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Old 09-10-2008, 09:11 AM
GMR GMR is offline
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vacuum leaks and vacuum gauge

Will an internal or external vacuum leak effect vacuum gauge readings?

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Old 09-10-2008, 09:35 AM
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It can go either way, depending on a few things.

If the leak is big enough, it may show low, providing you know what it used to read.

If a cylinder has a big leak and is misfiring, it will still be pulling vacuum and may not show. However, if that cylinder is missing bad, the idle speed will fluctuate and will give you a fluttering gauge reading.

One good way to ck for external leaks, is to let it idle WITH an RPM gauge hooked up, then spray around everywhere with carb spray, watching for an increase in idle speed, OR listening for the idle to smooth out some.

Internal vacuum leaks are hard to find, but are normally confined to where the intake meets the head. If this happens, you will usually find one or more oily plugs.

An infrared temp gun can be of great help too by reading each exhaust tube and compare temps. One that is low is most likely suffering from some sort of problem.

Mark
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Old 09-10-2008, 06:40 PM
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thanks Jmark
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Old 09-10-2008, 07:22 PM
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if by "internal" you mean rings and valves etc....definitely

here's a link to animated vac guages to show you what to basically look for:

http://www.users.bigpond.com/ergoff/vac1.htm

only caution is those Hg values on that site apply to a pretty much bone stock motor only....

a much better test for rings and valves seals is test drive with a vac guage in the car, read the Hg on a 50mph downshift 3rd to second so there is a actual load on the rotating assembly....

because a vac guage can and will tell you so much while driving it is a smart move to mount one in the car permanently....
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Old 09-11-2008, 08:18 AM
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Thanks red65mustang, the vacuum gauge site is the best I've seen.

I don't know what to look for if I do the test you suggested. I assume the manifold vacuum will be low because the engine is under load - yes. Can you elaborate on that please. My motor is a .040 over sbc 350 with, 222* @ .050 110* LSA cam, RHS vortec style heads. Vacuum at idle (850 RPM) is 15 -16 HG.

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Old 09-11-2008, 11:26 AM
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the key thing to look for on the test drive is that the needle is close to rock steady, at a stop light, recovering smoothly to higher Hg while accellerating, and doesn't change from what it tested at at idle or at hwy cruise (likely/hopefully 18Hg+)

on the downshift, no gas pedal, engine braking test, your looking for the highest Hg possible (impossible but a perfect sealing motor would pull a 32Hg, I have seen some truck "max TQ possible" cammed motors pull 29", WOW!!!...)
a plain vanilla stock motor, 190/200 cam in good shape will pull 25HG......
how much piston stroke travel is left when the valves actually do close (determined by the cam duration and installed advanced or retarded) to begin to pull a vacuum changes what Hg value you should see to know it is sealing excellent....
just my guess but with your cam I would look for atleast 23Hg, if it did read 25Hg+ I'd say Holy Sh## those rings are sealing great!!!!

Jmark made a key crucial point....
you really do need the "when new" reference numbers to evaluate from....
I write down the numbers after about 500 first miles just to be sure the cylinder hone and rings are broken in and there is a bit less assembly rotational friction...
many miles later,needle shows 2Hg+ less on a downshift, time to do a compression/leakdown test!!!

you can do a quicky (no load) test, vac guage hooked to manifold vac same as for tune up, just blip the throttle really quick to around 3000 rpms and observe how high the Hg reads as the motor slows down
(I think that's shown on that site)

a few more, of many, vac guage actual driving tests:

what is your actual carb WOT Hg, most carbs need atleast/approx 1Hg....if yours shows "0" the carb isn't making the best A/F mix possible, shows 5Hg(?) it's past time for a new filter or a bigger element!!.....
want max hwy mpg, tune the timing TT and carb for max Hg possible (1Hg more at 70mph=about 2-3mpg).....
want better intown mpg,,,do not let the needle fall below 12Hg once you are moving from a stop sign.....
does the motor/car combo want a quicker or slower timing curve,,,,observe how quickly the needle recovers given the same amount of pedal after a cent springs swap.....
what's the actual Hg when the tranny modulator shifts gears....

footnote:
you can have days and days worth of "productive putzin" fun with just a vac guage....

my latest "brain storm test", I swapped in a .8ohm ballast resistor (I'm using a pertronix module) in place of the 1.5,,,,,extended the reg sensing wire to the resistor input terminal (gained .4V alt output)
60mph cruise Hg jumped from 18 to 21!!!! Hg needle recovery in town is much quicker with less pedal!!!
(WOT feels stronger by a bit, been to busy to test at the strip)
YUP!, vac guage results proved the motor wants more volts and amps ign supply....
(will the pertronix survive, time will tell, I am at the absolute max (8) amps allowed with a .6ohm coil)

Last edited by red65mustang; 09-11-2008 at 11:33 AM.
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Old 09-11-2008, 08:14 PM
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That's a really helpful post red65mustang. Thanks for taking the time. Yes, I agree, Jmark made a good point about establishing a reference point. I just finished building this motor and have it on a test stand and I am using it as a learning tool.

That's a heck of a "brain storm test" - would never have guessed, very interesting.

Anyway, learned a lot and your response will help me in further researching and trying different things.
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Old 09-12-2008, 07:14 AM
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your welcome....

but.....
LOL!!!!!!!!!!!

my test doesn't even hold a candle for "inventiveness" compared to what Jmark is doing!!!!!!

hey Jmark,
did ya ever get a "with poop" versus "no poop" Hg cruise reading????.....
did Taco Bell burrito's poop show a higher cruise Hg?????

TGIF

Last edited by red65mustang; 09-12-2008 at 12:17 PM.
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