I have posted before on this but still have not hunted it down. I am motivated again and need to finish this.

Here are the basics. ls2 402,-25cc dish pistons with .060 gaskets, stage 2 l-92 heads from Livirnois w/Patriot gold springs,l-76 intake,232/230 .600/.600 115,truck manifold kit w/ Turbonetics 76mm .96 ar.

Problem is the car makes no real power. I have been in faster heads/cam cars. I have put the timing all over with no real change in performance. Air/ fuel is 11.5 . I am seeing boost to the intake manifold. Where else should I look? What should I try? The gains I see from 8 psi to 15psi is really not that great. Maybe 75hp. Intake temps are 2* above ambient under boost. The only time It feels better is when the outside temps drop (60*ish) Then 8psi feels like 15 when it is 80* out. The only thing I have noticed is the engine sounds raspy under boost.
I am at the end of my rope here.I am about to pull these heads and slap on a set of stock LS6 heads and intake to see if it is the heads not working here.
Any suggestions are appreciated.

 

What does it do exactly? You need to provide a lot more info to get useful help. Have you looked for obvious things like leaks, turbo damage, and whether there something wrong with the wastegate and bov?

 

So just to confirm...

You are measuring 8lbs in the intake manifold.
Ignition timing has very little effect.
You are only seeing a 2 degree increase at 15lbs???
You do not feel a difference at 15psi, but the AFR is steady at 11.5.

I am not sure why you only see a 2 deg increase at 15psi boost, this seems to disobey the laws of physics. It might be worth it to check you boost gauge - just to be sure you are actually getting boost.

When I was playing with the VNT turbo on my 402, when you change the boost via the vanes you also change the engine characteristics, as it adds restriction to the exhaust and changes the VE of the engine. Needless to say this made it a BITCH to tune - but not impossible. The reason I am saying this is I could spike to 25psi by about 3000rpm BUT it did not pull as hard as it did when set to 10psi (both with a solid 11.5 AFR - why you ask - exhaust restriction! Boost is a means to an end, if you increase your boost from 8 to 15 psi and feel very little difference it means with only a little more air flow the boost goes up. i.e. mass of air makes HP not pressure. So if you look at your engine as a restriction to the turbo, just because you are putting 8psi in does not mean you are flowing alot of air. Take the exhaust off the turbo outlet and try again...

EDIT - BE CAREFUL, IT IS WAS YOUR PROBLEM YOUR AFR MIGHT SPIKE AND GO REALLY LEAN!!!

Other things to check...

As everyone knows if you have air, compression, gas and spark you make power.
Spark plug gaps - clean and set to 0.025".
Spark plug wires - ensure they are good (check resistance) and tight to plug and coil.
Compression - even 40psi will not make power in a 5:1 engine - although this is unlikely if you do not find the problem, you will have to check this eventually.

Hope it helps...

 

Boost is measured off the back of the intake.
I have a big intercooler. 4" thick
Yes the intake temps are only going up 2* if at all when boosting it.
Plugs are new gapped at .035.
Plug wires are stock and ohmed .31
I have figured compression and asked others and we keep coming up with 8.5 to 1
I will pull the exhaust just rule it out. Thanks.

Will a weak alternator cause problems with spark? My fans are always on high when running around and according to the gauge I am only at around 11 volts. I am just wondering if I am blowing spark out. Engine only gets raspy under boost. Even 6psi. I am going to put a volt meter on it to check.

Anyone else have suggestions or seen this before?

 

If it's making rasp when boosting I would check for some sort of turbo damage.

 

Check and make sure your turbo is not messed up. If the impeller is hitting the case the car could end up not boosting right. And if the blades are bent or missing some sections that will affect your boost too. Good luck.

 

I'm guessing all cylinders are firing.Shut the fans off while driving since they are not doing anything except wasting voltage.

Also guessing the exhaust has an open cutout or open downpipe for testing? I've personally seen a 250rwhp gain by just running an open down pipe over a free flowing exhaust(well,looked free)

Also,rolling it on a dyno might help narrow down the problem too. Just seeing what the graph is doing would be nice.

 

I am going to pull the exhaust off to check the impellar and to boost it with no exhaust on it. I did a backpressure test but I am not confident the results were totally accurate. Once the exhaust is completely off I can get it on a dyno and see what the graph looks like. I haven't put it on a dyno thus far because I wanted to fix it 1st but it looks like I will have to now.
What's hard to comprehend is it is building boost so the turbo most likely is not the culprit but I will check it to rule it out.

Also I did do a boost leak test and other than the one coming from the throttle body I am sealed.

To give you guys a idea about how low it is on power, I am still running the stock fuel pump without a issue.( I was planning on breaking the motor in and then seeing how it felt on 6psi before upgrading the system but it never ran lean)

 

Your turbo is too small for that large of an engine, is my understanding. At higher boost I can see where it won't make much more power since it's out of it's efficiency range right? Have you checked that out?

 

what is your altitude phil?

 

I know it is too small but there still should be way more power than I have.I plan on getting a bigger turbo but need to get this fixed first. If it winds up being all the turbo that's great but I don't think it is all the turbo. I am seriously making like 400rwhp on 15psi. I am putting it on a dyno next week so I will have #'s and a graph to go off of. Just trying to test everything in hopes of getting this thing fixed.

Thanks for your help.

Keep suggestions coming!

 

I am not too sure. I think we are 1000' above. I am in northern IL.

 

well for some info....i have a 366 with a T80..and the most i get is 14psi out of it.

i thought my turbo would have more in it too...but then again im like 6k above sea level

hopefully george at TTi can help me get more psi

 

A 76 should make at least 650-700rwhp on anything 360-408ci engine. Backpressure will vary greatly between those 2 setups, but it will still make power to a certain degree. 400rwhp at 15psi means their is a serious leak somewhere.

Also if you keep doing that you run a risk of blowing the turbo up. As you will be GREATLY overspinning the unit.

 

I understand that if there is a boost leak that the turbo will have to work harder to sustain a boost level but if I am seeing 15psi at the manifold shouldn't I be making good power?

 

What is the exhaust backpressure?
Did you do a compression test to rule out any obvious
mechanical issues?

 

Well we had to use a compression tester to test backpressure and I am not sure how accurate it was.( poss leaks) We came up with about 40psi @3500rpms@ 14psi of boost.

I did test the compression on a couple of cylinders and they were at 190 psi. I thought it was kind of high for 8.5 to 1 but high is better than low!

 

The backpressure/boost ratio should not exceed 2-1 in a well designed
turbo setup. Many setups only hit 2/1 at the upper rpm range not 3,500!
40psi at only 3,500rpm and 14psi boost is VERY restrictive!
Depending on the wheels of the turbo, you could be way outside
the compressor map.
Do you have info on the exhaust wheel used in your turbo?
If it is too small, it could be choking the motor even at low boost.
Even though you are FI, the engine is still an air pump and will not
make power even with positive manifold pressure if there is not
enough FLOW. The exhaust side can't be this restrictive and make good power. Running an open downpipe could minimize the problem, but with
40psi of backpressure I would not expect a big difference.
Who supplied your turbo?
Is it possible that is was built for a much smaller engine?

 

It was built for a stock cube ls1. Here is a picture of the tag on the box.

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If it was a backpressure problem wouldn't the waste gate get pushed open with the soft 6psi spring that I am running when I turn the boost up?

Because if that is the case then the problem I have with it not building more than 15psi with the wastgate reference pulled make sense now. The gate is getting pushed open.
I am just thinking out loud. What do you guys think?