Engine vacuum gauge test
Engine vacuum gauge test
(OP)
I've tested VW Jetta engine with vacuum gauge.
Here are results:
idle needle fluctuates fast between 15-16 in.Hg
at 2000rpm - 20.5in.Hg steady
at 3000rpm - 20.5in.Hg steady
released throttle from 3000rpm - needle jumps to 22in.Hg then returned to
15 in.Hg (idle).
Quick open/close throttle - needle jumps quickly to 0 then 22 and returns to 15.
It is my first practices with vacuum gauge. Would someone please explain how to evaluate
gauge readings.
Thank you,
Konstantin
Here are results:
idle needle fluctuates fast between 15-16 in.Hg
at 2000rpm - 20.5in.Hg steady
at 3000rpm - 20.5in.Hg steady
released throttle from 3000rpm - needle jumps to 22in.Hg then returned to
15 in.Hg (idle).
Quick open/close throttle - needle jumps quickly to 0 then 22 and returns to 15.
It is my first practices with vacuum gauge. Would someone please explain how to evaluate
gauge readings.
Thank you,
Konstantin





RE: Engine vacuum gauge test
sounds like your idleing at a low enough speed that you arn't producing much vaccume.
at a steady engine speed your vaccume should be about the same regardless of rpm's (unless an ecu steps in and changes something).
when you floor it the needle should drop to zero untill the engine starts pulling more air than the intake can supply at whitch point the vaccume will rise indicating the motor has to do more work to pump the air.
When you let off of the throttle at a speed above idle you get a high vaccume reading incating the motor has to work harder to pull in the necisary air.
hook up a vaccume gague in your dash it is cheeper than most stickers from performance companies and will impress all who see it :)
it is also a good way to know more about what is going on with your motor.
BTW if you let off the throttle going down a steep hill you probibly will get a reading of 26"HG or so
RE: Engine vacuum gauge test
1) Your idle reading is a little bit higher than I'd expect (ie excessive throttle opening) - was this with ancillaries on?
2) 2000 and 3000 are fine, although I might get as low as 22-24 inches in the same condition (on a different car).
3) behaviour on throttle release is what you'd expect - the engine is still turning, you have closed the throttle so it sucks all the air out of the manifold creating a bigger vacuum (hmm, a nasty phrase).
The main thing I would do with a vac gauge is use it to check that at WOT on the road that the MAP is as close to zero as possible. 1.5" of MAP is probably the most I'd like to see on an enthusiasts car.
Cheers
Greg Locock
RE: Engine vacuum gauge test
Rod
RE: Engine vacuum gauge test
Rod
RE: Engine vacuum gauge test
RE: Engine vacuum gauge test
This is actually the least efficient point in the engines operating range as the engine is working against itself. At low throttle opening-engine loads and RPM's, engine pumping losses can exceed the shaft BHP by a wide margin.
This is where the diesel engine really demonstrates its superiority over the Otto cycle (Oops, here comes the rebuttals!).
Franz