cannot make over 10 psi of boost

[camcojb]

well, one by one I've handled the little issues of a new car and I'm pretty much down to the final one or two on my list. Since I finished the car it will not make over 9.5-10 psi of boost, in any gear up to 6200 rpm or so.

Here's what I have:

402 LS2 turbo-specific build, ET Performance 245cc heads, PMC sheetmetal intake, Nelson Racing headers, twin T67-76RE Precision ball-bearing turbos with twin 44mm Tial wastegates and single 50mm Tial blow-off valve. The 3.5" in/out intercooler was in the car when I bought it and the old motor could run 15+ psi of boost without issue. I have 3" mandrel bent downpipes and full 3" mandrel-bent exhaust out to the rear of the car.

The wastegates started with 13 pound springs and I run a manual boost controller on top of that. I swapped to 19# springs in the gates and tried with and without the boost controller, no change. I added a boost line to the top of the wastegates directly off the turbos, no change. I shimmed the blowoff so it cannot open, no change. I removed the air filters and then the inlet tubes (3" mandrel-bent) and no change. I pulled the inlet/outlet tubing out of the intercooler to see if there was a rag inside or something stupid, nothing. I swapped from the W2W turbo roller custom cam back to the stock LS2 Z06 camshaft; picked up idle vacuum, but no change in boost.

The turbos have .81 exhaust housings, but spool fine. Even if those were too big once it made boost it should go nuts. My laptop shows 160-165 kpa which matches my boost gauge at 9.5-10 psi, so I believe the boost reading is correct. The car runs great, but I'd really like to be able to make more boost. I am out of ideas at this point. I've spoken with Precision and Tial, no answers.

The boost comes up fine and then it's like it hits a wall and stops at 10 psi; that's why I thought the gates or blowoff were opening, but apparently not (I know the blowoff isn't as it's shimmed shut). The only other thing I can think to do is add a boost gauge off one of the turbos before the intercooler just to see what it shows. Does anyone have any other ideas?

Jody

[sacarguy]

1st off have you verified your boost guage is not faulty or has a leak in the line leading to it ?

tap off the turbos outlet scroll and run a dedicated boost gauge there to see if there is something restricting them such as a rag in the intercooler etc.

check your turbos exhaust and intake wheels for damage
verify you have no large exhaust leaks
check to be sure your turbos are spining freely and have no bearing damage
check the waste gates to make sure they do not have damage to the valves even if shut if the valve or the seat for it are damaged it can cause issues
check the blow off valve diaphram and seal/valve
check all silicones they may look ok but have a tear un the underside
check the intake and intercooler for any large holes or cracked welds

the welds can crack and be just fine under vacume but when boost is applied they will open up the seam and allow large amounts of presure out
< have seen this happen with liquid to air intercoolers alot >


thos turbos should have no problem pushing 25 psi into that engine so im going to go with most likely your waste gate or turbos themselfs have a issue.

[projectile]

Check your exhaust pressure before the turbo. Might be an easy way to find a serious problem(Wrong turbine wheel for your housing, damaged wheel,open or defective wastegate,...)

Do you have an easy way to check exhaust backpressure before the turbo (o2 sensor bung or something)?

I can't think of anything else (at the moment) that sacarguy didn't mention or that you haven't tried.

[camcojb]

Originally Posted by projectile

Check your exhaust pressure before the turbo. Might be an easy way to find a serious problem(Wrong turbine wheel for your housing, damaged wheel,open or defective wastegate,...)

Do you have an easy way to check exhaust backpressure before the turbo (o2 sensor bung or something)?

I can't think of anything else (at the moment) that sacarguy didn't mention or that you haven't tried.

no, there's nothing to hook into between the engine and the turbo; you don't want an O2 on the pressure side, so the bungs are in the downpipes.

