I'm having a very weird issue (at least in my eyes) with my custom made rear mount twin set-up on 05 GTO. I have a pair of T3's (.42/.48) being fed by the stock piping (no cats but there is an "X" pipe where the factory resonator was located) feeding a individual 2" pipes to right below the rear axle where they merge into a "Y" and feed a 2.5" up to the intercooler. It leaves the intercooler and leaves 3" and steps up to 4" at the throttle body.
The car will not make over 5# when W.O.T. in first, second, or third but will go straight to 8 to 9#'s immediately in 4th & 5th.
I originally built the system with .60/,63 T3 units but thought the lack of consistent boost was caused by the large housing so I switched to the .42/.48 units.
I thought it was possibly a leak in the piping, so I literally took every section of piping off and leak tested each section individually. I used a pvc plug on the end of the piping with a valve stem on one end allowing me to fill the pipe with compressed air. Each section was tested to 25 psi (was scared to go higher as I made "bomb" when one the cap flew off the end of one I was doing while not paying attention to the amount of air I was injecting into the pipe). I only found 2 pin hole leaks in the piping, which I repaired. I then re-installed all the piping and pressure tested each section I pressured the entire system piece by piece as I re-installed it (I installed a section, capped it, pressure tested it ... couplers and all) as it was placed on the car.
I pressure tested all the vacuum hoses to/from the wastegates and not an issue was found.
I can measure the pressure from the compressors themselves when I measure boost from the compressor housings (have the nipple from each turbo housing "Y"d together and have the line ran to the cabin which allows me to plug it into the boost gauge when I want to see what they're putting out) and they will spool to 18 or higher with the wastegates disconnected, yet intake does not see higher then the reading mentioned earlier.
I'm going to go back over the intake bolts and areas related this weekend as well as put the .60/.63 units back on.
Any ideas on what I could/should look over??

 

What type of wasegates are you using? Where are they getting a pressure source from? What size plumbing from the manifold to the turbos? Whats the tune look like?

 

Internals

Back of the intake manifold

I have a 2" from each turbo that feeds to a "Y"



"Y" that feeds the 2.5" that travels the length of the car. 3" from the cooler to the intake

On the dyno the first time (with the .60/.63 units) it was running out of fuel with (over 90% duty cycle)
with stock LS3 injectors at only 4# of boost (I have CNC TSP LS3 heads, LS3 Intake, TSP cam, UDP ) and only 448 hp. So I had Fuel Injector Connection resize my stock LS2 units to 60's and also changed the turbos to .42/.48 turbos. Took it back to the dyno last week, it wouldn't make much over 5#'s before my oil pump started showing it's G-string (Shur-Flo at the time ... went to a large stainless shaft/bronze geared oil pump ... absolutely one of the best things I've purchased for this adventure) and had oil coming out the turbo on high RPM pulls. I took it home, replaced pumps and thought it best I start looking for the boost issue before trying to re-dyno it again.

 

18+ before the intercooler and 5-8 in the intake? Bypass the intercooler with a straight pipe and see what happens.

 

if it goes to 8-9 in 4th may just be me but it sounds like not enough load in the first 3 gears to build over 4-5lbs. What gears?

 

You have 18+ but 5 ag the intake. Sounds like a restriction to me

 

Check in the intercooler piping and the intercooler for rags and other obstructions

 

seen that happen a few times

 

EDIT ....Never mind .

 

I may do that but I doubt very seriously if the intercooler is a restriction at this point. In 3rd WOT the intake sees 5 but switch gears up to 4th and it goes directly to 9# .... If I ride down on the clutch a bit and release after a second or two in gears 1, 2, or 3 I can also ge the boost up to 9# at the intake. It just does not go to that point by throttle alone outside of 4th and 5th which is baffling to me.

Yeah, possibly but where? As I stated above I literally took each section of pipe off the car and pressure checked it individually. I then pressured the entire system piece by piece as I re-installed it (I installed a section, capped it, pressure tested it ... couplers and all).

Nothing in there... the intercooler pressure checked to 50 psi ...which is when the capped off side blew off with a vigor I might add, LOL!

 

They are talking about having a RESTRICTION, not a leak. Blockage. Pressure testing the intercooler won't tell you anything about how much air can FLOW through it. Thats why you replace it with straight pipe that you know will FLOW air.

 

When I used to work on heavy trucks we would use a very large shop vac and listen for a change in pitch when checking for obstructions ... simple but effective.