Vacuum canister

[Tom L.]

The vacuum canister on my '72 LS-5 is shot and I need to get a replacement. The engine was rebuilt some time before I bougt it and don't know if the rebuild was totally stock. Rather than simply order a stock replacement I checked manifold vacuum to see what it was to be sure I got the right one.

I read throught the archives and found that there are basically 3 different VAC's:
"B28" 0@4" 16@8" (idle vacuum less than 14")
"B20" 0@6" 16@12" (idle vacuum 14-17")
"B22" 0@8" 16@15" (idle vacuum greater than 17")

At idle I have 14" of vacuum, which would be the best choice? thanks!

[John H.]

Re: Vacuum canister

The vacuum canister on my '72 LS-5 is shot and I need to get a replacement. The engine was rebuilt some time before I bougt it and don't know if the rebuild was totally stock. Rather than simply order a stock replacement I checked manifold vacuum to see what it was to be sure I got the right one.

I read throught the archives and found that there are basically 3 different VAC's:
"B28" 0@4" 16@8" (idle vacuum less than 14")
"B20" 0@6" 16@12" (idle vacuum 14-17")
"B22" 0@8" 16@15" (idle vacuum greater than 17")

At idle I have 14" of vacuum, which would be the best choice? thanks!

Lynn -

Following the "2-inch rule" (VAC should be fully-deployed at least 2" Hg. below normal idle vacuum), you want the B20 VAC.

[Duke W.]

Re: Vacuum canister

Kudos for researching the archives to find the information, and then running the idle vacuum tests. That's the way to select a VAC.

Follow John's advice.

Duke

[Tom L.]

Re: Vacuum canister

Using the 2" rule, that's what I thought, just wanted to be sure, there were no helpful markings on the old one (just a B9).

One thing I forgot to ask and thought of after posting...Is 14" manifold vacuum correct for a healthy LS-5? I found no data on this. Thanks!

[Joe L.]

Re: Vacuum canister

Using the 2" rule, that's what I thought, just wanted to be sure, there were no helpful markings on the old one (just a B9).

One thing I forgot to ask and thought of after posting...Is 14" manifold vacuum correct for a healthy LS-5? I found no data on this. Thanks!

Lynn-----


A "B9"-stamped vacuum control is equivalent to a GM #1115357 or GM #1973436. It requires 7-9" Hg to start the plunger and produces 20.5 crankshaft degrees of advance at 16-18" Hg.

[Duke W.]

Re: Vacuum canister

One thing I forgot to ask and thought of after posting...Is 14" manifold vacuum correct for a healthy LS-5? I found no data on this. Thanks!

That sounds about right, maybe a little low, depending..., but I'd like to see some input from others. The LS-5 has the same camshaft as the L-36/68, and valve overlap is a big determining factor in idle vacuum. The other big factor is idle speed. Increasing idle speed increases vacuum and vice versa. Spark advance is also a factor and ported advance will usually yield slightly less vacuum at the same speed than full time advance.

So you need to specify exact idle conditions including RPM and whether a manual or auto transmission.

With a manual transmission vacuum @ idle speed should be specified with the trans in neutral. If an automatic, in Drive.

Duke

[Tom L.]

Re: Vacuum canister

The car is a automatic, I took vacuum readings in neutral- 14" at 800RPM and in drive- 14" at 650RPM. I saw no change so I didn't note it.

[Duke W.]

Re: Vacuum canister

That's odd. You should lose an inch or two idling in Drive from the idle in neutral condition. It may be he ported advance.

I recommend you check the VAC before installing to be sure it meets spec. If you have a Mightyvac, take it to the store with you.

Duke

[Tom L.]

Re: Vacuum canister-HELP!!!

Duke, I took your advice and using a cross refrence (taht I believe was from a post by you) crossed it to a B20 and found that it should be a Standard #VC-181. I oredered it (it wasn't on the shelf) and when I opened it after getting home it is stamped B26. Is this still a correct VAC for a '72 LS-5 auto? I'd hate to put it together only to find that it is not correct. Thanks.

[Duke W.]

Re: Vacuum canister

To the best of my knowledge B20 and B26 have the same specs, but test it with a Mighty Vac before you install and on the engine after it's installed.

Duke

[Stuart F.]

Re: Vacuum canister

Lynn;

Yes, do as Duke suggests: Test it. There are a lot of generic "one number fits all" VAC's out there and others that are stamped correctly for your application to suit judges, but do not perform as they are supposed to. I've had to send a number back that I ordered for that reason.

Stu Fox

[Joe L.]

Re: Vacuum canister

To the best of my knowledge B20 and B26 have the same specs, but test it with a Mighty Vac before you install and on the engine after it's installed.

Duke

Duke and Lynn-----


Yes, the "B20" and "B26" have the same specifications. However, the "B20" used to be what you found in a NAPA/Echlin box marked VC-1765 and the "B26" was marked VC-1808. I believe that the "B20" was originally designed to be equivalent to the GM #1115355 whereas the "B26" was a replacement for the GM #1973448. Curiously, though, the 1115355 and 1973448 had the same specs, too. I don't know why GM did this since both were simultaneously available. And, I don't know why Standard Motor Products (manufacturer of all the "B"-stamped vacuum controls) followed tham and had available two controls of the same specs. Maybe they finally figured that out and are now using the "B26" only.

[Duke W.]

Re: Vacuum canister

I'm not surprised. Duplication of parts is not that uncommon. Whoever speced out the ...438 didn't look at was was already released.

The aftermarket just copies the OEMs - mistakes and all!

Duke

[Tom L.]

Re: Vacuum canister

OK, I had a chance to bench test the B26 VAC to see if it matches B20 spec as noted and found the following... I slowly applied vacuum to the canister while observing the linkage that connects to the breaker plate. Movement began at 8" and it reached its maximum at 12". This closely matches the specs that I found here on the TDB: "B20" 0@6" 16@12" (idle vacuum 14-17").
I did not check to how advance was effected by slowly applying vacuum to the VAC from an outside source but did check the timing after re-connecting to a full time vacuum source. Timing was about 23 degrees at a 800rpm idle. It should have been 24 (8 initial + the 16 that the VAC is supposed to provide) but I feel comfortable knowing I am either close enough to spec or my measurements are a tad off, there is little room to see on these motors!

I took it for quick ride and it seemed much better than before, I'll be taking it for a longer run tomorrow. Thanks again for all the input.

[Duke W.]

Re: Vacuum canister

Good job on the testing. It's within reasonable production tolerance. The fact that it's a little "slow" to start is okay as this will help mitigate part throttle and/or transient detonation.

The most important spec is the vacuum required to achieve full advance, and it's right on the money.

Duke