brucep

The vacuum advance cannister that was on my L-79 when I bought it was stamped 437 15. Can anybody identify it in terms of performance specs and suitability for an L-79 compared to a B28 which I have on there now?

Duke Williams recommends a B20 or B26 over a B28 for the L-79 application (see the PDF article "Vacuum Advance Specs" by Lars Grimsrud and Duke Williams on BarryKs website). I'm wondering if this 437 15 is closer to a B20 than the B28.

Thank you

Powershift

Bruce:

I think this vacuum can is GM Part Number #1973437. It lists as being for a 1971 Corvette with 350 CID engine and distributors #1112038-50. It crosses over to AC DELCO # D-1308-A, which is no longer available. However, I do not have (all) the specifications for this unit.

It appears to provide a vacuum advance of 15 degrees versus the 1967 327/350 HP (L-79) GM vacuum can #1116236 which provides 16 degrees. Unsure of vacuum range. We need someone with a 1971 GM Service Manual to lookup the Corvette distributor #s above with 350 engine to get the specs.

You can also use a Mighty-Vac or engine vacuum gage to see where this can begins to pull in and where it is full in. Your numbers may not be exact, but will be close.

The #236 can is rated at 0 degrees of advance at 4"Hg vacuum and 16 degrees of advance at 7"Hg vacuum.

By comparison the B28 can (or Delco D-1312C equivalent) provides 0 degrees of advance at approx 5" Hg and 16 degrees of advance at approx 8"Hg vacuum.

I believe a B20 can provides 0 degrees of advance at approx 6" Hg and 16 degrees of advance at 12" Hg.

Larry

brucep

Thanks. I'm wondering if anybody with an L-79 who switched from a B28 to a B22 or B26 would report on the differences in performance they experienced.

JohnZ

It's not a matter of "performance" - it's a matter of idle timing and idle stability, and matching the VAC to the engine's idle vacuum characteristics. The VAC should be fully-deployed at 2" Hg. less than the engine's idle vacuum.

The B28 is fully-deployed at 8" Hg., the B26 at 11"-13" Hg., and the B22 at 14"-16" Hg.; select the one that's closest to 2" Hg. less than your normal idle vacuum level.

brucep

Thank you. I should have referenced a B26 or B20 not B22. What about the "Starts @Hg" spec? the B28 starts at 3-5 Hg versus the B26 and B20 which start at 7-8 Hg. Does this have any influence an on drivability or performance, and if so what?

JohnZ

That "437" VAC is from a '68 327/350 distributor, but I don't have the specs on it.

The starting point isn't that important - what matters is the vacuum level at which it's fully deployed; that point must be at least 2" Hg. below the normal idle vacuum level in order to have a stable idle and idle timing.