MBC tuning question

[jb talons]

I just installed my mbc - standard Turbo XS. I installed it with the hose coming off the compressor housing going into the mbc and the outlet going to the waste gate.

I did a couple of wot pulls through 2nd and 3rd gears and got the mbc adjusted so that it peaks at 15 psi.

My question is:
Is it normal for the boost to fall off at higher rpm's or should it stay at the max setting? What happens is, I get the 15 psi at about 3k, then it starts to fall to about 10 to 12 psi at 5 to 5.5K.

I was under the impression that it should go to the max setting and hold there.

Anybody got any input that can help me out.

Thanks,

JB

 

[Mike 99GSX]

If you're on the stock T25, then the answer would be yes that is normal.

 

[jb talons]

I was under the impression that the mbc would allow the boost to hold steady at a certain level, and that the car's stock boost control is what made it drop at higher rpm's. All that it seems to be doing is keeping it from going above a certain level.

So you're saying that the T-25 can't maintain 15 psi through higher rpm's. Is this the case? Can it be affected by better ic piping or upgrading the exhaust?

thanks for any input.

 

[soul]

you'll just have to get rid of the t25....it's too small to hold boost at high rpm's

 

[psychlow]

Originally posted by jb talons
So you're saying that the T-25 can't maintain 15 psi through higher rpm's. Is this the case?

That is exactly the case. The T25 is too small to push enough air to keep pressure steady at higher RPMs. At high RPMs, the engine's demand for air increases faster than the turbo can supply it, so boost pressure descreases.

 

[nine5raptor]

Originally posted by jb talons
So you're saying that the T-25 can't maintain 15 psi through higher rpm's. Is this the case? Can it be affected by better ic piping or upgrading the exhaust?

A pump simply adds energy and for every pump (which a turbo is) there is a mass flow vs. pressure curve. What you are trying to do is go beyond the curve, so something has to give.

Yes, better piping in/out can help some, but you won't gain a whole lot because head loss should be fairly low for our systems.

 

[NOSLO2PT0]

bigger piping in any part of the system will make the boost reading even lower. All boost is is a measure of backpressure through the system. Think of blowing air through a straw. You can feel the output at the end of the straw pretty good. Now blow through the center of a roll of paper towels (the cardboard center). Same airflow, just a LOT less pressure. Taking a reading inside that tube will yield a lower reading, similar to what the boost gauge is measuring.

 

[nine5raptor]

Originally posted by NOSLO2PT0
bigger piping in any part of the system will make the boost reading even lower. All boost is is a measure of backpressure through the system. Think of blowing air through a straw. You can feel the output at the end of the straw pretty good. Now blow through the center of a roll of paper towels (the cardboard center). Same airflow, just a LOT less pressure. Taking a reading inside that tube will yield a lower reading, similar to what the boost gauge is measuring.

Not quite.

Boost pressure is created by a limit in flow, but it is the head and valves that are the limiters, not the piping. You can use a straw or a paper towel tube, but if you gotta blow both of them through a pinhole, you will have the same pressure in both to get the same flow through the pinhole.

Along these same lines, you will get greater efficiency out of the system by increasing the piping size (I can figure the theoretical optimal tubing diameter based on a set airflow if you want -- it is pretty basic fluid mechanics) by minimizing losses in the system.

 

[jb talons]

Originally posted by nine5raptor
Not quite.

Boost pressure is created by a limit in flow, but it is the head and valves that are the limiters, not the piping. You can use a straw or a paper towel tube, but if you gotta blow both of them through a pinhole, you will have the same pressure in both to get the same flow through the pinhole.

Along these same lines, you will get greater efficiency out of the system by increasing the piping size (I can figure the theoretical optimal tubing diameter based on a set airflow if you want -- it is pretty basic fluid mechanics) by minimizing losses in the system.

Thanks for the input. Your speaking my language now. No need to do the calcs. Your explaination was right along the lines that I was thinking.

Thanks,
JB