Im not saying youre wrong brain, but the theory behind the quick step up to 3" is completely in line with what you posted about turbulent and laminar flow. It is basically making the assumption that you will get higher velocity from turbulent flow than you will from laminar flow that is rotating through the downpipe at a really shallow angle.

It may not be true, or the benefit may be totally canceled out by stepping back down in diameter, but I dont think anybody can really say for sure without testing.

To be more precise, you will get higher velocity in the direction of the pipe with the turbulent flow but lower absolute speed of the air compared to the laminar flow. But you dont really care about the absolute speed, only the vector component of speed in the direction of the pipe.

all I care about is evacuating the exhaust gasses as quickly and efficiently as possible. This means doing whatever it takes to get gasses away from the turbine, so it uses the least amount energy having to push gasses away and more on spinning.

Right and what we're saying is that according to accepted theory the turbulent flow resulted in a higher mass flow, and higher meaningful velocity which means lower back pressure.

ho-hum.

It's not about turbulent flow versus laminar flow with regard to the quick step up coming out of the turbine, but radial flow versus turbulent flow. What you do after that is your business. Obviously, keeping the larger diameter would be optimal but having a larger diameter versus not having it seems to make a difference at the outlet. But I don't know anything other than the stuff I read here and there. I have seen that radial to turbulent explanation somewhere else many years ago, too.

Isn't the cat the most restrictive part of the exhaust system?

Would it be dumb to step up the cat to 3" and then go back to 2.5" for the exhaust? Reason: because I have 2.5" exhaust already

Anyone ever try this foolishness?

The high flow cats I'm familiar with have 4" cat sections whether the weld ends are 2.5, 2.75 or 3" diameter. So, no. You end up with the same actual cat section anyway.

In my view, Beanireck has the right idea.

corky