Engine vacuum gauge test

[LKV]

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

 

[carnage]

looks like a good engine to me.
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

 

[GregLocock]

I disagree about the utility of a vac gauge on your dash - but it does look cool.

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

 

[evelrod]

Just a short note---Stopped in at the auto parts joint yesterday and saw a vacuum guage kit REALLY CHEAP, less than $5 !!! It came with a little pamphlet that gave a COMPREHENSIVE tutor on how to use it and what the readings mean. Go get one, Tiger!!!


Rod

 

[evelrod]

I just re-read my post of a couple days ago and realized a small error. The cost of the KD Vacuum guage was $15.95 (on sale). Sorry, I was not thinking very clearly that morning. LOL


Rod

 

[carnage]

I find my vaccume gague to be very valueable, but only because my motor needs tuneing so often. when I notice that my average reading has dropped a few inches of mercury I know it is time to check and make sure a plug wire hasen't shook loose from the last trip in the mountains, or maybe it is time to regap my points. my vaccume gague is actually how I discovered my vaccume advance had started leaking. I can't wait to save up enough money to get a new motor built for my truck. :}

 

[franzh]

Its ironic that most people use a vacuum gauge to determine how "efficient" the engine is operating. The instructions state that the vacuum must remain as high (sorry Greg, but I agree with you!) as possible for maximum fuel economy!

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