what should vacuum be?

[BerniniCaCO3]

Found a vacuum gauge in my tool box, what do you know. Not sure where/when/how I got it

Anyway, figure I'll hook it up and knowing the vacuum could help me diagnose my rough running rebuild.

What SHOULD vacuum be?
If I misplaced a vacuum line or left one open, I presume vacuum would be weaker-- what sort of range would suggest a vacuum leak then?
If I had a smoke machine, that would be best, but, I don't


Finally, more just curious. I looked, and indeed, sunpro sells a vacuum gauge to put in your car's dash. Probably not something I need regular input on though?
Anyway, so it looks like they're really sold for people running boost, turbo/superchargers.
Which got me thinking... when the engine sucks in air it creates a vacuum, which drives all sorts of things. But if a supercharger rams in air, it creates HIGHER pressure.
Does this mean that if I ever supercharged my station wagon, with boost, I'd play holy hell with everything that depended on vacuum to run? Can boost be high enough to even override vacuum (keep in mind I have NO idea what numbers are normal for boost in psi, and what numbers are normal for vacuum. If normal vacuum is -40psi and boost not more than 10psi, then you end up I presume with -30psi going to all your vacuum lines...?) (maybe not that straightforward).

[1990LTD]

17-20 hg

[1987cp]

It's occurred to me to wonder that about vacuum on a blown car too. Most cars seem to have idle vacuum between 10 and 20 inches of mercury (why in. Hg? Who knows?), and the blown car I saw a gauge in was a T-type making about 15psi boost. It was pretty goofy watching the gauge read first vacuum and then boost.

I would ASSUME that check valves in the vacuum lines, along with a good vacuum reservoir, would be sufficient for keeping normal vacuum accessories working when you're under boost.

As far as things like the MAP sensor, I have a couple of thoughts: one, that as a MAP sensor it'd have a rather different job to do once the engine management is reworked to deal with boots, and two, that on a MAF car the mass airflow meter would just sit and happily measure the volume of whatever air is sucked in. Probably a bit more complicated than that, though. Turbobuick.com might be one place to go reading about that sort of thing.

[gadget73]

17 inches or better. More is better.


The reason they use inches of mercury is because the first pressure gauges were literally a tube filled with mercury. The amount of vacuum was measured in how many inches up the tube the mercury column was. A more sensitive measure is inches of water, or inwc. Same principal, different fluid.

[johnunit]

Under 17ish indicates some sort of issues. Instability(at idle) is also indicating an issue.

[BerniniCaCO3]

Thanks-- I'll see what it is tomorrow or Sunday. There's a definite possibility I missed something.
Is the a way to make my own homemade smoke machine to see where it's leaking from, if the vacuum is suspiciously low?

Yeah, gadget beat me to it-- Hg is just historical. Same with the Fahrenheit scale for temperature; no guy sat down and decided to make 32 the freezing point and 212 the boiling point for water. Wikipedia has a neat little article on it; uses ammonium chloride salts which results in a lower freezing point (0F<0C) than just ice water, and then a horse's body temperature for 100F to set the spacing of the scale. Go figure.
I still think Celsius makes more sense

[BerniniCaCO3]

still curious about boost and vacuum-using accessories and gauges.
A turbocharged street car --just using light boost for fuel economy as much as for power-- will run with the turbocharger active for much of its cruising time I should think.
But it might be that the boost is too light to overcome vacuum?

And those gauges that go from vacuum to boost, are for racing cars whose turbos are much bigger than some VW that came with one stock?

[johnunit]

From my experience most turbo'd cars don't make boost below half throttle.

[1987cp]

And those gauges that go from vacuum to boost, are for racing cars whose turbos are much bigger than some VW that came with one stock?

No idea what you're getting at there. The gauge I saw was an AutoMeter installed on an '87 Regal T type. Fun car.


Turbocharged cars make boost whenever the exhaust velocity is sufficient to spool up the particular turbosupercharger that's been installed. Logically, the circumstances under which this happens (throttle, RPM, etc.) should vary based on a handful of variables. If you're looking at designing your own turbo system at some point down the line (after you have an engine that makes good power NA, I would presume), you'll likely be wanting to take a great many things into account.

[BerniniCaCO3]

for this car, no, I don't think I will... I'd need dish top pistons wouldn't I? Would I need to rebuild the guts of the engine, again?

But anyway, take a gauge like this:

http://www.amazon.com/Sunpro-CP8203-StyleLine-Mechanical-Vacuum/dp/B000CPAVUM/ref=sr_1_4?ie=UTF8&s=automotive&qid=1305397795&sr=1-4

It seems to suggest that running enough boost, you can overcome/override vacuum and actually go to positive pressure. Which means things running on vacuum now don't. Hech, a high pressure might actually fight your break booster (unless one-way check valves prevent that).

That's what I was curious about.
But that might only be an issue for very large amounts of boost. And I know the turbocharger(s?) on my volvo were not very large ones; boost probably never could overcome vacuum, and that gauge there would never swing into the positive/pressure range.

[BerniniCaCO3]

Now since vacuum is also just dependent on the engine's strength and rpm and all that, a low vacuum could also be a weak engine, not a vacuum leak.
If a single 1/4" hose is off, or dry-rotting apart, will it show up clearly in idle vacuum?

[86VickyLX]

Out of a new engine with a not ridiculous lopey cam, I'd expect at least 20. If not you have vacuum leaks or timing issues.