98 Cobra Precision 76/75 Not making power above 5500rpm

[mlaib1984]

Hey everyone. Looking for some advice on an issue im having with my car. I have a 98 cobra with a 4.75 MMR shortblock, FR500 cam kit with valves and springs, IMRC deletes, C.O.P. conversion for the Holley terminator x standalone. The turbo kit is an On3 with a reworked y pipe on the hotside for better flow. It has a brand new Precision 76/75 turbo with a .96 housing. I have pulled the car back apart to verifiy the cam timing is correct and it is set to 0. Basically the car stops making power at 5500 rpm and starts to fall off. Even the fuel table on the Holley is pulling fuel. We are not finding any blockage on the hot or cold side. We reworked the y pipe on the hot side for better flow since the on3 dead headed into the other side as opposed to flowing up to the turbo. It still has the twin wastegates that have been upgraded to precision wastegates. Also hot compression we checked while on the dyno all cylinders are 182 to 190. Im curently at a loss and am wondeing where to go now. Most people are saying with a setup like this 700 to 800 whp is possible. We can only get it to 500 and then it doesnt matter if we add timing or boost the power isnt going up. The big thing that I dont understand is the car runs fine? All of the spark plugs look good there isnt one that screams dead cylinder of engine problem. Any input would be appreciated. Thanks

 

[LS WUT]

@03redtr has one, maybe he can shed some light.

 

[Deceptive]

Spark issue?

What does the logs say?


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[mlaib1984]

Spark issue?

What does the logs say?


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I dont have a copy of the log. Tuner says the spark was good and it didnt have any missfire. The rpm would climp all the way to 6500rpm where the rev limiter was set but the power level at the wheels peaked and dropped at 5500rpm. None of the spark plugs were burnt up and all looked the same.

 

[I_like_turtlez]

There's no possible way someone could diagnose this without physically being there

One thing is for sure, you're lacking air and/or fuel either upon entry or exit; big restriction

I saw one of those little 2.1L blowers make 470whp @ 10psi without a tune on the stock injectors, it had more left in it with more fuel

You said you nose-dive at 5500rpm, sounds like it's running out of breath, looking toward the intake plenum down to the valves is the first thing I would do

 

[01yellercobra]

What's the boost doing when it noses over? Are you running a controller or just the wastegate?

 

[mlaib1984]

There's no possible way someone could diagnose this without physically being there

One thing is for sure, you're lacking air and/or fuel either upon entry or exit; big restriction

I saw one of those little 2.1L blowers make 470whp @ 10psi without a tune on the stock injectors, it had more left in it with more fuel

You said you nose-dive at 5500rpm, sounds like it's running out of breath, looking toward the intake plenum down to the valves is the first thing I would do

I understand its impossible to diag without being there. Im looking for suggestions since ive been scratching my head for the last two weeks. We were thinking it might have jumped time on a cam so i tore it down and it was still set to 0. I'll keep looking through the intake and valve train area. With the intake off valves still look good. As far as fuel goes its still a return style with 3 pumps in the rear a 10an feed and 8 an return with 120lb injectors.

I do appreciate any suggestions anyone might have.

 

[mlaib1984]

What's the boost doing when it noses over? Are you running a controller or just the wastegate?

 

[mlaib1984]

We're running the holley single solenoid setup through the holley ecu. It has 2 of the precision 40mm waste gates with 12lb springs. To get to 17psi we have roughly 50 percent duty cycle which is high based on the springs in the gates. We didn't see anything crazy with the Boost curve however i don't have a copy of the dyno sheet to verify.

 

[01yellercobra]

It's sounding to me like you might have a boost leak. Turbos will work to hit a boost target. But they may not meet the flow requirement.

IE, I had a boost leak on my truck for a while. I noticed it felt sluggish, but thought it was in my head. No codes and it still hit 25psi. Finally one of the times towing I could actually hear the leak in the cab. Swapped out charge pipes and the truck was like new again.

I'd start simple with a smoke test and going from there.

This video talks about opening pressure on wastegates. I'm not sure what your knowledge level is so my apologies if I say something you already know. Just trying to cover the bases.

 

[01yellercobra]

Nevermind.

 

[MG0h3]

Exhaust restriction?

TB not opening all the way?


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