Just had a new exhaust manifold put on and experiencing less boost. My Boost gauge is reading 14-15psi at max boost. I'm running a MBC and a Skunk2 exhaust mani. I also noticed more backpressure. I'm going to do some pulls in 3rd to see what's going on. I guess boost pressure is relative when you are going the MBC route? Any insight would be nice.

 

What do you mean that you noticed less back pressure? Does this mean that you are equipped to monitor exhaust pressure? If so, how? Do you have a pressure sensor mounted in the turbine housing, or the exhaust header?

 

I am wondering what motivates you to ask this question? Did you install MBC and header at once and thereupon notice a decrease in peakboost level down to 15 PSI?

You mention that you are going to try some WOT tests in 3rd gear. This is good. At some point I would like you to rotate the adjustment knob on your MBC out counterclockwise(-) to the absolute lowest boost setting possible through its range of rotational adjustability

Do a WOT run with the MBC set this way and note the peak boost reading attained. This is pretty much going to be the base wastegate spring pressure. The reading will be somewhere between 11-15 PSI.

P.S. Also, can you tell me what peakboost level you were seeing before the manifold swap? Did you make any other changes to the exhaust such as turbo elbow, down pipe, or test pipe? Did you already have the MBC installed before the manifolds were swapped?

 

Thanks for the response; the MBC was there before the swap. I was getting 20.3psi. The guys at the shop didn't take the intake apart, they did remove the filter (Injen) to get better access for the swap operation. I wonder what is going on.

I'm going to turn the pressure down just as you said and see what I come up with. Can you think of any reason why there would be less pressure? I mean the other manifold had a cracked bolt that basically fell out during the swap. You would think that I would have more pressure.... Oh well, that's why I'm asking the experts.

Oh and I didn't mean backpressure I meant vacuum, sorry.

 

Maybe your manifold isn't sealing. Warped flange, bad gasket, broken bolt can all cause exhaust gasses to pass before the turbo and cause boosting issues. Does it spool any slower than usual? Also try running your wastegate to your boost source. That will allow you to run wastegate pressure to see if your mbc is at fault.
-Edwin

 

Well, I took Sparky's advice and went for the MBC and to lower the boost all the way down and low and behold it had been messed with by the guys at the shop. It was 1/2 a turn from the lowest setting. I did a few pulls, parked at the AGIP Gas Station, and turned it all the way up and then on whole turn back down.

2 more pulls showing 20.5 boost pressure. And I noticed I'm running a bit leaner; I need to lean out my maps anyway. 20.5psi @5875/228 load/ 10.2 AFR--> avg of 5 runs.

Thanks Sparky.

 

Yeah, after I read that the shop messed with your MBC and vacuum/boost lines, I was gonna suggest replacing all the MBC hoses with new rubber hose and using new 1/4" hose clamps. This is cheap enough insurance.

MBC's are simple little devices but they are not idiot proof. You have to be checking boost level frequently as your peak boost level can be different when days are hot and nights are cool. Usually that is more of a problem now heading into the fall months when diurnal/nocturnal ambient air temps can differ more than 30*F. I usually figure about 1 PSI increase per 10 degrees F. of temperature drop. I mean the boost will creep up on its own when ambient air temps fall....So, keep an eye on it.

It can become an inconvenience as I can recall on my Buick I used to run 17# in September daytime heat and at night the air temps would drop down over 30*. So for night time driving I'd raise boost up another three pounds. Every day I would have to road test and reset.

The problem is that sometimes I would forget to readjust the knob on the MBC in the morning. So, I would be boosting too high in the daytime summer heat. So, MBC's are not idiot proof.