have a 91, want to add flanges, and what not to the exhaust right after the cat. It's 2.5" right after the cat. Long story short, I have reasons to run it 2.5" all the way back. Question is, is this going to be just too big of a pipe for the 1.6 8v?? basicallt 2.5" from the cat, to a 2 chambered muffler, and I'm going to have a shop bend up a 2.5" tailpipe to the stock location.

Several years ago it was discussed, and I think it was Zaggy that studied it and said 2 1/8" or 2 1/4" was just right. 

2.5" will deffinatly kill your back pressure, I looked into this not to long ago when I was trying to squeeze more power from the engine.

My thorley header came with a 2 1/2" collecter so I I just continued it all the was back. Since I have a automatic trans. I moved the cat back to just before the muff. Trying to keep the trans a little cooler. As an added bonus makes it easer to service as well. I have no complaints.

Can you tell me what issues I would have with low back pressure. 

One idea I have is to fab a ring that will insert into pipe just after collector. Slightly restricting flow, creating a ventri effect.

Any thoughts?

Unless you're going crazy with forced induction, and ONLY want high RPM - 2.5" is just way too big.

opening up the exhaust as soon as possible used to be a good thing.
straigth pipes were best, just illegally loud.
why would "back pressure" be a good thing?
just curious.
richard

Your valve timeing overlaps a little.  Meening, the exhaust is open a little when the intake starts to open. 

If the exhaust is too easy flowing, part of the new air/gas charge can flow out with the exhaust.  This can cause backfiring, and lose of power (less charge) on the lower end/crusing speeds.

Why do they over lap?  At a higher RPM, the overlap is needed to get the exhaust gas out of the cylinder more completley.  So, a open header is the best for RPMs at the high end.

This is why a race engines don't idle well.  Too much over lap, but a higher RPM the engine comes to life.