Setup:

2003 gmc sierra (4400 lbs w/ driver)
lq4/4l80e
springs,pushrods,timing chain
gt45 turbo 69mm compressor/77mm turbine
3'' coldside and 31x12x3 intercooler
80lb siemens deka/walbro 400
stock ecu with hptuners

Currently on 10-11psi on pump 93. Running around 11.4afr and 12* of timing. I really have noticed this thing fall over early. I do realize that with the small compressor on this engine, that I would not be an rpm king. But, it seems like I am losing power faster than expected. Im shifting at 5200 rpms, and I can still feel it fall over a little. Is there anything simple that I'm missing? Can a boost leak, or spark blowout cause this? Haven't got to check backpressure yet. This video may help explain.

 

I would say your turbo is just too small. Both the compressor side and turbine side. Turbine seems really small for that big of an engine.

 

lets see pictures of your hot side.

 

The waste gate is in a terrible location. Its basically a copy the kbracing turbo kit.

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I dont think the pre-turbo hot side is hurting anything, but the down pipe looks to be small, 3"?, if it is I think that might be a restriction which could hurt upper rpm performance

 

3" downpipe, 3 " Gibson muffler that's as big as a stock one, and the gate dumped into the exhaust. Think this could be an issue with post turbo back pressure?

 

I dont know for sure, but I highly suspect that it is alot of your high rpm problem.
the other thing is that the oem LQ4 truck cam is done past 5500 for sure. but dont bother changing it untill you step up the downpipe and muffler.
personally, I run a 5" down pipe that splits into 2 3" pipes and 3" mufflers. You could go up to a 4" Downpipe and split into 2 3" pipes/mufflers, or full 4" exhaust. many ways to do it.

 

My money is on the low timing try adding a few degs timing after peak tq


If you want be on the safe side for the time being add some good fuel.

 

That turbo is the mascot for Sloppy Mechanics, and they have ran it on everything from 4.8L to built 6.0L. I would NOT put any timing in it until you know it needs it, and instead would look at the backpressure of the exhaust first. Also check the turbine inlet pressure to give you an idea of how the housing is working. As 3pedals mentioned, the stock truck cam is pretty well done for by 5k, and a correctly sized turbo won't add all that much to the hp peak.

 

I can't see the DP being too small at 3". I as well as many others run that size and don't have problems at our HP levels.

 

what valve springs are you running? IMO the exhaust will be a restriction if its a full system from front to back

 

I'm also going to say 3" is small, and would check pressures pre, and post turbine. Also curious about valve springs. The turbine is not huge, but at 87/77, I don't see it as a horrible match to a stock cammed, and headed lq4.

 

Your turbine is fine and the downpipe isn't the problem. Sure, it's a little too small but it's not killing it. You need to get a cam. It's going to fall off no matter what with the stock cam.

 

i have a question about your build ......with the gt45 turbo the ones i have seen have twinn scroll housing flange is this tru for your turbo......i see that your kit has a round port on the flange where the turbo mounts........enem though i have seen people run a single tube up to a twinn scroll flange i was told that for the turbo to work the best it needs to have 2 smaller tubes feed it rather than one and with the roundish turbo flange on the hot side into a normal t4 flange i would think the flow path is not right and causing the turbo not get all of the exhaust or creating turbulence

 

you have any pics of the set up in the truck

 

Normally caused by poor oil supply, maybe. Turbo cars are so sensitive and so powerful.

 

Huh? Please explain.

 

Do you have a dyno graph at all?

 

Yeah.... Don't talk.

 

It doesn't matter that the banks aren't separated. It's not hurting anything. As for the round flange, I hate it. The exhaust is just crashing into the long sides of the turbo flange and causing turbulence. There's no reason to use a round flange.