vacuum psi

[emilo2008]

i have mine right now at 12 psi. i want to know what everyone else has theirs out.im trying to tune it. but runs good at idle and when i shift it to drive the psi drops.i know theres suppose to be a air hose from the transmissin to the engine but where.i tried getting vacuum diagram but i could not find one.please help tuning my carb.

[Ranchero50]

vacuum and pressure are two different things. Pressure is pounds per square inch, vacuum is inches of water column or mercury. The level of vacuum depends on the engines ability to draw air vs the restriction of the throttle plates / air inlet.

Healthy engine = more vacuum

Large bumpy cam = less vacuum

More RPM = more vacuum.

Correct air/fuel ratio, better vacuum.

More timing advance (within limits) and point dwell = better vacuum.

Make sure each is to spec before wondering about the vacuum too much.

Usually healthy stock or mild mild cammed engine correctly tuned 13-15" of vacuum at 900-1000rpm, maybe more. The vacuum will drop when the rpm drops when you shift it in drive. Disconnect the vacuum diaphram from the back of the intake (line going to the tranny) and suck on it (with your mouth) . it should not flow air or tranny fluid. If it does the line could be cracked or the modulator is bad. If it shifts fine most likely nothing is wrong.

Jamie

[ArizonaDan]

Follow this link for good info on tuning with a vacuum gauge.

http://www.classicinlines.com/Vacuum.asp

[TNIceWolf]

Tuning off a single gauge isnt all that reliable in my opinion. What I do is use a trio. A vacum gauge..a Tach/Dwell meter..... and a timing light. By going back and forth through all the adjustments the gauges monitor you can dial an engine in pretty close to optimum performance and fuel mileage. This method takes a few rounds through all the possible adjustments.......but leaves little to chance if the engine is in a tuneable condition. Not every engine is. The slack in a worn timing chain can be seen with the three gauge setup easily if you know what to watch for and are aware of the difference in the above sea level height of the engine.

[rd100]

I just did this. I also used a vacuum gauge, a dwell/tach meter and a timing light all at once.

I'm getting 20" Hg on my 1970 302 with 82k miles.

When adjusting the carb, initially I was using the tach for the mixture, but then switched to using the vacuum gauge.

The gauge let me see the difference between 19" and 20" while the tach was still reading pretty much the same RPM over those adjustments.

[emilo2008]

hey thanks guys i got it start and running ok but it still pops which cause the butterfly to stay open.and also everytime i shift it in reverse or drive it drops pressure from 15psi to 10 psi? am i missing another bacum hose from the transmission and the engine.i heard there is a hose from the transmission the the engine.

[rd100]

Are you reading 15psi? Or do you mean 15 inches of mercury? 15" Hg.

I don't think you are getting 15psi.

1psi=2.04" Hg so that would be about 30" Hg.

Assuming you mean 15" Hg, my gauge says this:

14-17" Hg is Late Ignition Timing.

0-10" Hg is Late valve timing or leak at intake manifold or heat riser.

17-22" Hg is normal motor.

Are you sure you are using the right gauge/a known working gauge?

[emilo2008]

I MIGHT BE THE MERCURY.IM GOING TO TRY TO ADVANCE THE TIMING AND SEE IF THATS IT.I CAN LET IT RUNNING AND TURN IT CLOCKWISE? YESTERDAY IT WAS AT 11-9 NOW I HAVE IT AT A STEADY 15 I DIDNT NOT HAVE THE BRAKE BOOSTER HOSE HOODED UP NOW I DO AND IT BUMPED UP THE PRESSURE

[rd100]

Yeah if you are looking down from above the distributor, clockwise would advance the timing. It doesn't take much of a movement though to get something happening.

Though have you checked to make sure your points are ok? Do you know what dwell angle you are getting? You need to make the dwell correct before messing with the timing.

[emilo2008]

it its really hard to start it but once its turn on it idles real good.its just it drops to 10 when i put it in gear i hook up the hose from the transmission to the port but now i can get it running.????/ can some one school me on how to time the truck.thanks guys

[70_F100]

hey thanks guys i got it start and running ok but it still pops which cause the butterfly to stay open.and also everytime i shift it in reverse or drive it drops pressure from 15psi to 10 psi? am i missing another bacum hose from the transmission and the engine.i heard there is a hose from the transmission the the engine.

First, get your TERMINOLOGY right. IF you want answers, you need to at least have some sort of idea what you're dealing with.

Vacuum SUCKS!! It's measured in Inches of Hg (mercury).

Pressure BLOWS!! It's measured in PSI (pounds per square inch).

If you're using a vacuum gauge, it's calibrated in (In Hg). Intake manifolds SUCK, exhaust manifolds BLOW!!!

Never heard of BACUM!!!

[emilo2008]

sorry ive seen the hg on the gague underneath the psi on top.well the hg drops to 40 when i shift and it idle back great when i but it back in park.

[70_F100]

sorry ive seen the hg on the gague underneath the psi on top.well the hg drops to 40 when i shift and it idle back great when i but it back in park.

A picture of the gauge may be helpful.

[R.Smith]

OK...now I'm confused, so I'll just say this. I put a vac guage in my truck because I figure you can never have enough guages. I don't own a cell phone or wikipod so I need other things to distract me from the people I'm about to rear end. Anyway, my guage reads about 20 Hg when it's idling and when I'm cruising at a steady clip on the hwy. When I pull off from a stop or hit an upgrade on the hwy, it will drop down to about 10...unless I really get down on the accelerator, then it drops below 10 to around 7-8Hg. When I back off the gas it goes up to about 28.

I had assumed this was pretty good according to the paperwork that came with the guage but that paperwork was pretty general.

Is this where my 390 shoud be...suckin wise?

[70_F100]

Based on those numbers, you're doing pretty good.

Usually, about 18 in is about as high as you'll see, but it's all dependent upon the circumstances. Throttle position at idle, idle speed, camshaft lift/duration, altitude all affect those numbers.

I guess since you're pulling 20 at idle, your 390 really sucks!!!