legacycentral bbs

So I have been running with the Tdo4 and 06 forester tmic for a few months now. I am currently running an mbc that is hooked right to the turbo and bypassed the stock boost controller. I have the mbc set to 11.5 just under fuel cute. I have started to notice that when I go WOT once i get up to around 4.5k and up i start to fall off of that set boost level and its goes from 11.5psi down to 8psi when the trans shifts around 6k. I do notice it more in 3rd and 4th gear which im guessing load has to do with it. I know when I did the compressor map for the turbo before I swap to it. From the map I should still be with in my peak efficency for the boost levels that im at and the turbo would be good up to 15psi so im wondering if it is a flow issue in the mbc were it doesnt alow the turbo to maintain that level or is it a head flow issue or exhaust issue. I'm running a 3inch catless divorced wastegate down pipe by nividia to my factory exhaust plus i'm running headerwrap on the manifold and uppipe

I'd suggest checking all the vacuum/boost lines to make sure there isn't any obstructions. If you can take apart or inspect the MBC for the same, that would be good. If everything checks out ok, I would suggest it might be a turbo issue.

Has everything else stayed the same on the car and how it runs? How long have you been running the MBC set at 11.5 psi?

When the boost drops, do you feel a loss in power, or is it just a loss of pressure on the gauge?

I was thinking of taking my vacuum lines off and checking to see if i have any blockage. I did noticed that my boost gauge only reads 11" of vacuum at idle. I dont know if it makes a difference but I do have my boost gauge t-ed into my vacuum line going to the bov and after doing some investigation I noticed when I blocked off the vacuum line to the bov my gauge was now reading 15" of vacuum. As far as in the car running the car seems to run great no misses and good power. I just noticed that the car does seem to fall off power wise at higher rpm like i have good power between 2500 to about 5200 then it start to fall off and I noticed that boost falls off to, but thats only on the highway in 3rd and 4th in the 4eat.

I would suggest checking your vacuum lines. Your vacuum reading at idle should be close to 19" assuming the motor has stock internals. Have you used a separate gauge to check engine vacuum?

As you discovered you may want reroute the vacuum line to your boost gauge so it's not the same line as the BOV. Also, the BOV, if atmospheric may be leaking, which may be why your vacuum at idle isn't where it should be.

I have had 2 different gauges connected to the same spot and they both read the same 11hg of vacuum at idle. At first I thought it was a cheap gauge but my autometer gauge have the same reason.I found it intresting when I first installed the boost gauge in that location it read 22 hg of vacuum then it slowly started to go down and now sit at around 11 at idle. The car does seem to run absolutly fine. I do have stock internals the bov is going 100% recerculation. Could it be possible that the bov might be leaking internally causing the low vacuum issue to the gauge. I dont know were a better place is to plump the gauge into. Any suggestions?

You can tee into the vacuum line going to the MAP sensor. It will be located on the front passenger side intake runner. If you still have 11" of vacuum, you've got a vacuum leak some place or a loss of compression or something else is up. At which case, it couldn't hurt to do a compression test or leak down check.

This is very true I will check my compression over the weekend and see were I'm at and I'll tee the boost gauge into the map sensor plumping never though of that before good spot to get full manifold pressure