That last thread on boost gauges was getting too far gone from the topic....

I notice you are all using Boost gauges.. that is, the photos that WERE posted, showed "Boost" with a Boost/Vacuum (+/-) sot of thing.... would not a Blower always be "boost", so there would be no vacuum? at worst it should be 0 (zero) right.. wrong..? testus anum?

 

There can be no boost if the throttle butterfly is in its zero position, as the supercharger cannot compress air that is not there.

15 in-Hg is typical for idle, supercharger or not

 

>>That last thread on boost gauges was getting too far gone from the topic....
>>
>>I notice you are all using Boost gauges.. that is, the photos that WERE posted, showed "Boost" with a Boost/Vacuum (+/-) sot of thing.... would not a Blower always be "boost", so there would be no vacuum? at worst it should be 0 (zero) right.. wrong..? testus anum?

The supercharger has a sort of bypass valve that allows air under normal pressure to be recirculated back into the intake stream without compression. This results in a 'normal' state of vacuum. My car idles at around -8 psi. Normal cruising is around -10 psi. You will only get into boost under acceleration, and not too much unless you are hitting the throttle fairly hard. My car only has an intake added so far, and the max boost I've seen so far is 12.1psi @ about 200' above sea level.

I don't think I have seen any gauges specifically labeled for supercharger, or blower. However, some are labeled turbo. These will work with a supercharger, as well. As long as you T into the right vacuum line, or directly tap the manifold.

-joe

 

FYI:
Area A: the part of the intake system between the throttle body and the blower intake will see pressures from ambient (at WOT) to 15"vacuum (idle and closed throttle).

Area B: The output side of the supercharger (actually the output snoot connected betwen the intercooler and the intake manifold) has a vacuum operated bypass valve. When this valve controller sees vacuum through a tube connected to A, it will open the butterfly and shunt the blower output back to A, thus producing near zero boost. A spring must be overcome by the force produced by the vacuum; otherwise (like at larger throttle openings), the bypass is closed by the spring (at any near ambient pressure in Area A) and there is no shunting.

 

Yep, even without a bypass valve, the manifold still sees vacuum most of the time due to the throttle body. As long as you are measuring downstream from the throttle body, it doesn't matter if you have a turbocharger, supercharger, naturally aspirated, etc. Vacuum is the order of business during most of the engine's operation.

The only situation in which you wouldn't see vacuum would be in a diesel (no throttle) or in a system where you are measuring boost upstream from the throttle body (like the boost sensor in the intercooler of VW 1.8T's).

 

Even without the bypass valve functioning, vacuum ranges from 10 to 20". When cruising or decelerating the blower does not compress enough air to create "boost". Labeling the gauges differently is pointless when they measure the same thing, air pressure within the manifold compared to ambient atmospheric pressure.

 

I made an interesting observation after installing my boost gage. You can see the Yo-Yo effect on the gage. The boost pressure will cycle between 0 and 3-4 psi while the yo-yo is going on. This supports Ryan's theory on the bypass valve being the root cause.

 

Giddayup! I LOVE getting answers! Thanks folks.. I feel smarter already! Okay, I'm off to get a boost/Oil Temp gauge pod setup I saw elsewhere.