C1 vacuum advance again

[Howard M.]

Duke:
Working with a 60, 283, base motor set at 4* btdc. Made the following readings with the vacuum gauge tee'ed into the advance line (removed the PG modulator line): rpm=525 in park, in hg=18", adv=19*. Max adv is 42* @>2000 rpm. Previous owner installed a B1 can. Based on your previous posts, the B1 sounds marginal. Should I go to a B28 or other can? Don't know the distributor number-tag is gone.
Also made the following measurements without the vac can: 500 rpm, 16.5-17 in hg, 4* btdc; 1000 rpm, 19 in hg,14*; 1500 rpm, 20 in hg, 19*.
Thanks for your help.

[Donald L.]

B22 16* @15" HG *NM*

[Duke W.]

Re: C1 vacuum advance again

Chevrolet diddled with the base cam vacuum advance control until they finally settled on a nominal 12-incher in '66, which carried through to '67. After that is was all about emission control so the rules changed.

I figure the 12" (11-13" with production tolerance) VAC was finally settled on because PG idling in Drive at 475 looses a couple of inches relative to a manual trans pulling about 17-18" at 500 in neutral.

The B1 VAC requires 16-18" to pull to the limit, so it doesn't pass my "two inch rule" even for a manual trans, and definitely not for PG, but that's what's in most parts books.

For base cam engines with manual trans the nominal 15" (figure +/- 1" tolerance on all nominal data) B22 (NAPA/Echlin VC1802 or equivalent) is fine. For PG I recommend the 12" VC1765 (B20) or VC1808 (B26). They both have the same specs. I consider these also to be "optional" for base cam engines with manual trans.

The 8" VC1810 (B28) is required for all SB mechanical lifter cam engines and was also used on L-79, but since L-79's should pull about 14-15" @ 750-800 idle speed, the 12-incher is optional for them. The 8" VAC too aggressive for a base cam engine and may cause part throttle or transient detonation, but mechanical lifter SBs only pull 10-12" at idle, so they need the 8" VAC or they will have idle stability and hot running problems.

The 12" VAC is also best for all BB through '67. Those with ported vacuum advance should be converted to full time vacuum advance and the VAC on L-78/71 replaced with a 12 VAC since OE was 15.5". L-36/72 had 12 VACs as OE, but you never know if it's right or if it works unless you test it.

You can cross reference the NAPA/Echlin part numbers at the Standard Ign. Products, Echlin, or BWD web sites. Since SMP now owns all these brands, it's the same part as long as it has the correct stamped ID number regardless of the name or part number on the box.

Delco also offers VACs, but I don't trust their catalog callouts or cross references, and whatever they sell, unless you happen on a part that's been in stock for about 15 years, is made by SMP and will have the same ID numbering scheme, not the original GM/Delco scheme.

Duke

[Howard M.]

Re: C1 vacuum advance. Thanks, on my way to NAPA *NM*

[Mark H.]

Same recommendation?

use the recommended base can for an L75 midyear w/manual trans?

[Duke W.]

Re: Same recommendation?

Yeah, except it changed just about every year.

Duke