I can't hold boost :(

[blcknspo0ln]

I've tried everything to hold 28 psi and I can't with my 50 trim. It hits 28psi, but then bleeds off to 23 psi-ish by 8k.

-PTE 5031RE with mitsu housing
-TiAL 38mm off of o2/ 1.4 bar springs
-Hallman MBC off of intake manifold
-Boost leak tested to 30 psi
-Forge BOV with red spring
-BC 280/280 + JMF SMIM

Any insight?

 

[gsxtasy]

Your MBC boost source is off the intake manifold? Faulty MBC? Turbo should be able to hold more than 23lbs to redline, I would think. Hmmm...I'm not sure, thinking outloud....

 

[romeen]

Wastegate could be blowing open. Maybe try shimming it to see if that helps.

You know more about turbos and cams than me but do you think it's possible that the 280's are moving too much air for the turbo to keep up with at the higher rpms? Sorry if my understanding of how that works is flawed.

Edit: Nevermind comment about wastegate. Just reread your setup.....I need to fully wake up before I start posting.

 

[DSMunknown]

When do you hit 28?

 

[Defiant]

The motor's out-running the blower.

 

[1992awdlaser]

The only thing I can think of is the 280's and intake manifold flowing too much air. My G50 holds 28psi fine. But, I am on fp2's and stock intake manifold.

 

[bluetlr2001]

Why is your MBC coming off the intake main? Shouldnt it be coming off your piping before the bov closer to the turbo?

 

[1992awdlaser]

Why is your MBC coming off the intake main? Shouldnt it be coming off your piping before the bov closer to the turbo?

Mine is off the intake manifold. Putting it at the compressor cover could cause boost variatons. The air starts out fairly cool at the compressor cover boost source. Once the car gets moving the air heats up a good amount though. This variation in temperature can cause the boost controller to open sooner once the air heats up, which would cause less boost. This is why you want to put it after the intercooler. Most people won't want to weld a piece on the upper intercooler pipe to put the line so the intake manifold is the only choice.

 

[gsxtasy]

Mine is off the intake manifold. Putting it at the compressor cover could cause boost variatons. The air starts out fairly cool at the compressor cover boost source. Once the car gets moving the air heats up a good amount though. This variation in temperature can cause the boost controller to open sooner once the air heats up, which would cause less boost. This is why you want to put it after the intercooler. Most people won't want to weld a piece on the upper intercooler pipe to put the line so the intake manifold is the only choice.

There is also a pressure drop across the intercooler. If you have your boost source at the J-pipe, and you set it at 20psi, once it crosses a FMIC and gets up to the motor, you may only be seeing 17psi. So even though your MBC is set for 20, you're actually only getting 17 into the motor. The pressure drop will be bigger, with bigger turbo's as well.

 

[blcknspo0ln]

There is also a pressure drop across the intercooler. If you have your boost source at the J-pipe, and you set it at 20psi, once it crosses a FMIC and gets up to the motor, you may only be seeing 17psi. So even though your MBC is set for 20, your actually only getting 17 into the motor. The pressure drop will be bigger, with bigger turbo's as well.

Exactly why I moved it to the intake manifold. I originally had it on the compressor housing, but moved it to the SMIM in hopes that the pressure drop was the cause of my ailments, but that didn't work. I too am starting to believe that I just need a bigger turbo. I never thought I'd say that Thanks again guys, I knew my answer, just skeptical that everything didn't work together as planned.

 

[CanadianTSi]

Take off the vacuum hold to the wastegate and see if she will hold boost up top

 

[turboglenn]

YOu could always just run a line from the intake to the top port on the WG and see if it would hold PSI that way (just as a test) but it does sound like you're flowing more than the turbo can hold. God i wish i was getting hear that point

 

[blcknspo0ln]

Yea, i've done that. With the direct line to the WG, she holds ~22 psi just fine. I don't get it

 

[Black95TSIawd]

Is it possible the bolt-on turbine housing is causing a restriction up top? I know you like to rev your motor to the moon.