Duke, What do my original Vac numbers tell you?

[Perry M.]

I finally decided to pull the distributor cap off my very original 63 split window FI car in order to see what the vacuum canister numbers are. Posted are the pictures of my original, they have been on the car since the mid-70s and I assume is factory, and also pictured is A replacement Airtex B28 vac. The car runs great, always has, but what can you tell me about the numbers on the original vacuum canister?

[Duke W.]

Re: Duke, What do my original Vac numbers tell you?

201 15 (start @ 8", 15 @ 15.5") is the VAC used on ALL '63 engines. It doesn't pass the Two-Inch-Rule for the FI engine, but that doesn't really cause problems in the OE configuration because for some inexplicable reason '63 L-84 is set up for ported vacuum advance. It definitely caused problems on the 340 HP engine, which has full time vacuum advance.

My 340 HP SWC suffered from idle instability when new. It would stall within 30 seconds at idle unless I blipped the throttle. The dealer and so called "experts" I talked to were clueless or said just disconnect the VAC, you don't need it.

I finally figured it out in 1965. The Duntov cam pulls about 12" Hg at 850 or so, but the VAC required 15" to fully deploy. As I approached a stop I always downshifted and pushed in the clutch at about 1500. This caused enough vacuum to pull the VAC to the limit, but as engine speed dropped down to idle the VAC began backing off. This caused a reduction of spark advance, which caused revs and vacuum to drop more that caused more loss of revs as the VAC kept pulling advance. It was a positive feedback loop that would cause the engine to stall fairly quickly.

From this I coined the term Two-Inch Rule as he primary criterion for selecting a vacuum advance. (Total vacuum advance is a secondary criterion.)

In 1964 Chevrolet modified the spark advance map on SHP/FI engines although early SHP/FI engines had what were basically carryover distributors and 375R FI units. The second design distributor had a 236 16 VAC, start at 4", 16 @ 8", and the modern B28 replacement has the same nominal specs. Chevrolet also set up the 380 FI system for full time vacuum advance. In addition they considerably quickened the centrifugal with the max 24 centrifugal all in at 2350 rather than 4600.

So I bought a '64 ...236 VAC and the companion centrifugal weights and springs, installed them on my engine and it made a HUUUUUGE difference. Not only did it idle stably (with the typical Duntov cam lope due to the high overlap), but the quicker centrifugal made noticeably more low end torque, which really helped around town because I ordered my SWC with a 3.08 axle because I wanted a 150 MPH top speed and easy cruising at 80 MPH and up.

So what I recommend to you is convert to full time vacuum advance, install the B-28 VAC and install the black springs from the Mr. Gasket 928G spring kit, which should bring full centrifugal in at no more than 3000. Use a dial back timing light to set total WOT advance (VAC hose disconnected and plugged) as high in the 36-40 range as the engine will tolerate without detonation. This must be done with the engine revving above the RPM for full centrifugal.


Trying to set initial timing on mechanical lifter engines can be very difficult because you have to get revs below where the centrifugal starts (700 on '63 to '65 distributors) and these engines may not idle stably long enough to get the job done before the engine stalls. Also with the 8" balancer the timing tab is not accurate. Like 12 degrees is really about 10, and 16 is about 13.

Duke

[Perry M.]

Re: Duke, What do my original Vac numbers tell you?

Thanks Duke. I will follow your advice.

[Mark E.]

Re: Duke, What do my original Vac numbers tell you?

Duke,

Brilliant narrative; I always learn from your posts.

[Lawrence S.]

Re: Duke, What do my original Vac numbers tell you?

Great thread! Thanks Duke

[John D.]

Re: Duke, What do my original Vac numbers tell you?

Duke, If you don't mind I have a question for you. When you said the Duntov cam pulls 12" of vacuum or you referring to the Duntov 097 or the 30-30 cam.
I see the B28 is not made elsewhere. Thanks, John

[Duke W.]

Re: Duke, What do my original Vac numbers tell you?

The ...097. My recollection is that my OE Duntov cam pulled about 12" at 800-850, but it's had a LT-1 cam since the seventies that's about the same. I know you say higher vacuum for the Duntov. Engine Analyzer says they both have about the same effective overlap in sq-in-deg, which would indicate similar idle behavior. 30-30 cam typical idle behavior is 10" @ 900 due to the greater effective overlap, which is what makes this cam so soggy down low.

I don't use the term "Duntov 30-30". It's a misnomer. I don't think Duntov personally designed it, but he was certainly involved in the overall design philosophy. Drawing number 3849347 was drawn by Wachowki and checked by K C Kraemer, dated 4-12-63.

Drawing number 3736098 was drawn by Ed Loo and checked by Pebbles and another name in script rather than printed, something like J Skupin, which is tough to make out because the drawing copy is reduced size from original. The date is 22 FEB 56.

I think it was in Jerry Burton's Duntov biography that he stated Duntov's cam design for the Chevy small block incorporated the same lobe as the cam he designed for his Ardun OHV heads for the Ford flathead V8.

BTW the ...347 and .098 drawings are the finished camshaft drawings, not casting drawings/number, which are cast into the cam, and in addition to listing lobe lift, in inches to five decimal places every cam degree they have data for the rough cast lobe shapes for the tooling engineers who produce the tooling drawings.

The ...346 and ...097 drawings are "assembly" drawings consisting of finished camshaft and pin, which is what was sold over the counter.

Lots of SMP replacement parts are made in Mexico, and AFAIK those plants meet Tier 1 OE quality requirements, same as SMP's US plants.

Duke

[Ed D.]

Re: Duke, What do my original Vac numbers tell you?

Duke, very informative. I always enjoy your narratives and technical expertise.
Ed