I am looking for something efficient;

I have 01 s2k; I feel like the map is very rich and there is a potential there. Also it is possible for me (I dont have legal issues in the place that I am living) to replace all the exhaust system with a custom 2.5" or 3" mandrel bend-single exhaust, no silencers, no cat, no nothing (it would lean it a little bit.)

did anybody try to install such an exhaust? What kind of power could I get with this (+vafc+toda header)? What would be your estimation about the 1/4, with 4.77 gears and this setup?

 

Lets tackle this one thing at a time:
1. The A/F map does go very rich after 8200 rpm on an AP1. Opening the exhaust up will help this a little, but not enough. Having no silencing in your exhaust will make for a VERY loud car. If you want to see what it will sound like, just remove your exhaust after your header and drive around for awhile. If you like it that loud, then go for it. A single 70mm straight as possible exhaust with no mufflers or cat would produce somewhere in the neighborhood of 10-15 rwhp. None of us here are running such a setup, but a few do have headerback 70mm exhaust singles with some form of muffler attached, and it is still very, very loud. A VAFC tune will indeed help the fat A/F ratio, you can expect anywhere from 5-15 rwhp gains from that. A toda header is a very nice piece, and will also provide very nice gains with your planned exhaust. Basing your stock dyno numbers near 195 rwhp (based on a good average of AP1's), those modifications will put you into the high 220's, possibly even 230 rwhp.
As for a 1/4 mile time, Most good drivers can land a 13.9 stock out of an AP1. Gears have reported 13.3 in the 1/4. I would say with everything you have mentioned, there may be a 13 flat in there for you, but probably 13.1~ish.

 

^^Well said.

I would look at some other headers, Toda makes a good header but from what you want to do with your exhaust, it sounds like you're going for all-out top end horsepower, in which case you would probably be better off with a J's Racing or Tracy Sports header, the Toda is a little more of a balanced-power design. 2.75" exhaust is what you want if you're going to stay N/A, and get a silencer and a muffler on it, it will still be loud as balls. With a 3" straight exhaust you would literally have tons of hearing damage, plus without a good muffler or silencer of some sort you would get a lot of resonance into the cabin, and it would just end up being a PITA to drive.

 

yeah i totally agree with you, i had 3" exhaust on my all motor h22 with no resonator, and you could hear that thing 3 blocks away...thats ok for a street/drag bubble, but not for a nice car like the s2k, i wouldnt go bigger then 2.75 unless your gonna do a serious all motor build