Wastegates + MBC's
 

I bought a 2560 that came with what I believe is a 12-14 psi wastegate actuator. I googled the part number (448734-7) and all I could find was this.
http://t2.frsimg.com/FRS-Internal-Wa...8R_p_9631.html

This is too much boost for me right now, and I'm under the impression that if I set my MBC to anything under 12 psi, I'm still going to boost to at least 12 psi because of the actuator, along with shitty spool, etc etc.

It seems to me like the ideal setup would be a weak wastegate actuator (like 6 psi), and then running whatever boost I want (read: anything above 6 psi), and when the actuator sees my desired boost level it should slam open.


Opinions/thoughts/flaming/spoonfeeding? Anything bad about running a weak spring with higher boost? I searched for this and just found people bitching about boost creep.

A boost controller is only truly effective at a higher pressure than your base wastegate is set to.

MBC will work great at lower than wastegate pressure. But of course once it opens, the wastegate won't, there in lies the problem.

Weaker spring at higher boost is a great option, that gets you the good spool.

With just a wastegate, it'll slowly open up as boost builds towards your wastegate's opening setting, wasting spool.

With a MBC triggered wastegate, the wastegate stays completely closed until the MBC releases the boost, at which point the wastegate will snap open. Hence better spool.

I had a 12psi wastegate and was able to make as little as 8psi on my turbo back in the day.

One thing to consider is if the spring is too weak, it might not be able to hold the wastegate closed under high turbine inlet pressure. So don't go too far on the side of the weak spring.

Isn't that what the MBC is for?

lololol, in that case all of our wastegate springs are too weak. lololololz.

Rule of thumb is WG spring should be no less than 50% of the target boost, otherwise you can have bleed off.

EG: You want 12 psi, WG spring should be no less than 6.

Of course, this doesn't hold 100% true and depends a lot on what kind of WG you run, the diameter of the gate, design of the manifold if its external, etc etc etc.

Jeff's got it...

The case for the too weak spring is when the spring can't hold back against the pressure delta on either side of the wastegate itself. Turbine inlet pressure vs. turbine outlet pressure basically, then your WG diameter (both of the hole, and of the poppet valve) the lever arm on the WG arm, and the crank, all factored in to compare against the spring rate and preload in the actuator.