say "this is a decent upgrade for poor kids like me" and probably due in part to the amount of exhaust leaking where they met. If you think you need that size piping, then run it all the way back.

So from my theory based perspective and my real world experience, I think you should just get a whistle tip. They go woo woo.

Re: Proof of larger DP/ Fluid Dynamics
 

Technically it just keep exponentically expanding pressures across the board. You say pot-a-to, I say po-tat-to. I believe it is a restriction, because if you measure pressures before and after the turbine, there is a difference, no matter what's going on in the intake tract.

True it would take a helleva lot of power to max out a 3" exhaust. I'm not trying to see huge gains. Just gains or not. I'm questioning if it helps on spool time. I really don't need multiple good answers. Just one, since I understand that not many have a 4" DP. I know of 2. I'm not really willing to ask these people, since I really don't like them.