Vacuum advance and timing adjustment

[Duke W.]

Re: Vacuum advance and timing adjustment

There is more than one "total advance".

TOTAL IDLE ADVANCE should be equal to initial timing plus full vacuum adv.

TOTAL WOT ADVANCE should be equal to initial plus full centrifugal.

Cruising speed advance will be a sum of initial plus some centrifugal plus some vacuum depending on the specs of the centrifugal and vacuum advance curves.

On SHP engines with quick centrifugal, such as the '64-'64 SB mechanical lifter engines that achieved full centrifugal at 2350 and full vacuum advance at 8", the cruise advance over 2350 RPM will sum to 52-54 degrees with 12-14 initial timing.

Duke

[Scott Marshall]

Re: Vacuum advance and timing adjustment

Hello Duke,
I have replaced the factory distributer with a Mallory Unilite without a vacuum advance on it. So according to what you are saying, the only real difference I should notice is less fuel economy? I have only started the car, not driven it yet. I went with the Unilite for the breakerless circuit and did not notice it had no vacuum advance until sometime after I bought it. I was wondering, what the effects would be by not using the vacuum advance?
Scott

[Duke W.]

Re: Vacuum advance and timing adjustment

You will likely notice poorer fuel econonmy, poorer idle quality, and unless the cooling system is in tip top shape, hotter running or overheating especially in stop and go driving in hot weather.

The Delco single point is a very simple and reliable ignition system. They were somewhat indifferently assembled, and most have seen nothing but new points, condenser, cap, and rotor since they were assembled by GM.

A good "blueprint" overhaul will make them better than new, which is always what I recommend rather than some generic aftermarket unit, many of which are not really suited to street driven cars - like not having a vacuum advance properly matched to the engine's manifold vacuum characteristics.

Duke

[Patrick H.]

Re: Vacuum advance and timing adjustment

Jack,

To make life easy, what I did is purchase and rebuild a second Corvette distributor. That one I had recurved to 1965-ish SHP specs, and run on unported vacuum. It's what I now run in my 71, and it works very well. I also installed a vacuum can that matched my new setup as well as my engine's vacuum characteristics.

My original 1971 distributor and the TCS solenoid are tucked in my basement. I've got them for show or judging, and it is FAR easier to swap out a distributor for whatever reason than to try and swap curves on an as-needed basis.

Patrick

[Jack Corrao]

Re: Vacuum advance and timing adjustment

Patrick, are you saying the curve in the 71's distributor won't work so well when the TCS is bypassed? I was going to hook it up to non-ported vacuum and run it that way. What would the drawbacks be? Is the mechanical curve and vacuum can that different on this distributor that older ones. Would new springs that bring all the advance in sooner help? It seem this is all pretty important to get maximum driveability and performance from this dinosaur.

Thanks again

[Patrick H.]

Re: Vacuum advance and timing adjustment

Jack,

There are a couple if issues:
1. The vacuum can is set for the engine's original configuration.
2. The top of the distributor shaft has a special configuration or "autocam" on it that will not allow the full mechanical advance you'd want. As a result, you may swap springs to alter the curve, but won't be able to "match" the curve you'd like with regard to rate of advance AND amount of advance. The original shaft will limit the amount of advance the distributor can deliver.

So, I just purchased another distributor and set it up to match what I wanted it to do. I checked my vacuum specs at various rpm levels (as per Duke's recommendations) and, using a chart of vacuum can characteristics, found one that matched my engine and new distributor curve.

As I mentioned, a lot easier than just running straight vacuum and dealing with a lousy curve OR swapping and swapping springs and never being happy.

Patrick

[Scott Marshall]

Re: Vacuum advance and timing adjustment

Hello Duke,
OK so what you are telling me about the distributer w/no vacuum advance that I am using is that there are no advantages. Just poor fuel economy, poor idle, and overheating? Just wondering how bad I need o get another distributer. Thanks
Scott

[Duke W.]

Re: Vacuum advance and timing adjustment

My suggestion would be to completely disassemble, inspect, and overhaul your
original single point distributor, then reinstall it. Single point overhaul and "blueprinting" have been discussed numerous times on the Board, so you might want to do a search and find some previous threads on the subject. It's a fairly easy task for the DIYer unless the housing bushings need to be replaced.

Duke

[Jim T.]

Re: Vacuum advance and timing adjustment

Tim I cannot reference all the GM manuals about plugging the vacuum advance line when adjusting engine timing, but my 70 with its TCS factory operating system would normally not allow vacuum to the vacuum advance at idle, operating in reverse, in first, and second gears. However this was not always the case 100% of the time. The system had a temp sending unit in the right cylinder head. If the temp got hot enough system would allow full vacuum until the temp was reduced. My 70 could operate with full vacuum at idle if the engine got hot enough while adjusting the timing. But like I have posted, my 70 operated with the TCS controlling vacuum advance for about a month when it was new so I always plugged the vacuum advance line when adjusting the timing.

[Timothy B.]

Re: Vacuum advance and timing adjustment

Thanks for all the help with this. I switch my car to full time advance at idle from ported advance and it runs better now at idle than ever before. Now that I understand how this system works, it all makes sense!!