TurboTommy

Well, there's 3 different kinds of MBCs that I know of and they all function differently.
But, really this exercise is about taking the method of boost control out of the equation for a moment and realizing that the amount that the wastegate valve lifts off its' seat is determined by the effective pressure it receives. Since the origin of this pressure comes from the charged system, how is it supposed to be kept constant by varying RPM(remember; valve lift is determined by effective pressure at the diaphagm; not varying amounts of exhaust volume like when RPM changes)

I know what you mean by the valve lift due to exhaust pressure; it has a much lesser effect once the valve is off its' seat.

In any event, I ran boost pressure directly from the charge pipe to the wastegate (no boost controller or cycling valve) as a test. Boost was initially at spring pressure and then gradually rose with RPM, rising rapidly the last 1000 RPM. This is with 46mm wastegate and custom (more direct) wastegate plumbing!

I've done some digging into this phenomenon and had to draw a conclusion:
we know it as a truth that no matter what supporting mods we do with a stock turbo, boost always seems to roll off at the upper RPMs. Well, I can imagine the opposite can happen as well; if the whole system is efficient enough, with hardly any pressure drops from the air filter all the way back to the tailpipe, then boost will be generated easily with the least amount of energy needed from the exhaust. Result: runaway boost pressure coming from an ever increasing supply of exhaust from increasing RPM.

We need some turbo gurus to chime in here to set me straight. Maybe I'm missing something; that's why, after I thought about it some more, I was wondering about how the wastegate actually keeps steady boost. JohnKoaWood mentioned wastegate flutter; I guess meaning at lower RPMs major flutter (between almost closed to almost fully open, cycling rapidly) to no flutter at all, at high RPM?

Laust Pedersen

If you with “boost creep” mean that the boost goes above 17 psi for the direct connect, then your 46mm WG should be fully open and it still cannot get rid of the excess exhaust pressure.

Assuming that your WG valve isn’t stuck or the WG plumbing isn’t under dimensioned in any other way, then it looks like you need more WG flow capacity, a bigger turbine and/or a smaller compressor … or just set the boost pressure to the “runaway level”.

It is really as simple as that.

TurboTommy

Yep;
that's pretty much what I figured.

I really don't want to change my turbo and wastegate configuration for the third time, though.
I wonder if one can get an exhaust pressure blow off valve. An adjustable valve that one could tap into the cross-over and it would relieve excess pressure when it hits the setpoint.
Does any body know if something like that exists? They might have been around back in the '60s and '70s as a crude stand-in for a wastegate. I would supplement my system with that.

beebone

As I know, the stock WG is inverted comparing with other external WG. I beleive the WG will barely open when the boost building up, then open more when heavier boost and regulate the amount of boost.
I just found from my EBC, when I set the boost offset in lower value, the boost come soild, otherwise, the boost feels like lack of power. What I mean the boost offset is how soon/late to open the WG when the boost start to build. I can obtain the target boost in both ways, but the feeling is different.