Confusion with AMA vacuum specifications

[Larry M.]

I've looked at the AMA specifications for L46/L82 Corvettes to determine an approximate amount of vacuum I should expect from that camshaft in our car, and I'm confused with what I found.

If I'm reading the specifications correctly, by year, the same camshaft produces quite different vacuum results. Here's what I see:

1969 - 12 in
1970 - 17 in
1973 - 24 in
1974 - 12 in
1975 - 4 in
1976 - 8 in (10.5 with M40)
1977 - 8 in
1978 - 8 in
1979 - 8 in
1980 - 8 in

Assuming I'm reading the charts correctly, why would the same camshaft produce those different figures? Or am I misunderstanding what I'm reading? And to satisfy my initial reason for looking into this question, what vacuum reading should I aim for? The car is a 1972 LT-1, stock with the exception of the camshaft, and the AIR and TCS equipment removed.

Larry

[Duke W.]

Re: Confusion with AMA vacuum specifications

I don't recall that the AMA specs list idle manifold vacuum. It's looks like what you have presented is vacuum required to pull the VAC to maximum advance, except '75. The 4" is probably the start point. Typical L-46/82 idle behavior, manual trans in neutral would be about 14-15" @ 750. TH400 idling in Drive at the same speed would be a bit less due to the converter load. If it still has OE ported vacuum advance, convert it to full time and install a B26 VAC.

[Larry M.]

Re: Confusion with AMA vacuum specifications

Duke,

Yes, the figures I listed are that max vac figure. Thanks for supplying me with the number I was looking for.

Larry

[Larry M.]

Re: Confusion with AMA vacuum specifications

Interesting; I see that the NAPA VC1808 (B26) and VC1765 (B20) have identical specifications - 8 degrees at 11-13". I wonder what the difference might be.

Edit: A google search turned up a number of references that these two advances function identically, and are interchangeable.

[Duke W.]

Re: Confusion with AMA vacuum specifications

The B20 and B26 specs are virtually the same - within production tolerance of each other. Years ago the NAPA number was consolidated to VC-1765 and when the B20s were exhausted the B26s showed up in the box. The 8 deg. is distributor so crank advance is 16. Start @ 6" 16 @ 12" are the nominal specs. These same VAC are available in many "brands" and all are manufactured by Standard Motor Products.

Duke