five_ohTurbo

91 5.0 stock motor except mild cam, unknown specs. Car has full length headers, dual 2.5 into single 3, T7 turbo ( T3 flange, .84AR hot side 3" outlet, 4" inlet 2.5" outlet .70AR cold); 12Gal fuel cell, full -10AN fuel system, aeromotive fuel pump, filters, and regulator, stock ignition...
anyway, idle AFR is roughly 14.5, and about 11.6 under any kind of load. Normal driving is fine, nothing really to cmplain about, but when the car makes about 3psi it starts bucking rather violently. AFR remains about 11.5 through boost, so i don't really expect a fuel issue. Any suggestions would be nice, but I won't really have any more details till tomorrow when we go back to testing it. Thanks

nitrous_bob

hows the meter look ? i'm assuming it is not stock, but maybe it is.
2nd guess possibly the tfi module ? but typically they just totally go, and the car won't start.

dan_gra

Be very careful putting that motor under any stress until you find out what is going on. I have owned a few turbocharged cars over the years that I have modified and any bucking issues need to be taken care of before you end up with a blown engine. Any engine be it boosted or not needs three things gas, air, and spark. With a boosted engine the spark is the one that can cause the most damage any detonation can lead to engine failure. I would assume with the stock ignition you have you are having a serious spark problem. It could be your timing is way off but most likely I would think with that setup you should be pursuing a msd box with the adjustable boost timing. For every pound of boost you should be retarding the ignition by 1-3 degrees to prevent detonation. I hate to say this but many of the tuner shops for imports have people that work there that are very knowledgable about turbocharging n/a motors and they may have the resources to run you on a dyno to see what exactly is going wrong with your vehicle as it spools up. Right now I own an 89 notch and a 88 Saab 900 turbo that I'm putting about 17psi through with fms 24# injectors and a 3.0 bar regulator with an upgraded APC computer. I'm going to have to modify that notch more to catch my swedish beater. You will have a blast when you get that sucker dialed in. My Saab from rolling at 30mph in second will break the tires loose till I reach about 45+. I couldn't imagine what it would be like in my notch if I had that type of boost going into it.

Good Luck and be Careful. Are stock motors do not have the bottom end to take to many mistakes with boosting.

five_ohTurbo

timing is backed way off, but like you said we have no box (6al boost box) so timing is basically the same thoughout the boost curve... my boss developed and owns a twin turbo c5 vette so its not like we're completely retarded, however this one is making me have second thoughts about that lol. MAF is just a reman piece, nothing special. Based on the AFR, timing is very suspect at this point. we'll be playing around with it a bit today so any new info i get i'll post up and hopefully will spawn more response. thanks for the tips.

five_ohTurbo

ok... removed the intercooler and fixed some minor boost leaks, car seems a little better. It boosts fine and pulls nice if you ease into the throttle, but if you just go WOT from say, 1500-2000 rpm it will bog out when it makes boost. How is the stock intake at flowing at high CFM? I've never really worked with the 5.0 before so i dont know if its choking the motor out or what... back to the drawing board today.

aode08

drop it off for a tune, sounds like several calibration tables are off.

five_ohTurbo

no aftermarket pcm control at all, stock ems, ignition, and maf. Fuel is adjusted via boost reference pressure regulator to 35psi, wideband is dead on at 11.5 and we've been told that we don't need a tune chip as long as timing and AFR are compensated for. Do we need an aftermarket chip or should stock be able to run 7psi?
91 fox body, fox body, fox body, fox body...
cookie monsters available?[&:]