General Boost Questions

[gijoe985]

Ok, I've got a 96 GSX with catback 3", BOV, And CAI and an aftermarket boost gauge.

I've been coming up with some general questions about boost. I have only driven it for a few days and I am naive to what my boost should be doing.

I've gathered that you get vacuum under decel. And should get around 0 at coast?

I've found that my readings seem to vary. I'm in the mid to upper 20's inHG when I'm decelerating. If I'm barely on the throttle I'm still at 0-10 inHG. If I'm in third and go WOT my boost gauge builds up to about 15psi and then drops quickly. It never really holds a good boost. Also, I don't seem to be getting good boost until 3500+. I'm sure I probably have leaks.

Could someone walk me through what boost reading I should be getting in different scenarios. That'd be a huge help. I've searched a bunch, but so far it has only helped fill in a few blanks.

 

[jrohner]

Mine:
Idle -18
cruise: -8 to -15, depending on speed.
slight incline: -5 to 0

A turbo 16G or smaller, should get full boost by 3000 or sooner in the higher gears. If the boost falls really fast, maybe it's just a boost spike and your boost is set much lower. How much boost does it have at higher RPMS?

If yours doesnt read much vacuum idling, you have a leak after the throttle -- intake gasket, injector seals, EGR valve/gasket, etc. Leaks before the throttle don't change your vacuum reading. When my car had HUGE boost leaks, it didn't even seem to effect the spool up much.

If the boost is building really slow, you may have an exhaust leak before the turbo -- head to manifold gasket or manifold to turbo gasket. You will be able to hear an exhaust leak, it will be kind of a ticking sound by the exhaust, you may be able to feel it with your hand also.

Take off your MAS & intake tube and find something to clamp over your turbo compressor inlet to plug it. Then put air in with an air compressor, but keep the pressure low (about the same pressure as you run for boost).

 

[force_feed]

in general:

When you car is ideling you should be reading vaccume only (-15 +/-) This will also happen during decle, etc.
When you start to accel as your turbo starts to spool the guage will climb from a negative number to 0.
As you know once your turbo spools that's when the guage reads a postive number

 

[FORMONTOYA]

Mine:
Idle -18
cruise: -8 to -15, depending on speed.
slight incline: -5 to 0

A turbo 16G or smaller, should get full boost by 3000 or sooner in the higher gears. If the boost falls really fast, maybe it's just a boost spike and your boost is set much lower. How much boost does it have at higher RPMS?

If yours doesn't read much vacuum idling, you have a leak after the throttle -- intake gasket, injector seals, EGR valve/gasket, etc. Leaks before the throttle don't change your vacuum reading. When my car had HUGE boost leaks, it didn't even seem to effect the spool up much.

If the boost is building really slow, you may have an exhaust leak before the turbo -- head to manifold gasket or manifold to turbo gasket. You will be able to hear an exhaust leak, it will be kind of a ticking sound by the exhaust, you may be able to feel it with your hand also.

Take off your MAS & intake tube and find something to clamp over your turbo compressor inlet to plug it. Then put air in with an air compressor, but keep the pressure low (about the same pressure as you run for boost).

I will agree with most of this. The only exception is the location for a boost leak test.

The one thing you will quickly find is that a lot of turbos (even new ones) will leak around the turbine seals since most are a "pressure activated" type of seal, meaning that you have to have the car running and oil pressure for them to seal properly.

With that being said, I do my test from the IC inlet as a majority of the time any leaks will be after the IC anyway...mainly under the BOV and throttle shaft seals on a well maintained car.

my 0.02

 

[jrohner]

For the first ever boost leak test, it would probably be best to test at the throttlebody, it will make it easier to find those leaks. Next would be to check the BOV.

 

[FORMONTOYA]

in general:

When you car is ideling you should be reading vaccume only (-15 +/-) This will also happen during decle, etc.
When you start to accel as your turbo starts to spool the guage will climb from a negative number to 0.
As you know once your turbo spools that's when the guage reads a postive number

With stock cams I would be highly pissed if I didn't run at least 18inHg.

During normal cruise conditions (under about 90-100MPH) you will run vacuum of some magnitude (at a steady 70 I run about 10inHG).

If you run a T25 (stock turbo) boost will start to drop off around 5K RPMs rather quickly.

A stock 2G will peak at about 14PSI and drop to around 11-12PSI because the 2G BOV is made from plastic and just leaks.

How the boost reacts will depend on how far your foot is in the "go" pedal. If you go WOT it will hold higher boost longer than if you were at 50% throttle.

On decel (foot off the gas) you should see around 20-25inHg.

Of course all this is under the consideration that it is a well maintained car.

 

[gijoe985]

So, if I go WOT, I should see my boost go up and spike and then drop down and hold a number for a while.

I.e. 15psi spike and 10psi held...

How long should it hold that 10 psi?