low boost

[irish91983]

Hi all
So I noticed recently that I'm not getting full boost anymore. The gauge use to be pinned at full throttle and now it will not go past 7 psi. What could cause this

Thanks, Keith

[J Wikoff]

7 psi sounds like the normal level.

Have you made any changes recently?

[irish91983]

I did a new set of belts. But how come before it would be pegged at 10?

[01bonneSC]

Yeah 7ish is stock. Mine never ever pegged.

[irish91983]

That's so strange. My was always up around 10. Now it's only at 7 with all my mods

[J Wikoff]

Higher-than-stock boost can be caused by exhaust restrictions, like a plugged cat or smushed pipe. A drop in indicated boost can be caused by mods that free up the airflow, like headers, higher ratio rockers, or hi-flow/no cat.

[Wes]

What John is describing is called boost stacking.

Here is a good description of it from http://www.gpona.com/forums/boost-stack ... t8975.html

One way to look at it is, you motor is a big pump. It can only put out as much as it takes in. Therego, the more efficient that the air can get into the engine, the better. Only if you can have the same efficiency coming out of the engine. If you put a CAI, ported throttle body and blower on your car with a 3.5" pulley and you have little to no exhaust mods, that is bad. Sure, you let a whole bunch of air in, but it has no where to go. That's where headers come in. They are much less restrictive than stock exhaust manifolds. Ultimately your goal is to match the output to the input.

Boost stacking is when you have too much air trying to get into the engine and it has no where go. This actually causes the blower to heat up which also heats up the air coming in. This is bad. You want the coldest air possible coming in, this makes for better combustion. Now the Gen V blowers are a little bit more free flowing than the Gen 3's, I believe that is why you guys don't have to drop pulley sizes as low as the Gen 3 guys to get the same results. Also, the 97-03 guys have to worry about the bend in the stock DP which causes a lot of exhaust restriction. The 04+ guys don't have to worry about that fortunately.

[irish91983]

So pretty much I'm gonna need to do rockers? Cause I already have fwi and headers with cat.delete.

[Wes]

Not necessarily. The 7 psi you see is actually better than the 10 you previously saw if boost stacking is the issue. It means that your engine is moving more air, and not stacking excess pressure, and heat.

I don't understand why the belt change brought it down though. I would think that if you had a belt that was slipping, and you replaced it with a new one, your pressure readings would be reversed from what you stated.

And 7 psi is what a stock m90 is supposed to output. If your sig has all of your current mods, then 7 psi is where you should be if everything is working as it should.

Do the 01s still use a map sensor for the boost gauge reading? Is that maybe gunked up, or on its way out?

[01bonneSC]

So all you did was change the belt and now it only shows 7psi?

[yourgrandma]

Temperature has a real effect on boost levels as well. I've seen as high as 21psi on a 3.4" pulley on a -35*F day. If its warmer now than last time you noticed, that could be the explanation.

[irish91983]

It has been very hot and humid as hell here. I'm not sure when it actually started to read the 7 psi but I didn't notice it until week or 2 after my new belts. It could have been doing it for a while I don't really watch the boost gauge all the time.

[01bonneSC]

It has been very hot and humid as hell here. I'm not sure when it actually started to read the 7 psi but I didn't notice it until week or 2 after my new belts. It could have been doing it for a while I don't really watch the boost gauge all the time.

Ok. I wouldnt really worry about it then. Does it seem any slower?