You need to check what you just wrote. A boost guage reads in bar or psi.
I think you are talking "vacuum" guage. A boost guage is just that, I assume you mean a combination guage. I only have a boost guage but my EVC V reads both.

 

it looks like Defi's electronic gauges read -100kPa to +200kPa

but their mechanical gauges read inHg

any relevance to that?

 

bar can be converted to in Hg. Or vice versa it is just not commenly done.
1 bar = 29.54 in. Hg

 

i actually had typed boost/vacuum, but i edited it since its not really vacuum...thats why i wrote "automotive" boost gauge

haha im all confused now

 

i wasnt aware of that

i was always taught in electro-mechanical class that there is no such thing as negative pressure, only vacuum

 

It seems like all mechanical guages are marked in in Hg.
I can only guess that it is the easiest way to calibrate the guage since it is mechanical so it will be acted on by atmospheric pressure.

 

Yeah it's rediculous.
That's why I used the term absence of pressure and not negative pressure.
Negative pressure is a vacuum. An absence of pressure is just less pressure then atmosphere but it's not negative because it's still pressure.

 

Damn you guys for being so technical.

Instead of thinking of "negative" pressure, just think "switch over from vacuum to positive pressure" or something.

Or just create your own scale in your head, where 14.7 = 0 and anything > or < than that is (+) or (-) respectively.

 

i think what he wants know is, what psi the boost guage is indicating if you are doing 75-80 mph(3500-4000k rpm). no uphill and no down hill. just maintaining a cruising speed on a flat freeway.
since full boost kicks in @ 3000k rpm (guessing). what is the boost reading would be at that speed?
a lot of people are saying stay off boost to save gas but you have to drive below 3000k rpm to do that right? pls explain.

 

boost requires load...if you are just trying to maintain speed at 3500rpm, you probably wont see any boost at all...maybe 1-2psi

 

From my short experience with a boost guage, it seems like boost is more dependent upon how much throttle is being administered than anything else. You'll have to specify how much throttle, otherwise it isn't an accurate comparison. Regarding boost, most people are concerned with how fast u get to full boost and if it will hold full boost or it tapers.

 

Question..When i'm idling my boost guage is showing 16 - 19in hg. Is that normal?

 

Here is how you can help. Drive on a flat part of the freeway and maintain a constant 70 mph, then 75 mph, then 80 mph and then tell me what is the reading on your boost gauge. It is as if you have cruise control and you are maintaining that speed on a FLAT section of the freeway. I do not have a measure of how much throttle application is needed, but if I had to guess I would say you should have the bare minimum throttle application to maintain that constant speed.

 

Whoa, this thread had a whole bunch of confusing information.

a numba 1) A boost guage is simply a pressure gauge. The pressure it measures is INTAKE PRESSURE RELATIVE TO THE ATMOSPHERE. So, if it's reading zero (as it always should when the engine is not running) then the intake pressure is the same as the ambient air. When boosting intake pressure goes up and the needle reads postive. When under low load (like idling) the intake pressure is actually lower than the atmosphere. People are apt to call this 'vacuum' although it's rarely a true vacuum. So if your see 15 in.hg below zero, then your intake pressure is 15 in.hg less than atmosphere. If you see 14.7 psi below zero, then your intake manifold actually IS a vacuum (no air), but this will never happen. Remember that all of these readings are dependent on the local air conditions. My perfectly normal evo, at idle, only sees about 8-10 in.hg of vacuum at 11000' elevation. At see level, I see closer to 16.

a numba 2) Boost pressure is regulated by, among other things, load on the stock evo. You'll see no boost under cruising conditions.

a numba 3) At cruising speeds, the stock ECU is already keeping the combustion at stoichiometric and you're not boosting. The factors that are going to affect your mileage there are things like coefficient of drag, rolling resistance, etc).

a numba 4) When people say "stay out of boost", what they really mean is "don't accelerate swiftly". When you mash on the throttle all the time, you keep the engine out of it's efficiency range, the car has to run richer, and more energy is wasted from building excess speed.

d

 

I will say it for like the 10th time.
Cruising on a flat surface at say 75 mph constant you will have exactly 0 boost.