Should i runmy vacum advance from the full manifold or ported vaccum. I have a 390 wita holley 4160. What if any damage can come from running either of them? when I have the engine hooked up to ported and hit the gas hard aor going up a hill I get a gringing noise. when its hooked up to full manifold I don't. Yall have helped me more than you know. need some schooling once again

 

ported vacuum only allows the timing to be advanced when under a load or anything past idle. Manifold vacuum is a constant 17 inches or so of vacuum to your dizzy which causes your base timing to be way more than what it should. If your initial timing is 10 and its hooked up to manifold vacuum then you've got anywhere from15-25 degrees of timing at idle. If you've already advanced your timing at idle, theres no timing to advance in order for the ignition to keep up with the quicker moving pistons at higher rpms. Bad for valves and seats as i learned the hard way at high speeds on running too much timing.

 

ported vacuum only allows the timing to be advanced when under a load or anything past idle. Manifold vacuum is a constant 17 inches or so of vacuum to your dizzy which causes your base timing to be way more than what it should. If your initial timing is 10 and its hooked up to manifold vacuum then you've got anywhere from15-25 degrees of timing at idle. If you've already advanced your timing at idle, theres no timing to advance in order for the ignition to keep up with the quicker moving pistons at higher rpms. Bad for valves and seats as i learned the hard way at high speeds on running too much timing. so ported is what you want!

 

run the ported source!

 

"gringing noise"? I'm just not understanding that one, just part of keyboard diagnostics. Run the ported source like everyone says.

Vacuume changes with the throttle:
Throttle closed (idle)-high vacuume in manifold, no vacuume at timed port.
Throttle opens-vacuume in manifold drops, air rushing through carb venturi creates vacuume in timed port.

When you are charging up-hill with your foot in the gas, you actually have less vacuume in the manifold, more at the timing port.

With all that said, take a hard look at the timing. Is the distributor pretty old?

 

My schooling must of been different, when under load or charging up a hill with your foot in it vacuum fell off and timing retarded to prevent detonation. Vacuum advance was for high timing under light loads for fuel economy.
All this with a ported vacuum siginal.
.....=o&o>.....

 

how does one put up a new post? i have only been able to reply!

 

new thread !

 

i take that back it's new topic. I am a new user too

 

Yeah Carl, and I've NEVER been wrong-just ask my wife! OK, so I played hooky that day, now I'm trying to decipher your notes. LOL

All this time I thought vacuume advance was transition from initial to centrifigul. At some point you should reach a limit for total advance?

Here's what I can't get past:

If you open the throttle wide, the intake vacuume should drop-load or not, as kind of a somewhat equalization between demand and supply.

The venturi velocity should increase, although resistance (loading) to the engine should reduce flow through the venturi. Isn't that vacuume signal directly related to the venturi velocity?

How does the load/no load affect the comparison between manifold and venturi? I don't dispute what you are saying, just trying to get some free skoolin myself.

OH! FWIW my truck makes a "gringing" sound on the highway also. It's the shift lever for my transfer case-good steel. Somehow, I don't think it's the same sound albcasummit hears. LOL

 

Not even close.

There are three total advances. Mechanical advance, which is the total of initial and the mechanical advance, generally 36 to 38 degrees in FEs.

Vacuum advance, which varies depending on how hard the engine is working. Ported vacuum shuts off all vacuum at idle. When throttle plates open the ported vacuum ports sees manifold vacuum as long as the throttle plates are open. Is advance is limited by the amount of advance allowed by the advance unit, usually 10 to 15 degrees. Vaccum advance is used to improve gas milage while at cruise. At WOT and while accelerating the vacuum advance is static and does not advance the timing. The vacuum port is below the venturi and just above the throttle plate so does not sense the increased vacuum in the venturi.

Total advance is the is the total of machanical advance (36-38 degrees) and the total of vacuum advance. This advance can be as high as 45 degrees when engine is loafing while cruising down the highway (high vacuum and and whatever mechanical advance is in at your cruise speed) RPM, generally 2200 to 2800 and 25 to 38 degrees depending on the advance curve in the distributor.

If you don't believe me, just put a "T" in the carb to vacuum unit hose and watch the gauge.

 

Bear, as always, right on the money.

With any motor that can generate "good" vacuum at idle, that is, one without too lopey a cam (like the 941 or bigger, I'd think), you should run ported vacuum.

If you're like me, and had only 10 inches of vacuum at 900 RPM at 10BTDC timing, you reverse it, and run manifold vacuum for the vacuum advance. This kicks the idling timing way up to get more vacuum, but the motor still starts easy because the vacuum advance isn't doing anything when cranking. Then, under load, the vacuum advance backs off retarding the timing so it doesn't ping like crazy (11:1 compression - static, and a 4 degree advance on the cam for low-end). But that's just me. DO NOT try this at home

 

Extremist alway have to live life differently. It's the only way than can get things to function to their satisfaction. I ought to know, did the same kind of thing for years.

 

"Extremist", there is no better word to replace that one that seems to apply to many members. Off subject a little, the wife before she became mine has a 69 912 Porsche and that was screwed up by many because the motor has vacuum retard.
I don't know when it began (didn't care as i'm a 60's motor person) that Ford's had dual vacuum vacuum advance units. Push pull? I recall mid 70's ported went to manifold vacuum to idle up when over heating besides restricted or delayed ported vacuum.
.....=o&o>.....

 

Beemer, I remember those advance/retard puppies. Not one of Ford's better ideas. The timing retard kicked in under decelleration, for a rudimentary pollution control. Some vintage Mustangs (67?) have this type distributor.

Albcasummit, to answer your original question, the distributor should be plugged into BOTH ported and manifold vacuum, if you have a PVS (ported vacuum switch) on your thermostat housing. The PVS allows manifold vacuum to go to the dist until a specified temperature is reached, and then it switches to normal carb ported vacuum. (On a 3 port PVS, center port goes to dist, #1 to manifold vacuum, #2 to carb timed vacuum)

This allows for a slightly higher idle speed during warm-up, and gives better drivability when cold. Most people bypass the PVS as being an unnecessary pollution control, but actually the engine runs much better with it in the loop (especially in cold climates).

The yellow PVS is 160*, blue-green is 125*. There's many other colors and temps, but those are the main temps used for vacuum advance.

This is probably more than anyone wanted to know, and way too much info on PVS's......

TLR