my primitive vac guage

[bgd73]

I glanced over at my pile of junk and spotted the hand controlled vacuum pump, I kept it to bleed brakes.

on the top was a guage... I did not need it to pump brakes. Moving guage by hand something was sticking and I took it apart to clean. here is what it looks like (simply wanted to share). I calibrated it back to zero, and will use it soon enough. I have never checked my old sube, but do remember a comment on the carb versions, having one of the highest readings ever..will find out soon enough.


that copper flat curve seems to be an airtight channel, it responds to vacuum pressure...


and this is where it shows a reading.

cheap.. and now have two useful tools from one.

maybe get an LED light in there somehow, run a hardline into the car. so I can monitor...

[bgd73]

here is the readings for a 1987 carbed subaru



250 rpm (not 2500.. this is really 250, and even less)



this is maximum, 22 in Hg of vacuum. I first saw this in the 80s.. waddya know, it is still there. Super Stoich... gotta love the real thing

and on another note, one to learn safe max psi for boost for an old sube, can learn it from converting inches of mercury to psi (psi because I am american)

22*.4912=10.8064 psi
In the little sube world I am in, this was a safe rule of thumb I never got to. (never ran a turbo)
and 10.8 psi is .744 bar
interesting.
of course today and boost and max is like a crack addiction to many, but above is the rule I would follow...I never learned if that worked, but it sounded smart enough. Simply to stay safe within oem mechanical setups.

[elhigh]

That's about where my Toyota 22r idles - inches Hg, not revs. 250rpm? Srsly?

Dang.

[mwebb]

at hot idle
any flutter in the vacuum gauge needle is bad

it may be related to the engine speed of 250 rpm ?

retest ;
at around 700 rpm , if there is still flutter you have a valve sealing problem

does your car have EGR ? if yes , have a look with your eyeballs and make sure it opens around 2k rpm , if it does , your intake vacuum should be
lower than 22"
if EGR does open , your passages are clogged .
disregard this if you do not have EGR

and quick quick
translation of inches of mercury to PSI is 2 to 1
30"hg = 15psi , more or less so
15"hg = 7.5 psi more or less

[bgd73]

Originally Posted by elhigh

That's about where my Toyota 22r idles - inches Hg, not revs. 250rpm? Srsly?

Dang.

I can't keep it there because of a common alternator, but in the summer, during the day, it is quite alright (the lights dim below 500 rpm)
it is a HUGE fuel save on the old carb.

I only got a quick read on specs typical throughout downdraft vac readings, (all cars) 22 inches HG is a maximum typical for gassers. I do not know what happens beyond that.

Getting too much could be as bad as not enough for the range of stoichial decency. 22 is the high end.

[mwebb]

expect
18 to 21 " at hot idle
18 to 22" at 2k rpm
18" with EGR open
22" with no EGR or EGR not open

0" at WOT 25" at decel following WOT snap .

but
the single most important intake vacuum measured value is
steadiness / smoothness , if there is flutter in the intake vacuum
you have a leaking valve and that is never good

sometimes
at low RPM valve overlap will cause flutter
so
what is your intake vacuum value at 750 rpm , is there flutter or is it smooth
the answer
will let you know if your engine has problems , or not .