The vacuum advance is extra advance on top of the mechanical advance.

Howdy
I think Schornack tunes his distributors for mechanical advance only, not to be used with vacuum advance.

Doug, at what throttle opening are you having the problem? I assumed that we were talking full throttle pinging between 2300-3500 rpm. Any other info about when the pinging occurs? Was the Pertronix installed before or after the distributor was reinstalled?

Unplugging the vacuum advance will affect the part throttle timing, not the full throttle timing, as it adds timing in response to vacuum. Light throttle, lots of vacuum and vacuum advance, full throttle, very low vacuum, no vacuum advance.

It was knocking with the throttle in any position.
Whether I was taking it easy or getting into it.
It was knocking between 2300-3500.
Noiw thatI've unplugged the vacuum advance as Jim S. suggessted it stopped.
I've emailed him for a answer as to why and I'm awaiting a response.

Doug, did you get the specs for the distributor when it was rebuilt, and were you connected to ported or manifold vacuum source? Adjustable or stock vacuum advance canister?

The Pertronix is simply an ignition trigger. It replaces the points. If you've wired it according to the installation instructions and have the proper voltage to the Pertronix ignition (which MAY be different than the voltage to the coil, depending on the coil) then it should work just like points but with a non-adjustable dwell and no high speed bounce. It does not affect the vacuum advance, but it is often necessary that if you modify your factory distributor curve that you must also install and adjust an adjustable vacuum advance canister. And, as noted, some modified distributors use no vacuum advance at all.

Leaving the vacuum advance disconnected may prevent your engine from running optimally under light loads.

Specs I'll have to look.
But when I sent it to Schornack for recurving I also provided him with all the required info.
Rear gear, trans, cam size etc.

I'm just trying to snoop some specs for my own reference. ;-)

Doug, you're at about the same place as Rommel was a short time ago. Check the "Timing Questions" thread. You should find some useful info.

Sorry that I didn't start off this thread with the disconnected vacuum advance test drive suggestion, it might have reduced the length!

When you install a Pertronix, recheck your initial timing. In a perfect world, there wouldn't be a change but ...

One of two things.

When you depress the throttle the vacuum quickly drops, but is not instantaneous. Therefore the time lag will add some timing on tip-in from the vacuum advance and you would get knock as you tip in under throttle.

Another possibility is the spring load on the vacuum advance is so little that if you are getting only a couple inches of vacuum is adding 1 or 2 degrees timing and is just enough to cause pinging. This would be the case if you keep the throttle fully depressed from 1500rpm and then hear knock as the engine speed increases through the 2300rpm to 3500rpm range where you have noted the problem.

If you don't have an adjustable vacuum advance already, buy one (Crane is nice) and keep adding spring preload until the knock goes away. Then you can have your performance curve and better gas mileage.

Very useful info in Rommel's thread there is no doubt. I do need to point out that there is a difference between Doug's and Rommel's though. Doug has a 10-1 and Rommel's is 9.5-1. This 'can be' a significant factor. Come on Doug, pony up the extra $46 sents a gallon and see if the spark knock goes away. If you use octane to end the spark knock then you may find that you have more power because of it. I say this because it's the way my engine is. And I have to address the good point made that all engines do not perform better just because you add octane. BUT, some do.

It already went away with the disconnected advance. Therefore if the advance is still disconnected and he notes an improvement that would indicate he is still getting detonation that he is not detecting (I am assuming he is going by ear). This is entirely possible because under WOT you probably won't hear it. In this case you may be stuck with paying extra for fuel. Before you give in totally, I would try a colder range spark plug first (drop down in 1 range increments under you get fouling under part load conditions).

But if he does not have a problem with knock without vacuum then while it may "cure" the symptom to use a fuel with a slower burn rate, it's not addressing the root cause. Instead of spending 46 cents a gallon, spend $15 one time for an adjustable vacuum can and fix the problem.

You cannot increase mechanical timing and not compensate on the vacuum side for driveability.

This is an excellent point that doesn't get stressed enough. Even if you can't hear it you may still be causing damage. That's one reason to err on the side of higher octane. Especially when you are on the ragged edge with a 10-1 engine. What if your tune 'slips' a little and you are getting detonation you can't hear because you have pushed the envelope? Yep, you guessed it, the engine will need to be rebuilt......

The other excellent point (missed above because I updated as Fbody posted) is that spark plug heat range is not a one size fits all.

Not only have we owners changed lots of things on our engines (a/f mixture, compression, camshaft, intakes, cylinderheads, etc.) but the fuel has changed since these cars were developed as well. As a result what worked in 196_ on these engines may not be optimal for today. Make sure your carburetor is properly tuned and then experiment with colder heat ranges. As long as your spark plugs are not fouling under light driving conditions you should be running the coldest plugs possible.

Not sure if I should revive this thread...

My MaxLead2000 showed up on my door step today. I filled the tank with 93 octane and added 12 oz of the stuff. I did a little driving and got on it a few times and once the treated gas came through the knock mostly went away. I'll have to drive a little more to see if 12 oz is enough.