i'm trying to figure out why my power dropps off so early. i have a feeling it's because my exhaust system absolutely sucks, but i'd like to hear what you guys think. here are my dyno graphs:



and



my mods are:
HRC s16g (used to be stage I) at about 14 psi, 8.6:1 Howell shortblock, 1G SMIC, blitz electronic boost controller, S-AFC (0 correction at WOT, i have a little correction at part throttle because i'm too lazy to perfect the S-FMU settings), AF/X UDP, AF/X 55mm throttle body, S-FMU with standard spring and 2.72" disk (about 70 psi of fuel at 14 psi of boost), accel 415 cc/min injectors, flowmaster 60 series delta flow axle-back, OEM NGK's, and some other crap (plug wires, coilpack, etc...) that doesn't really do anything.

your a/f does not look very healthy to me

You're running quite lean while the turbo is spooling, and pretty lean overall. I'd fix that.

I would say that there are three main things holding your numbers down

First as stated above your a/f ratio fix that before anything

Second is your exhaust. Its probably too small for the amount of boost your running, this is causing the turbo to run hotter and possibly even in the high rpm's causing your engine to eat exhaust gasses. I would say run a 3" cat back or possibly even run a 2.5" down pipe to a flex section and run 3" from there back eliminating the cat.

Third is the 1 smic. The 1g is doing a horrible job in both flowing that amount of air and also cooling it down. Your intake temps are most likely very high and this would also lead to slightly higher egt's. A nice fmic setup would give you cooler intake temps and a larger safety room for detonation.

This is just coming from my limited knowledge so if any of it sounds wrong correct away
:wary

People have made much more power on 1G smic's and 2.5" exhaust. I would start with the tuning before replacing parts. How bad is the exhaust when you say it is bad?

it's real bad. i have the drunken HRC downpipe, (which is dented at the bend down by the oil pan, the previous owner ran over a stop sign :wtf) a stock cat which i feel is on it's last legs (it smells funny every now and then) and i have the stock cat-back piping with just a ghetto flowmaster muffler.

yeah, my A/F ratio isn't exactly perfect. i'm going to give the hard S-FMU spring another try, the first time i tried it i had much better results with the silver spring. it would be nice to start off at like 12:1 for A/F and then wind up around 11:1 around redline. much safer that way.

That is definitely a ghetto exhaust setup. Time to start looking for a whole new setup from the dp back. :-)

yeah, that's what i plan on doing in the next few weeks. getting a 3" GReddy Evolution cat-back for my birthday, then i'll take it to the muffler shop and get a 2.5" or 2.75" downpipe that goes to a new cat. i'll have them make me a test pipe too for race day (i'll end up taking out that damn cat 5 minutes after i get back from the shop}()

but in the meantime, i'll mess with the S-FMU a little.

Considering what it looks like now, a new exhaust should make a night and day difference in power up top.

Isn't the optimal fuel ratio 14.7:1 (14.7 grams of air to 1 gram of gas)? Wouldn't you want the air fuel ratio graph to maintain this ratio? I've heard/read that optimal power is made at 12.6 to 1.

Correct me if I am wrong but I though a higher number means lean (19:1) and a low number mean rich (9:1).

Kurt

14.7:1 is stoich, higher is lean and lower is rich.

our cars seem to make the best power boosted when running 11:1-12:1 on the dyno. Remember also, typically there is less load on the dyno than on the street, so ur numbers on the street are usually a tad leaner.