Vacuum Advance Can

[Richard D.]

Re: Vacuum Advance Can

That's an old, obsolete version of Lars' paper. The latest version has a revised application list that we developed together and additional explanatory information (written by me) on what makes a suitable spark advance map for a particular engine configuration and why. The latest version is here:

http://www.lbfun.com/warehouse/tech_...ance_Specs.pdf

For example, we eliminated the B1 applications such as base cam Corvette engines because it's really not the best choice, but is based on some ill-conceived parts substitutions done by GM long after our cars were built.

There are three VACs that meet the needs of any OE Corvette engine with full time vacuum advance or ported advance engines that are converted to full time. All are based on applying the "Two-Inch Rule" to typical idle vacuum and are the first three listings in the latest revision.

15", B22 - all base engines/optional engines that use the base engine camshaft (283-220/230/245/250/275HP; 327-250/300HP) with manual trans

12", B20 or B26 - all big blocks, all base engines/optional engines that use the base engine camshaft (as above) with auto trans, L-79, L-46; L-82 w/ MT.

8", B28 all mechanical lifter cam small blocks; L-82 w/ AT.

One application I'm not sure about is big blocks with auto trans, since I've never gotten a reliable report on their manifold vacuum/idle speed idling in Drive. If they pull less than about 13-14" they should use a B28.

Thanks for the update on that Duke.

You gave me the advise on my distributor a few years back, and following that advise, I was able to get it straightened out.

Thanks again, Rich

[William C.]

Re: Vacuum Advance Can

A VC-1765 will either be a B20 or a B26. The latter was formerly VC-1808, but it's essentially the same specs at B20, so both were consolidated under the VC-1765 part number, and either is okay.

As previously advised, I suggest you check before leaving the store that maximum advance is achieved at about 12". Then you can check on the car to be sure that it yields about 16 degrees at idle.

Duke

Replaced vacuum advance can with the VC-1765 which is B26 on the can.
The car has great driveabilty now and no ping at all on 93 grade fuel. The max advance is @ 12" and is 16 degrees at idle. Thanks to all...Bill

[Duke W.]

Re: Vacuum Advance Can

Did you get detonation under some conditions with the 436 VAC - like light acceleration?

With the B26 do you notice any improvement in idle stability/quality?

Duke

[William C.]

Re: Vacuum Advance Can

Did you get detonation under some conditions with the 436 VAC - like light acceleration?

With the B26 do you notice any improvement in idle stability/quality?

Duke

Detonation when accelarating from a cruising speed then it would stop when the vacuum advance retarded it to mechanical advance with the 436 VAC. The idle is pretty much the same. It idles very well at 750 RPM and will idle down to 600 RPM without stalling...Thanks Again, Bill