95batmobile

iTrader: (2)

The car is a stock lt1, with a rear mount system running only 6lbs of boost. It has plenty of fuel with the walbro 255 and a hotwire kit plus 60lb seimers. Originally i thought it was just the tune, so when we went to get it tuned it kepting holding at 4500. The tuner said it could be the spark plugs, so it was tr6's gapped at 35 but he changed them to 28 and it didnt help, also it has the msd opti, coil, and 6al box. So what he is thinking now is that the valve springs are bad. My hesitation with this is that is did the same thing on the old motor before it blew up, so i dont think its a valve spring problem if its going to happen on the exact same rpm (with lower boost) and do the same thing on a newer motor.

So, what could the issue be? i read about maxing out the maf, but thats about as far as anything i could find. Could vaccum or the trans do this? im lost and dont want to keep throwing thousands dollars at it.

Purple Poncho

iTrader: (1)

Maxing out the MAF is a problem obd2 cars have. Yours is a 95 so will not have that problem unless you have done a PCM swap.

WhistlingZ

Disconnect the maf wire and run it and see if it pulls above 4500.

If that doesn't work, disconnect the hose before the throttle body and do a pull with no boost, see if it pulls above 4500.

IDriveChevy

iTrader: (5)

Can you datalog the car? Sounds like something electrical is causing issues. Could be TPS or the 6AL box pulling a lot of timing.

Spartan7

iTrader: (7)

What exactly is it doing when it hits 4500? Is it breaking up, running rough, feel like you hit the rev limiter? Make sure you've got the right pill in the 6AL box.

Puck

iTrader: (17)

I'm assuming if it was on a dyno then it had A/F and wasn't going lean.

Assuming that, my first thing to look at would be the 6AL box. Remove it or test a known working one to narrow it down.

Ed Wright

iTrader: (1)

It's not a problem if the tuner has a clue.

bufmatmuslepants

iTrader: (3)

If your running a cat the cat could be clogged.

97Z28SS

iTrader: (5)

Could be a backpressure issue, that turbo is pretty small for a V8, also did the car rev above 4500 and now it won't?

Ed Wright

iTrader: (1)

Small turbos will make boost real soon. Not likely too small turbo. Clogged cats or muffler baffles broken loose, or factory pipes inner wall collapsed won't rev, won't make boost. How much boost does it make at 4000 RPM?

97Z28SS

iTrader: (5)

Ed I remember an article in GM High Tech about a 97 SS that had a single 60-1 t4 turbo and it had the same issue, wouldn't rev past 4500 or 5000 and the problem was the turbo couldn't flow all the exhaust out fast enough creating a bunch of back pressure that's why I asked.

96capricemgr

iTrader: (1)

My vote would be return the ignition to mostly stock components, your "upgrades" are at best unnecessary and at worst a very real suspect in causing this issue. The wires can begin to arc when torque is getting high and cylinder pressures are high.

What springs are on it?

Ed Wright

iTrader: (1)

Wow. I've never seen that. (Old as I am, I haven't seen everything yet.)
A plain old vacuum/fuel pump test gauge hooked to an O2 sensor bung would tell the tale real quick. Should not see over 1 psi, if back pressure is the problem pressure will go to the moon when it hits that RPM.
When he said "Rear mounted turbo" I foolishly assumed an STS kit, which normally run very well. I have seen so few home brewed turbo kits turn out well, I no longer even try to tune them. I decline that kind of work all the time.

moehorsepower

iTrader: (3)

Thank You Ed, I have been saying that for years, I still run a stock MAF on my turbo setup, even when it was supercharged..

onephatZ28

iTrader: (8)

Do you have a rev limiter on the 6al box? A bubby of mine had a after market ignition box on his 95 z28. don't remember what brand. put it on the dyno it would not pull past 5k lol took 3 days before someone thought to check it