My 66 elky runs great without vacuum advance. Do you need it? (350 crate)

No. You don't need it.

You'll get a little better gas mileage while cruising if you have it, and if you have it on manifold vacuum, a smoother idle.

If those aren't issues with you, you don't need it.

How exactly does vaccum advance work? :confused: redface.gif

It simply pulls in more timing advance when the engine is at full vaccum. The vacuum dashpot has an arm attached to it, that pulls the distributor plate when engine vacuum is applied, moving the timing ahead. So at WOT, when vacuum drops off, it adds nothing. But at idle, part throttle criuse, it adds anywhere from 8+ degrees of timing to make the combustion more efficient at lower rpms.

Keep in mind that at high rpms, time delays are so small, that advancing timing alot isn't needed.

Lastly, you can either hook up the vaccum can to full manifold vacuum or the 'ported' vaccum ports on many carbs. The ported vacuum ports only come on when the throttle plates are open, not at idle. I believe they were needed for emmissions testing. Most usually run the vaccum advance direct to manifold vaccum to help cruise milage and idle quality.

Some stock GM HEI vacuum cans pull in as much as 20 degrees advance! For a high compression hot street motor, thats way too much! So always check how much your distributor has.

Keep in mind that at high rpms, time delays are so small, that advancing timing alot isn't needed.

What does this mean?

oh oh, , this is verbatim from my distributor man who has a "hall effect" thing going on in my case that dials back the timing 2-3 degrees at around 6000.

what he mentioned to me, is that things are happening so quickly at higher rpms, that ignition timing lead ( advance) is much less critical to propogate a good flame front. In my case, I actually lose a few degrees up there ( not that I have ever free wheeled the motor to 6000 to check it).

My logical assumption here, which might be incorrect, is that adding 10-15 degrees advance whiule the einge is at upper RPM is not only detrimental to your bearings, but completely unnessesary, while at a moderate 2000-3000 cruise RPM may help flame travel and therefore combustion efficiency.

I am misinterpreting here?

Would the carb adjustment be affected by the advance or retard in timing? As in too rich or lean after timing adjustment is made?

Originally posted by 427L88:
oh oh, , this is verbatim from my distributor man who has a "hall effect" thing going on in my case that dials back the timing 2-3 degrees at around 6000.

what he mentioned to me, is that things are happening so quickly at higher rpms, that ignition timing lead ( advance) is much less critical to propogate a good flame front. In my case, I actually lose a few degrees up there ( not that I have ever free wheeled the motor to 6000 to check it).

My logical assumption here, which might be incorrect, is that adding 10-15 degrees advance whiule the einge is at upper RPM is not only detrimental to your bearings, but completely unnessesary, while at a moderate 2000-3000 cruise RPM may help flame travel and therefore combustion efficiency.

I am misinterpreting here? When a current-carrying electrical conductor is placed in a magnetic field, a voltage sometimes develops between one side of the conductor and the other. For this to happen, the magnetic lies of force must be perpendicular, or nearly perpendicular, to the line containing the conductor. The voltage then appears at right angles to the magnetic lines of force. If the conductor is a strip, and the magnetic lines of force are perpendicular to the strip, then the voltage will appear between opposite edges of the strip. This is called the Hall effect.

http://math.arsc.sunyit.edu/projects/vector/hall.html

Did you ever wonder why an engine that works best with 35° timing at 3000 rpm usually seems to work just fine with the same timing at 6000 rpm? The mixture has half the time to burn to achieve the peak cylinder pressure at the correct time. Does this mean the flame front travels twice as fast?
The correct firing point for achieving peak cylinder pressure at the correct time ( typically around 15° ATDC ) varies widely and depends on many variables. The flame propagation rate in a combustion chamber is not constant. Here are a couple links to reasonable explanations of the combustion process.

http://www.nd.edu/~msen/Teaching/TFSeminar/1998S/Engines.pdf

http://toyotaperformance.com/ign_combust.htm


Vacuum advance operates on intake vacuum, which is inversely proportional to load. It is used to increase the volumetric efficiency under light loads. It does not affect the speed of flame travel, it only compensates for the lack of it. What kind of load do you have on your engine when you are running 6000 rpm? Do you have 15" of manifold vacuum? Do you think the vacuum advance is operating at this point?

Del my 350 in my truck runs great w/o it also.