Another vacuum question

[Joel T.]

I have been following the posts out here on manifold vacuum and the vacuum advance can on the distributer and have a couple of questions.

On my 1963, 327/340, I am running a Competition Cams stick which is slightly hotter than the stock solid lifter cam. When I run the car, the most I get by way of vacuum is 13 inches of mercury when measured out of the vacuum advance line at the base of the carb. I have been chasing this down as a potential intake manifold leak (this is the car which produces oil smoke out of the right cylinder bank when hot) thinking I should be seeing 18-20 inches. The distributor has a B1 vacuum can.

Two questions; what should I be seeing by way of vacuum?? and is the B1 vacuum can correct for this application?

Thx!

Joel

[Duke W.]

Re: Another vacuum question

Idle manifold vacuum is a function of valve overlap. Some think idle vacuum is affected by the inlet valve closing point, but that is a myth. The more valve overlap, the less idle vacuum, and "hot cams" need higher idle speed than mild cams in order to achieve reasonable idle quality, stability, and manifold vacuum. Overlap creates a "vacuum leak".

During the overlap period, exhaust side pressure is near atmospheric, but the inlet side it throttled. The lower the engine speed, the more time is available for exhaust gas to leak into the inlet port, which lowers manifold vacuum and dilutes the fresh charges with exhaust gas. The lower the idle speed, the less idle vacuum. Extreme cases are very high overlap racing cams, which may just barely idle at 1500-2000 and only generate a few inches of manifold vacuum. Idle is very erratic and unstable because of the extreme exhaust gas dilution.

Exhaust gas dilution of the fresh charge reduces flame propagation speed and causes combustion instability (misfires, flameouts). This is why high overlap creates idle "lope" and why high overlap cams need around 28-34 degrees total idle timing, which is achieved with the sum of initial plus full vacuum advance plus maybe a couple degrees of centrifugal if the centrifugal curve starts below typical idle speed. The 300 HP cam only needs 20-26 degrees total idle timing and should pull 17-18" at 500 RPM idle speed in neutral. (less with Powerglide at 450-475 in Drive).

The OE Duntov cam only generates about 12" at a 800-900 RPM idle speed with the suitable 28-32 degrees total idle timing. If your installed cam is "hotter" i.e more overlap then the Duntov cam it should generate LESS idle vacuum at a given idle speed, not more!

The B1 can requires 16-18" to pull to the limit, so it violates the "2 inch rule" with your cam as I outlined in the earlier post. Also review the summary chart (including the subsquent correction post - I mixed up a couple of numbers) in the previous post, which lists the proper vacuum can based on observed idle vacuum.

You need the NAPA/Echlin VC1820 (stamped B28) vacuum can, which pulls to the limit at about 8". The B28 can will be "locked" on full advance at idle, which will provide stable total idle timing in the correct range. It should result in better idle stability and maybe a little higher manifold vacuum.

This is the most "aggressive" vacuum can available, and it should be used on any engine that generates less than 14" at idle with suitable total idle timing. It will even tame (somewhat) a L-88.

Also as a reminder, idle vacuum should be checked by "teeing" the vacuum gage into the vacuum advance signal line. Some have just removed the signal line from the vacuum can and connected it to the vacuum gage. This, of course, changes the engine configuration - idle speed and manifold vacuum drop, and the readings are meaningless. Test equipment must be connected in "parallel" not series.

The first rule of connecting test equipment is that it must be connected so as not to change the configuration.

Duke

[Jeff Farrar]

Re: Another vacuum question

The NAPA vacuum can should be the VC1810 (B28). Pretty sure Duke just had a typo. Didn't want you to ask for the wrong part.

Good Luck.

[Duke W.]

I added a correction post, yesterday *NM*