boost spike with mbc, indeed!

[a67ssrs]

I have an '05 SRT-4, bone stock, until I installed a perrin mbc yesterday. I think I have it adjusted nearly where I want it (spike to 17 psi, I suppose I may turn it up to 18, but I want to stay safe). The thing is, I guess I didn't fully understand the term 'spike', or maybe it's my boost nazi '05 PCM. When it was stock it would boost 15 psi from 2500-5k rpm, then fall off towards redline. Now, it boosts to 17 for a fraction of a second, then falls to 15 immediately, and acts just like it did before. Is that all I get out of a mbc?
On a related subject, how worried should I be about PTB? It kinda worried me when I'm on it half throttle, and it'll spool up almost like I'm at WOT, then blow off, I think without me letting off the throttle (although I probably let off every time it does that, I do NOT want detonation, or compressor surge).

[a67ssrs]

I just figured I'd add that I tested a bit more on the part throttle boost, and it definitely will blow off repeatedly at a bit less than half throttle or so.

Also, I was only spiking to 16 before, now have it turned to 17, and it seems to hold a bit more boost (15-15.5 psi) until 4500, when it starts falling off. I suppose I should be pleased with the bang for the buck factor, and would be, if it wasn't for the doggone PTB.

[Scott@FM]

With a stock wastegate actuator you will not get much more then what you have now. It is one of many fail safes built into the car. If you are hearing a fluttering under boost that is most likely the factory bypass valve leaking. This is designed to leak around 16 psi of boost. You will also get some boost at part throttle since the stock turbo is so small or you will get excessive boost at part throttle if you changed/re routed your vacuum lines. If you have any other questions feel free to pm me.

[a67ssrs]

Thank you for your reply. I've been thinking a lot about this, and tinkering with the mbc, and it seems to get any appreciable increase in boost past the momentary spike, I have to turn it up to the point that it spikes to almost 18psi. That worries me, so I am going to remove the mbc, at least until I get some supporting (fuel and management) mods. It seems easier to launch the car without the mbc on it anyway, so I imagine my 1/4 mi. times would be approximately the same, although I still haven't run the darn thing yet.

How much extra h.p. are we talking here, anyway? 10? I could take out the spare tire and achieve the same result...

[turbokid]

Good idea! A couple extra pounds of boost isn't worth 1000 dollars in motor work!
I am not sure what the supporting fuel is for max boost on the srt4's really but better safe than sorry!
The quick spool up is a characteristic of the mbc!
I like it at some points and dont on others. Since gas is crushing me right now I have taken the MBC off since I can't seem to keep my foot out of it and I like having the part throttle driving with out hauling ass down the road

[1badsrt-4]

Take the mbc off and get rid of it. Buy yourself an adjustable wastegate actuator and adjust boost that way.

[bhazard]

The motor cant take 18 psi?

[1badsrt-4]

Quote:

Originally Posted by bhazard The motor cant take 18 psi?

Sure it can, the fuel system cant handle much more then that. Im still on a completely stock motor with a big turbo,and I run 25psi threw it all the time at the track.