Turbo cobra stops making power at 5300???

[qwik01cobra]

I just recently put my cobra back together with a precision 76 bw/bb turbo. I have a CG fab hotside, .05 over motor, with 9.3:1 compression. Stock 05 revised 04 cobra heads and cams. ported short runner, precision 46mm wastegate, open 4" downpipe, twin GT pumps, fore hat, fore ruel rails, 60lb injecotors, TR6 plugs gapped at .028, 14-15psi of boost. *im running the car with sunoco 100 unleaded pump gas

The problem Im having is the the car has a smooth steady powerband up to 5300 RPM and then falls off (negative,doesnt even plateau) Not bad but it doesnt keep making power to redline like it should. The A/F is spot on and sits at 12 the whole way to redline. The car doesnt hickup or misfire at all. The car did not respond at all to added timing either. It made the same power with 13 degrees of timing as it did with 16-17. The car did respond to added boost but would still fall off up top. The computer tries to add timing at the top end like its working to hard. The fuel duty cycle is only at 47% and it pulls so smooth even past 5300. I'm just confused why the power drops off. I do have the original COPs in the car and no ignition upgrades. My only concern is that maybe I'm not getting a good enough spark on the top end of the RPM. Maybe someone here can educate me better on this. I'm wondering if I should get a boost a spark, new COPs, and maybe even a colder plug like a TR7? Hopefully someone can shed some light on this subject.

Thanks in advance

Chad

 

[Modern_Muscle]

If youre having spark blowout, your A/F will not stay steady due to the unburned fuel passing by the sensor.

What turbine housing, and wheel size on that turbo?
That could be acting as a bottleneck or restriction causing your symptoms.

 

[qwik01cobra]

Ya I understand that. There is no spark blowout at all I'm just wondering if it could be do to a weak spark.

Precision 76mm PT7675 billet wheel ball bearing turbo

.96 a/r.

75mm, 84 trim turbine wheel.

This turbo has been proven in a lot of cobras making way more power then I'm pushing.

 

[Joyrider564]

Try br7ef plugs. I use em in my pump gas turbo cobra. I would richen the car up a bit too, but that's just me.

 

[cinch1985]

I thought if it fell off up top that the valve springs we're too weak. But at only 15 pounds I would think you would be ok.

 

[badcobra]

Post your graph. Is your clutch slipping?

 

[MaximumVelocity]

Can also be weak intake springs or bad cam timing.

 

[qwik01cobra]

Brand new heads less then 500 miles. Clutch is not slipping at all. I'll post the graph soon.

 

[01yellercobra]

Brand new heads don't mean the springs can keep up. I had to put stiffer springs in a buddies car because the valves would float otherwise. His was a 2v though.

 

[qwik01cobra]

Ya I have seen it before. But my car isn't making crazy boost or power. My novi 2000 set up was being pushed harder then this turbo set up. Does anyone have there turbo cobra dyno graphs? I want to compare

 

[01yellercobra]

I was told it had to do with the back pressure from the turbo. It wasn't boost related. I didn't look into it too much since I'm still a blower guy.

 

[watman02]

Ur backpressure b4 the turbin housing is getting to high and causing a restriction there for making more pumping work and losing HP. Ur hot side is too small A/R// sized too small if that is the case.

 

[qwik01cobra]

How about if I'm only running one wastegate off one bank? CG fabrication hot side. It's close to the merge but dominate to the passenger bank

 

[watman02]

If u have a single turbo the u would only have 1 wastegate to begin with.

 

[watman02]

Where is the dyno graph?
what intake ru running? What did IAT temps do any knock?

 

[qwik01cobra]

Well I was just curious about the WG set up because I've heard people going back a forth forever about where it should be located and it's not efficient to run it on one side and not at the merge ect.

Waiting on tuner to email me the file this coming Saturday. IATs were 75 degrees. It was a nice cool day . I'm running a ported short runner 01 cobra intake manifold. The clutch wasn't slipping because the rpm matched the wheel speed. No knocking. No hiccups. No spark blow out. Ect. Car feels really strong on the road too. Doesn't miss at all. No hesitations. Idles perfect, drives perfect.

Here is my wastegate/hotside set up

 

[99cobra09]

I been told that the WG should be right after the exhaust in the straightest position possible. Im thinking maybe the wastegate is possibly bleeding off some boost but your saying the boost is steady, correct?

Does your tuner have any ideas on the issue, im sure he has bit more insight since he is the one tuning and doing the datalogging.

 

[01yellercobra]

With that set up I see it bleeding off the passenger side, but not the driver side.

 

[qwik01cobra]

The boost is consistent through the RPM at the manifold and compressor housing. My tuner had thought that it could be a spark weakness issue up top cause that's the only time he has seen this issue happen before. I have the original 100k mile COPs and no ignition upgrades.

The power drops off after 5300. Not really bad but then tries to recover (via computer adding timing) and never fully recovers. It should be making about 50 more hp and the curve should still be climbing past 5000.

I'll post the graph tomorrow

 

[watman02]

I think the back pressure on the hot side is the problem pumping work is causing the power drop u are only bleeding off the pressure from the pass side bank the drivers side bank has to come up to the Y and the go backwards down the pass side pipe to equalize the preasure causing the air the crash into each other. My opinion is u need another wastegate on the drivers bank.
Valve springs could also be the problem too. U could have bungs welded on before the Y into the turbo and measure back pressure throught the curve. I would bet u see a big spike on the drivers side at 5300.