I was changing my vacuum lines and discovered by cycling valve was broken so decided to upgrade to a MBC.

I installed a Lindsey Racing MBC I also did all the vacuum lines at the same time and fixed all my exhaust leaks (stock system) a few weeks ago.

When I went for a test drive to adjust the MBC I noticed the boost drops from just under 15 psi to 10 psi at high RPM in second gear no matter what the MBC is set at.

From what searching turned up the boost roll off would be normal for a stock system with a cycling value but I would expect a MBC setup to hold full boost up to red line.

Is my waste gate bad and just opening when any pressure gets through the MBC?

 

thats common of the factory wastegate

 

This is a big debate, but my turbo couldn't hold the psi... It had nothing to do with my wastegate since I had already replaced it with a new Tial.

 

Yeah, the stock 26/6 is not able to hold boost levels at higher rpm, but you usually don't get issues with that until the higher gears.

 

Just buy a new wastegate... you will want one eventually anyway.

 

Lorax, I love the way you keep things simple!

"Just get the 62 Inch Plasma tv, you'll want one anyway..."

 

LOL! Funny you should say that, I just got a new one. It's actually 42 inch though...

back on topic....

 

Shoulda got the 62" youlll want one............
Naw.. Get both. Turbo and Wastegate.. But.. then you'll need some management.. Wait then Fuel to keep up, so injectors and pump.. Then you'll need bigger tires.. But then some sort of suspension to handle them... ... It's what we all go through. But... I say wastegate also.

 

LCD FTW.

 

I guess the next thing to do is get a new wastegate. I'll do some more testing in a higher gear and see if there is any difference.

 

the factory chip is designed to reduce boost pressure at the higher rpms. Your wastegate could be fine, and the stock turbo/wastegate can hold 15 psi to redline no problem, assuming neither is broken.

 

But since he is running a mbc the computer has no way of controlling boost pressure, aside from cutting the ignition.

 

Methinks your roll off could really be a little bit of both...the 26/6 and a tired wastegate.

My Turbo S with the stock chip, fresh stock 26/8, 3" stainless downpipe/bypass, dual port Tial WG and a Lindsey MBC holds whatever limit I dial into the MBC all the way from 3200 to 6000+ rpm in second and third.

My MBC lives at its lowest setting...staying around 12 psi...and spending the money other guys invest in head gaskets on big TVs and my track toy

 

my stock/shimmed wastegate holds 15 psi to redline

 

I am having the same problem. I have the Lindsey MBC, a 26/8, Russell's chips, a shimmed wastegate, no CV, and all new vacuum lines and my boost still falls off. Also, the MBC does not hold the same boost in different gears. In lower gears my boost goes to 13 -14 then in 4th goes up to 16 -17.