Boost drops off at high RPM (stock twins w/ MBC)

[dmadma60]

My near-stock 1JZ does not hold the selected boost level at high RPM. I set my MBC for approx. 13-14psi in 4th gear, but as I hit 5000+ RPM the boost drops by 1 to 1.5 psi. Is this normal? This happens in 3rd gear also.

I pressure tested my intercooler system and pipes but realize I could have a leak elsewhere.

My car:
1JZ from Soarer
Stock downpipe
3" midpipe, high flow cat, 3" exhaust
FMIC 19x12
MBC (ball&spring type)

Any ideas? Perhaps this is just normal?

Thanks,

Dave

 

[Boost Lee]

If you've got the stock downpipe, I'll assume you still have the stock Y-pipe as well.

Upgrade those two things...
And with assumption that your twins haven't eaten themselves yet,
That should really help out a lot.
Otherwise, with a standard MBC, this can be a fairly common issue.

Jeff

 

[dmadma60]

If you've got the stock downpipe, I'll assume you still have the stock Y-pipe as well.

Upgrade those two things...
And with assumption that your twins haven't eaten themselves yet,
That should really help out a lot.
Otherwise, with a standard MBC, this can be a fairly common issue.

Jeff

Thanks! I have actually been driving this way about 1.5 years but only added the MBC a few months ago.

 

[tissimo]

Check for boost leaks.

 

[87M-GTE]

Its the DP/Y pipe


-Sam

 

[dmadma60]

Its the DP/Y pipe


-Sam

Thanks all. I will post again after I get a 3" DP. Not going to wrestle with the Y-pipe.

Also, like I said, I checked the I/C and intercooler pipes for leaks. I am not sure how to properly check the other areas. I did have a nasty leak at my boost gauge fitting once.

 

[Boost Lee]

Thanks all. I will post again after I get a 3" DP. Not going to wrestle with the Y-pipe.

Also, like I said, I checked the I/C and intercooler pipes for leaks. I am not sure how to properly check the other areas. I did have a nasty leak at my boost gauge fitting once.

Boost leaks can be more evident than you think.
"Checking boost leaks" to most people consist of looking over the intercooler piping.
Anything from a leaky throttle body gasket to a punctured intercooler can cause these problems.

But I still stand next to the downpipe/Y-pipe.
3" Downpipe will certainly help, but that Y-pipe is just something else. :nono: