Vacuum Advance Specs

[Tim S.]

Here is a chart I found on another website that I thought would be of interest, considering how often timing and vacuum advance gets discussed. Please do your own due diligence as I have not validated any of this.
Tim.Vacuum advance specs.jpg

[Owen L.]

Re: Vacuum Advance Specs

That paper looks like it's from Lars Grimsrud & Duke Williams - or at least it's copy/pasted from their work. Lars and Duke have a lot of information and for the most up to date, ask Duke here or email Lars at: v8fastcars@msn.com and ask for it.

[Duke W.]

Re: Vacuum Advance Specs

That's Lars paper from at least a decade ago. Some of the numbers have changed or been discontinued.

Selecting a VAC is easy. There are only THREE currently available single point/TI VACs that meet the Two-Inch Rule for ANY engine configuration with full time vacuum advance, OE (either full time or converted to full time from ported) or aftermarket. Though not all inclusive the following are the THREE VACs and their general applicability to OE drive train combinations. ALWAYS verify the applying the Two-Inch Rule. Valve overlap is the biggest factor in idle vacuum, and the more overlap the less the manifold vacuum at the lowest acceptable idle speed. The numeric value is the manifold vacuum in inches Hg that provides max advance and figure +/- one inch tolerance. All provide nominal max crank advance of 16 degrees.

15" B22 - base engines with manual trans.

12" B26 Base engines with automatics, big blocks with manual trans, L-79

8" B28 - all mechanical lifter small blocks

ALL of the above are manufactured by Standard Motor Products, but they are available in at least half a dozen brands, including Delco. Any parts store should be able to cross the NAPA number to whatever brand part number they carry. The B-number is SMP's ID number for the different specs since they all look the same externally. Few seem to understand the importance of having a functionally best fit VAC, but it's a very important part of the engine control system to provide broad range performance, driveability, and fuel economy for road engines. Racing engines don't need one.

The B28 is getting hard to find, but there may still be some Airtex brand 4V1053.

Duke