Jody

[deuce_454]

your headers look VERY blue, my guess, (which is only a guess) is that the turbos are on the small side, you could be at the surge limit at 10 pounds, have you done the math and checked the turbo maps?? it could be that the housings are the limiting factor.

you could be moving the same mass of air at 10 pounds thru the bigger heads as you weer at 15 pounds with the old ones.. and its the mass, not the pressure that matters, the ET 245 heads flow like a MoFo and on a 402 with your super optimal intake and hot side tubing, you could be outflowing the turbos

[camcojb]

Originally Posted by deuce_454

your headers look VERY blue, my guess, (which is only a guess) is that the turbos are on the small side, you could be at the surge limit at 10 pounds, have you done the math and checked the turbo maps?? it could be that the housings are the limiting factor.

you could be moving the same mass of air at 10 pounds thru the bigger heads as you weer at 15 pounds with the old ones.. and its the mass, not the pressure that matters, the ET 245 heads flow like a MoFo and on a 402 with your super optimal intake and hot side tubing, you could be outflowing the turbos

The turbos were spec'd by Precision for my combo. They are T67's that flow like T76's according to Precision, and have the larger .81 exhaust vs the more normal .68 exhaust housing. They claim they are plenty big enough for 1400 hp. I actually was thinking they were on the big side as I didn't want a laggy engine, but they're not laggy at all, come up on boost nicely. They just don't make as much boost as they should.

As a comparison, I had this engine in my GTO with a pair of GT3072R turbos that made 16 psi no problem, could have made more but that was as high as I ran them. They were much smaller turbos than what I have now.

I'm going to hook a boost gauge to the outlet of one of the turbos first to see what the boost pressure is before the intercooler, just to eliminate that as an issue. If that's near 10 psi then I'm going to shim the wastegates closed so they cannot physically open and see what happens.

Jody

[Ketzer]

Stones of steel. I'd be terrified to block off the wastegates and try to force up the boost. I know you guys are all really knowledgable and this is easy peasy for you, but with the coin you've got invested, I'd be sweatin' bullets watching the boost gauge on that test run.


Jeff-

[camcojb]

Originally Posted by Ketzer

Stones of steel. I'd be terrified to block off the wastegates and try to force up the boost. I know you guys are all really knowledgable and this is easy peasy for you, but with the coin you've got invested, I'd be sweatin' bullets watching the boost gauge on that test run.


Jeff-

I think I could modulate it with my right foot pretty easy. I don't plan on going wot for a 1/4 mile pass, just load the engine and see if the boost swings past 10 psi. Going to check the boost on the inlet side of the intercooler now before any other changes, will post back on my findings.

Jody

[ohcbird]

"Hits a wall at 10psi"

Since you've eliminated all the other variables, I'd check the exhaust pressure going into your hot sides. If you're seeing over 20 PSI there (2:1), then you've found a restriction. The closer to a 1:1 ratio, the better. As a side note- do you have any EGT probes? I'm curious as to what your gas temps are hitting. Do you notice a more-than-normal rise in IAT when you're on the boost for a while?

I agree that your .81 housings are about right & should work fine, but you might be right at their limits already. I think that motor is trying to move alot more air than it currently can, hence the 'wall' effect. I think you can get 1.01 housings for those; that might be worth a try.

[deuce_454]

Originally Posted by ohcbird

"Hits a wall at 10psi"

Since you've eliminated all the other variables, I'd check the exhaust pressure going into your hot sides. If you're seeing over 20 PSI there (2:1), then you've found a restriction. The closer to a 1:1 ratio, the better. As a side note- do you have any EGT probes? I'm curious as to what your gas temps are hitting. Do you notice a more-than-normal rise in IAT when you're on the boost for a while?

I agree that your .81 housings are about right & should work fine, but you might be right at their limits already. I think that motor is trying to move alot more air than it currently can, hence the 'wall' effect. I think you can get 1.01 housings for those; that might be worth a try.

my thought exactly.. but check the boost before at the turbines just to be sure....

you could have another issue causing it.. some manual boost controllers need to be plumbed to both sides of the diagphram on the wastegates.. so they hold them closed by applying pressure on top to hold the valve closed till the boost hits preset, and then they release this pressure to allow the valve to open.... just make sure you havent got the hoses crossed (mounted wrong) on one or both sides so your boost controller is actually opening the valve instead of keeping it closed