How much extra advance does vaccum advance give?

[unclebilly]

I am trying to program a mega jolt with a RS curve - this is easy because these were mechanical advance distributors.

When I try to program the T S E curve, I need to know how much extra advance the vaccum pot gives over the mechanical curve. Anybody got any ideas?

[RSTarga]

I don't think the vacuum gives advance , it retards the ignition for idle. As soon as you crack the throttle no vacuum. All the advance is mechanical IMHO.

[unclebilly]

I think that is case for the mid year California cars. I could be wrong tho.

[Josh D]

It definetely could be year and tune dependant. My '80 RoW is definetely advanced at idle about 3* from vac advance.

[911pcars]

Prior to heavy smog regs., vacuum advance mechanisms typically advanced the spark, hence the term, "advance". As a guess, I would say about 10º. However, you could connect a vacuum pump tester and observe the timing difference with a timing light.

Sherwood

[snbush67]

You have to confirm what direction the RS vacuum pulls (advance or retard). I think you will find that the vacuum controlled lever pulls opposite of the mechanical advance. I measured the vac pod from my 78SC and got (-) 3-5 deg. at the distributor which I think would translate to (-) 6-10 deg. at the crank.

[Flat6pac]

All the pods through 77 were retard at idle. The 78 used the vacuum to advance the spark. In 1980 to 83 the dual vacuum pod did both so you need to know which hose is which. Hook them wrong and nothing is right...
Bruce

[snbush67]

Bruce,

My 78 rotates counter clockwise, but the vacuum pulls clockwise. Doesn't that retard?

[timmy2]

My '78 ROW doesn't have any vacuum module on the distributor so this whole conversation seems retarded to me!
J.K. guys, now you can get back on track...

[Flat6pac]

Shane, pull the distributor and on the table see what it does. The counter weights move the advance for about the first 20 degrees (ccw) and then the pod starts pulling the 6 upward pointing prongs clockwise so the last 10 degrees of advance is moving toward the advance mechanism.
The rotor is advancing and the pod is pulling toward the advancing rotor.
Bruce

[T77911S]

vacuum advance- it does just that. advances the timing for mid RPM use. at WOT, the vacuum has no effect and it is all mechanical.

vacuum retard- it moves the timing in the non-advance direction, but only at idle. so you can really think of it as an advance also. as soon as you crack the throttle, the timing advances.
retard was added for emmisons. basically it makes the combustion chamber hotter to burn the fuel more efficiently.

if the points plate moves in the same direction the rotor turns, that is retard.

[cmcfaul]

The vacuum effect on my E accounts for 10 degrees. At idle it's set at 5 degrees ATDC. Disconnect the hose (or crack the throttle) and it goes to 5 degrees BTDC. With this stock set up you can get max, safe advance at redline and a properly timed, smooth engine at idle. Thats why Porsche designed it that way. Without the vacuum you loose 10 degrees of the curve.

Chris
73 911 E

[snbush67]

Originally Posted by Flat6pac

Shane, pull the distributor and on the table see what it does. The counter weights move the advance for about the first 20 degrees (ccw) and then the pod starts pulling the 6 upward pointing prongs clockwise so the last 10 degrees of advance is moving toward the advance mechanism.
The rotor is advancing and the pod is pulling toward the advancing rotor.
Bruce

Bruce,

Thanks for the insight, I understand I was looking at it backwards.

[911pcars]

Originally Posted by cmcfaul

The vacuum effect on my E accounts for 10 degrees. At idle it's set at 5 degrees ATDC. Disconnect the hose (or crack the throttle) and it goes to 5 degrees BTDC. With this stock set up you can get max, safe advance at redline and a properly timed, smooth engine at idle. Thats why Porsche designed it that way. Without the vacuum you loose 10 degrees of the curve.

Chris
73 911 E

Slight correction. PAG retarded the timing at idle to reduce emissions at idle (smog specs at the time). The vacuum source at idle was below the throttle plate which only applied neg. pressure to the vacuum unit with the throttle closed, thus retarding the ignition timing.

At off-idle and above, vacuum drops and the vac. unit rapidly diminishes its effect on the ignition advance. The normal mech. advance curve applies all the way to approx. 5000 rpm and is calculated w/o any effect (additive or subtractive) from the vacuum advance unit.

If you're not sure what your single or dual diaphragm vac. unit does, check idle speed timing with a light while varying vacuum with a mech. vac. pump.

Sherwood