midpipe - more questions!

[S800Racer]

I'm all pumped up to do a 2.25" midpipe on my Si & crawled under the car to look at the stock midpipe. I notice that the tail end of the cat is the same diameter as the mid-pipe. It won't do much good to put a 2.25" pipe in downstream from a 1 3/8" constriction, right? Or is there some way to install that enlarges the opening from the cat?

[S800Racer]

People have been reading this post but no one has an answer so far. If you have installed a new midpipe on your car, let me know what the connection looks like at the cat. If there is still a 1 3/8" restriction at that point, the system won't flow any more than the stock system.

The volume of the exhaust gas in the midpipe will be larger, but slower with the overall flow rate remaining the same. The same laws of physics will affect the muffler as well. If the opening into or out of the muffler is smaller than 2.25", it will limit the flow.

[02-sir]

It necks down more after the flange. Someone posted what the cat flange ID was, but I can't remember the number. It's larger than the mid pipe though

[JSIR]

I actually put a bigger flange on my cat and then connected a corresponding flange to the new midpipe. I don't really know if it helped any though. I've kept the stock flange just in case I ever need to put it back on. The stock cat. conv. flange is perfectly matched to the stock midpipe flange, I think it is about 1.8 inches inside diameter. If anyone really wants the measurements let me know I can take my caliper to it.