What is a B1 vacuum advance?

[Patrick B.]

Re: What is a B1 vacuum advance?

Last year I used the VC1765 from Napa for a rebuilt stock L71 using manifold vacuum. It is marked B26.

According to the Napa link below, one of the photos shows B26 marked on the flange. I did a search for VC1765BC and it does not exist. Strange.
http://www.napaonline.com/Catalog/Ca...765_0195970883

Some results of vac advance testing in post#30, and specs from Duke's info in post#32 in this thread....
https://www.forums.ncrs.org/showthre...433#post533433

Note the M201 15 repro I bought was way out of spec.

For the record, I stayed with manifold vacuum using the VC1765 and never went back to ported vacuum. Engine ran great after final carburetor and timing adjustments.

Rich

Thanks Richard for the NAPA catalog reference to the correct part. The part I got from NAPA was VC1765SB not VC1765BC as I had written by mistake. I went to a NAPA store and asked for VC1765, and the guy said there was no listing for VC1765 but he had a listing for VC1765SB. Foolishly, I assumed they were the same thing. I will go back and show him the catalog listing you sent and hope they will take back the worthless B1's. They have no listings by B numbers, and these VC numbers are a little ambiguous.

[Michael J.]

Re: What is a B1 vacuum advance?

Michael -

The VC-1810 is the "B28" can, which is AC-Delco #D1312C or GM #88924985, and they're still available from GM; Rock Auto and GMPartsDirect.com have them too. I buy them regularly.

I had thought about trying those, but the NAPA guy said they were not engineered correctly, and that AC-Delco had messed up, to replace their 1810, and wouldn't work as well as the 1765 on my L71. Maybe I should try them.

[Richard M.]

Re: What is a B1 vacuum advance?

Hmmm, never saw reference to a VC1765SB but it clearly is shown on the Napa site marked as "B1".....
http://www.napaonline.com/Catalog/Ca...5SB_0312172580

The Technical Bulletin(attached below) written by Lars & Duke shows the Echlin part# for the B1 is VC680.

So I searched that one on the Napa site......
http://www.napaonline.com/Catalog/Ca...680_0168354755

Why Napa has two different part numbers for a B1 is confusing. The specs for each is not included on the site.

Rich

[Patrick B.]

Re: What is a B1 vacuum advance?

Richard-- Thanks for the great article and specs. NAPA took the VC-1765SB's back even though they were special ordered and ordered me the VC-1765's that hopefully will be B-26's as shown in the online catalog.

[Joe L.]

Re: What is a B1 vacuum advance?

Richard-- Thanks for the great article and specs. NAPA took the VC-1765SB's back even though they were special ordered and ordered me the VC-1765's that hopefully will be B-26's as shown in the online catalog.

Patrick------


I show the NAPA VC-1765SB to be a "B10". Are you sure the ones you got were "B1"? The specs for the "B1" and "B10" are completely different. The "B10" is equivalent to the old GM #1115360. The NAPA VC-680 should be the "B1".

The VC-1765 should be either "B20" or "B26".

[Patrick B.]

Re: What is a B1 vacuum advance?

Joe-- it was definitely B1. I returned the two units I bought and hopefully the VC1765s I will get will be B26's .

[Joe L.]

Re: What is a B1 vacuum advance?

Joe-- it was definitely B1. I returned the two units I bought and hopefully the VC1765s I will get will be B26's .

Patrick-----


I wonder if these were mislabeled? I just don't think the 1765SB should have been a B1.

[Patrick B.]

Re: What is a B1 vacuum advance?

Joe- They were definitely marked B1 and took a lot of suction to move the plunger. They could have been misboxed.

[John D.]

Re: What is a B1 vacuum advance?

According to a Delco Remy specification manual a 163 and a 201 vacuum advance are close but not the same.

'163 specs in laymans terms. : Starts at 8 to 10. All in at 16.25 to 18. Moves 8* which is 16* crank.

'201 specs ---------------------: Starts at 7 to 9. All in at 15-16. Moves 8* which is 16 crank

'236-16 specs------------------: Starts at 3 to 5. All in at 5.75-8.25. Moves 8.2*

Error factor in all of the above is 1 degree plus or minus.

Credit for the above information goes to Don Baker.
If I were you I would print it and save it.
JD

[Duke W.]

Re: What is a B1 vacuum advance?

The trouble with the 163 is that it may take as much at 18" to pull it to the limit. This is marginal - doesn't meet the Two-Inch rule for even base cam engines with manual transmissions that idle at about 18" @ 500 in Neutral, and it certainly won't work with an auto that idles in Drive at less vacuum.

The B22 is much closer to the 201, especially at the top end which is critical to stable idling.

Chevrolet used the 201 on the 327/300 in '62 and '63, changed the VAC in '64, again in '65, and again in '66, which carried over to '67, so it took them four years to "get it right".

The '66-'67 VAC is 12". Why? Because it works with Powerglide. Since a too aggressive VAC can cause detonation, I recommend the least aggressive that meets the Two-Inch rule. In the case of the 327/300 with a manual trans, the 15" B22 (or OE 201-15 on '62-'63 if it still works) is the best fit. For Powerglide the 12" B20 or B26 is the best fit.

Most guys don't understand how critical this simple ten dollar part is to idle stability, fuel economy, and detonation free part throttle acceleration, but those who switch to a best fit from one that was not properly matched to the application can't believe the difference it makes.

Duke

[John D.]

Re: What is a B1 vacuum advance?

Duke, A couple of weeks ago one of the gang here (Doug) was having over heating problems because a genius disconnected his VA. Remember that one? John

[Michael H.]

Re: What is a B1 vacuum advance?

But... JD, some engines were designed and built with no vacuum advance at all and they didn't over heat.

If my vacuum advance doesn't work exactly right, is my 425 HP 66 going to over heat?

[Duke W.]

Re: What is a B1 vacuum advance?

GM has many tests that a car must pass to enter production. One of them is likely an extensive idle test in hot weather. Engines that didn't have vacuum advance up to '62 might have required larger radiators or more low speed fan flow.

Engines that were designed for full time vacuum advance are more likely to overheat if the vacuum advance is disabled because they were designed and tested with a properly functioning vacuum advance.

The Duntov-cammed FI engines from the fifties had short centrifugal curves, but added more initial to make up for it. Strangely, this did not apply to the 270 HP engine.

Duke

[Michael H.]

Re: What is a B1 vacuum advance?

GM has many tests that a car must pass to enter production. One of them is likely an extensive idle test in hot weather. Engines that didn't have vacuum advance up to '62 might have required larger radiators or more low speed fan flow.

Engines that were designed for full time vacuum advance are more likely to overheat if the vacuum advance is disabled because they were designed and tested with a properly functioning vacuum advance.

The Duntov-cammed FI engines from the fifties had short centrifugal curves, but added more initial to make up for it. Strangely, this did not apply to the 270 HP engine.

Duke

[Duke W.]

Re: What is a B1 vacuum advance?

That's true, but they had a lot less cooling margin than if they had vacuum advance at idle. Big blocks always had a reputation for "running hot" even when new, and many here have completely solved L-72/71 hot running issues by converting to full time vacuum advance.

I'm sure you've heard the stories of the '68 Corvette press intro. Duntov was really sweating hoping the big blocks wouldn't overheat.

Duke