Piping diameter and turbocharged motors

[kazuo]

YES I USED THE SEARCH :mrmeph:

Moving on, I have finally gotten around to upgrading my exhaust system on my S13. Motor is a Redtop SR20DET. Going off what most people say/do, it would be best to get 80mm piping all the way through, and use a high-flow cat if you plan to have one (which I do). So I bought all this stuff, and here's what Iended up with:

75mm HKS Frontpipe (I thought it was 80? WTF..)
"3-inch" Catco Cat (from HT... we can assume if it is literally "3-inch" that it is in fact like, 76mm or so)
70mm 5zigen Border III Catback Exhaust

So not exactly 80mm straight through is it? My question is, is the fact that every part of the exhaust system has a different piping diameter going to affect performance in any way? If so is it going to be really drastic, or just like a ho-hum 2HP or some insignificant number decrease? Or I am just being stupid? Everyone else is like "AWW NAWW DUDE YOU NEED 80MM" and I dont think it'd make a big difference... but honestly I don't know. That is why I am asking around! :D

All help is appreciated. I was thinking about replacing the frontpipe at least, (so then it goes at least 80-76-70) but I'm kinda lazy now...

[240racer]

technically you don't want any cross sectional area changes on your exhaust, however this is difficult and not always possible. Since you have decided to run a muffler and cat, you will have area changes. The goal is that all of the Diameters you chose are larger then the ones that are chosen for you (like inside the cat and muffler) I would not worry at all about a 75mm downpipe, the air is hotter and still expanding more there, so it shouldn't make a difference. You can make well over 450hp on 75mm exhaust, as long as there isn't too much junk later on. I would be much more worried about the 70mm cat-back setup, is there anywhere in that part that the diameter goes below 70mm? If there is, skip it and get something else. If not, 70mm is only slightly smaller then 75 and not a big deal. However that pipe is a lot longer then the DP and will cause more restriction even if it was the same diameter. On my KAT I have 3" from the O2 elbow back to the dynomax race muffler that is larger then 3" I skipped the cats becuase I'm never going to test the car for emissions.

[VLOWS13]

good info

anyone like to add to this? i am curious about this as well