Vacuum cans (again)

[Dennis O.]

I know this has been gone over several times and I have spent a lot of time searching, but I can't find the information I need on Vacuum cans. The "Search" function seems to simply ignore arguments such as "B20" "B26", and "B21". I have tried numerous combinations of "vacuum", "advance", "can", etc. and seem to get all kinds of unrelated threads.

Anyway, here is my question. I have a can marked "B21" on my '67 L79. I seem to remember seeing that this is the wrong can, and I should be using a "B20" or "B26". Is this correct? Also in the stock configuration, is the can connected to manifold vacuum or ported vacuum? If connected to ported vacuum, would it be advantageous to connect it to manifold vacuum?

I also spent a lot of time searching the internet in general trying to find a chart or something similar that shows the relationships be between the "B" numbers, the parts currently available from the various vendors and the specifications of the can. This does not seem to exist anywhere. I think it would be nice to have this information in one place.

Thanks for any help anyone can provide.

[Duke W.]

Re: Vacuum cans (again)

The 12" B20 or B26 (NAPA VC 1765) is the "best fit" for the L-79 WITH ORIGNAL CAMSHAFT DESIGN. It should be connected to a full time vacuum source.

Google lars grimsmund duke williams to find the vacuum advance listing on the Web. John Hinckley also has a paper on the Web that explains how to set up a spark advance map and why for vintage Corvette engines, and my San Diego presentation available from this Web site (see the "sticky" section at the top of the TDB page) includes a siimilar discussion and recommendations.

This stuff is all over the Web and thoroughly explains the spark advance requirement for all speed and load condition including idle and why exhaust emission control engines usually have ported vacuum advance, but pre-exhaust emission control engines usually have full time vacuum advance.

Duke

[Dennis O.]

Re: Vacuum cans (again)

Sorry Duke, I don't know why it didn't occur to me to "Google" such obvious terms as "lars grimsmund" and "duke williams". I looked for arcane terms such as "NAPA vacuum advance", "NAPA B20", etc. I'll try to do better in the future. The fact remains that terms like "B20", "B21", and "B26" are not valid search arguments on this site and what I consider to be other fairly fairly reasonable search arguments did not bring me to the items you mentioned.

Thanks for the information; you did answer my question.

[Joe R.]

Re: Vacuum cans (again)

Dennis,

If the search engine won't take B20 etc. because the search string is too short, put a star after it, e.g. B20*. That will find everything that starts with B20.

Joe

[Richard M.]

Re: Vacuum cans (again)

Dennis, The search needs 4 characters or more, but you can add wildcards to get better results. In the future try this in the search.....e.g. *B20* *B26*

It'll find occurrences of any characters before or after. This will include spaces, so B20 or B26 will show up. Of course it will also find strings like.....
AABB20515, B200030, etc, etc. So you'll get more than you wanted but may at least help you find what's pertinent.

I just tried it and found some useful posts, including a link to the spec document Duke referred to. Here's one.....
https://www.forums.ncrs.org/showthre...095#post618095


Rich

[Dennis O.]

Re: Vacuum cans (again)

Thanks for the suggestions. Is there anywhere the rules for the "Search" function are documented? I don't like bothering you guys anymore than you like being bothered.

[Richard M.]

Re: Vacuum cans (again)

In the FAQ link above near the top, there's a link to the Search info.