technical... MBC overhaul...

first i want to appologise for this question b/s it is related to a VW PASSAT VR6, but overhaul its all the same thing. here's the story...


a friend of mine build his VR6 engine and turboed it. he changed the internals, get a HKS BOV , external WG , big FMIC , i'm not sure what exsact kind of turbo it is, but here what the problem is.

the car starts rising boost and everything is fine, but ones he reaches his high boost level (10psi) the gauge starts jumping everywhere between 10-13 psi and the car feels like it jurks. now the boy knows his shit and build the car from scrach, but when i asked him, on how much he setted his MBC he was like: "what , boost controler, i dont have one". ???

i mean isn't he supposed to have one. how the hell is he supposed to set boost levels w/out one. isn't he got control over the boost only from his external WG spring. i mean the spring will be his only determination of boost level , right? man thats crazy. what if his spring is set at 15 psi,woun't he destroy his engine that way? he already tried to change the spring in the WG with stiffer one, and it only got worst(i guess he set the boost level even higher) . another thing is that now, he says that there is a hissing noise coming out of the turbo( like a bad seal or something).

my question is this. isn't every turbo aplication supposed to have MBC or whatever, just some kind?

p.s. Kevin please lighten up the situation...at first sounded like a boost creap, but he has the external WG , which tells me that that is not the problem, but the MBC

There are at least a dozen different springs for my Tial WG, in like 2 psi increments. You certainly can control boost with just the WG spring, and its a little better because its so simple. But you have no easy adjustability. Takes a good 10-20 minutes to change the springs.

I just dont understand why he went to a stiffer spring if he was trying to lower boost. Shouldve gone lighter. IF 10 psi is what he wants boost at, and the fluttering between 10-13 psi is the problem, well, I cant remember what causes that but I have seen it in on car. Ah yes! Its usually from running too soft of a spring and trying to run double or more with the MBC. Running a 10 psi spring at 10 psi should cause any problems, something else is wrong. I would check for boost leaks, and if that is ok, look into the tuning. Being very rich could cause that I suppose. Or too lean. Yeah, I'd get a WBO2 on it. Could also try an 8 psi spring and see if it will hold that. Might be something about that specific boost pressure the setup doesnt like.

This will be a tough one to do over the internet, but I'm sure this guy will be able to figure it out eventually. :)