I don't want to sound like and ******* but your question is so subjective that no one really could goive you an accurate answer.

Per instance, is your question intended for the 5 speed EVOs?, as far is boost and RPM there are so many variations, per instance if I am slighty going downhill my car is going to show a lot of vacuuum, if I go uphill slightly my car is going to show a tiny amount of boost.

It depends of what gear you are on, it depends if you have a full exhaust, air filter, boost controller, BOV and the list goes on and on and last but not least, anyone who runs a different turbo will give you different PSI/RPM values.

Could you elaborate your question again and see what the people have to say to that?,,

Thanks!!

 

there has to be SOME sort of vacuum to make the gauge move and keep the motor running though right?

 

Because like any other automotive term it just used repeatedly and accepted.
Kind of like an alternator .

No if the gauge were absolute it would never see zero and always be in positive pressure. Put it this way, the reading in inches of Mercury is a absence of pressure below current atmospheric pressure or barometric pressure. If you truely had vacuum the intake manifold would collapse, including everything else.

When you stick your finger on a straw and suck you are lowering the pressure in the straw below atmospheric pressure. You are not creating a vacuum.

 

I want to know how much boost the car is running at different speeds while maintaining a constant speed. I want to know that since it might make a difference in gas mileage.

 

I just edited the initial question. Hope it helps.

 

Inches of Mercury is a measure of pressure. Buy a guage from any source other then automotive industry and it's considered a pressure guage.

I may have mispoken above, I'd have to re-read it. Guages are corrected to read 0 as 14.7 psi or 29.92 in. Hg. Therefore it is an absence of pressure because you have not reached below 0 psi or 0 in. Hg. You are only reducing pressure below atmospheric conditions.

Here is a good explanation of barometers
A barometer measures the pressure of the air pushing on it. On the average, at sea level, the air has a pressure of 14.7 pounds per square inch. This means a square one inch on each side has 14.7 pounds of air pressure on it. We don't feel it because the pressure is pushing with equal force in all directions. Instead of using pounds per square inch, barometers in the U.S. measure the pressure in inches of mercury. This is how high the pressure would push mercury into a tube that has the top sealed off from the air. A reading of 29.92 inches of mercury is the same as 14.7 pounds per square inch.

 

depends on how much you step on the pedal to keep it at xx mph. You could coast down to 60 mph and full throttle it back up to 75 mph or lightly lightly step on it to get there.

 

I think the point everyone is trying to make is that there is no boost at a constant speed because load is low to maintain a constant speed. Only acceleration creates enough load to cause boost. At any constant speed within the means of the engine's normal operation you will not have boost.

 

that makes more sense

thanks for clarifying!

 

tim you might be getting beyond the realm of discussion for this thread!

All he asked was what the gauge reads. It doesn't require an explanation of physics, though apparently some have asked for it.

 

well then, the gauge would read 22 in. Hg...not -5psi

 

I guess I haven't seen enough gauges to know what the labels look like. Mine sits at 0 until I start the car (electronic gauge) and drops into negative numbers indicating a pressure level lower than that of 14.7psi (0).

Or am I missing something here?

 

You are correct in the way the guage operates.
They just tend to express it in in. Hg. and not negative psi which it could be expressed in.
But that is too confusing.

Actually I believe they only do it because in of Hg. is much more accurate the trying to read it in psi.

 

Come to think of it my gauge doesn't even have the damn negative symbol next to the numbers.

But it's bar, bar for sure!! Even harder to convert on the fly for us standard-learnt folk.

 

^^ what kind of gauge do you have?? i dont think ive ever seen an automotive boost gauge that doesnt read in "Hg

nvm just saw your sig