On my one car it holds at about 18 -psi at 900rpm idle on my friends car it's about 12-13ish at the same rpms at idle, this is after we "fixed" the vac leak. The intake gasket on pas. side was toast but it seems that we still have another leak?

around 20 is good for a subarur. my xt6 only sucks down to 22 lbs

-20'ish, oh man we got a bit to go then, it's not running all that hot either, if you downshift coming into a corner hot then it's got lots of snot, but if you come into the same corner but on the brakes with no hard downshift it drives like crap with sluggish rythmic acceleration. I'm going to check the disty but I think we have vac issues somewhere else.

well, i dont know if you can go strictly off of vac. timing will effect vac. and compression, and mileage can effect it, alot

My RX sits at about -20psi at 1,000rpm idle.

 

-Brian

17-20 is considered normal at idle for almost all cars.

if your friends car is at 12-13 psi then yes... there's probably a leak somewhere.

 

 

-Brian

You guys all have supernatural vacuum.Atmospheric pressure is only 14.7 psi.

LOL thats what I was just thinking. When I stick my hand over the TB of a car idling there certainly isnt a 20psi difference in pressure sucking my hand in.

LOL thats what I was just thinking. When I stick my hand over the TB of a car idling there certainly isnt a 20psi difference in pressure sucking my hand in.

Most people don't understand that a boost gauge shows boost(positive manifold pressure) in terms of PSI and measures vacuum in inches of mercury.

if 1 bar is 14.whatever. and your car is creating vaccume, it should be more than atmospheric pressure. space is around 30lbs of vac.

if 1 bar is 14.whatever. and your car is creating vaccume, it should be more than atmospheric pressure. space is around 30lbs of vac.

Atmospheric pressure is about 15psi. When your engine is not running, there is 15psi in the intake and 15psi outside the intake. When the motor starts running, the pressure will be a bit less than 15psi in the intake. The difference in pressure between the outside and the inside causes air to be sucked in. The difference in pressure cannot be more than 15psi, because 15psi -15psi = 0 which is total vacuum (like space) and there is no such thing as negative psi in this kind of application. The psi difference on an engine is never more than a few psi. Like Calebz said, you guys are reading 15 or 20 or whatever inches of mercury.

so the boost is measured in lbs, and the vac is measured differently?

 

if you have a gauge thats set for bar, is it the same way?

so the boost is measured in lbs, and the vac is measured differently?

 

if you have a gauge thats set for bar, is it the same way?

 

Yes,like calebz said,normally boost in psi and vacuum in inches of mercury.No reason the same units couldn`t be used for both though, like gauges calibrated in BAR do.

There is an almost endless number of different units for pressure measurement.Here is a very handy covertor:http://www.fortunecity.com/meltingpot/portland/508/tools/convertor/pressuretxt.htm

 

Furthur confusing matters is the difference between gauge pressure and absolute pressure.Gauge pressure is the difference between the measured pressure and atmospheric pressure.

eg. When your motor is off,boost gauge reads 0,even though it sees atmospheric pressure of 14.7 psi.At 10 psi indicated boost the absolute pressure is 24.7 psi.

A vacuum gauge reading of 20 inches of mercury is equilivent to an absolute pressure of 4.88 psi or a gauge pressure of -9.82 psi.