B28 VAC (vacuum advance control)

[Duke W.]

The OE part number is 1115236 and the nominal specs are start @ 4" Hg, max 16 deg. crank advance at 8", and this VAC necessary for engines that idle at 12" Hg or less in neutral if a manual trans, and DRIVE if an automatic if they have full time vacuum advance. This will ensure that total idle advance is "locked" at idle speed. Otherwise, if there is not enough manifold vacuum to keep the VAC locked at full advance, total idle advance will be lost, which will further reduce manifold vacuum causing more idle speed loss and so on and so forth until the engine stalls.

GM stopped producing VACs for the single point and HEI distributors in the nineties, but Standard Motor Products has produced most of them since then. They are available in numerous "brands" including Delco, but all are manufactured by SMP and carry a unique ID for their various specs. A dozen or more may still be available, but ONE of the following three will be best fit using the Two-Inch Rule for any engine with full time vacuum, and they are the 8" B28, 12" B26, and 15" B22.

In recent times the B28 has dried up in most brands and the last remaining stock as of a couple of years ago was the Airtex 4V1053. But now this part has become the WVE 4V1053, and there are many reports that it doesn't meet spec, specifically requiring materially more than 8" Hg vacuum to pull to the limit.

So I did a simple web search "WVE auto parts", and got their Web site, https://wellsve.com/ ,wrote them a note about the problem and what happened to Airtex. It's tough to keep track of all the acquisitions/mergers in the auto parts industry. Here's the rsponse:


Hi Duke.

Thanks for reaching out. Wells and Airtex unity predate me. We do still get some folks that are looking for Airtex info.

Yes we have had several inconsistencies on the part number 4V1053 and its specs. This is something that was conducted some time ago however, it does take time to sort out completely. The verified parts have a stamping on the plate of "B28".

Hope this helps.

Dan Shreve
Technical Services Advisor

Wells Vehicle Electronics



So it looks like they understand the issue and are working on it. Rock Auto may be the best source, and they have a good return /exchange policy. See this nearby thread about a quest for an in-spec B28:

https://www.forums.ncrs.org/showthread.php?132872-69-vacuum-advance-for-L71

Duke

[Dennis D.]

Re: B28 VAC (vacuum advance control)

Ordered 2 from rockauto. Will test and report back.

Btw. I have a b28 in my 70 LT-1 now. I pulled the vacuum hose, (manifold vac),. There is vacuum sucking at the hose, but no change in idle when it's pulled .

[Duke W.]

Re: B28 VAC (vacuum advance control)

LT-1 engines are exhaust emission controlled engines and OE is ported vacuum advance. So either your vacuum source is not full time or the VAC is dead.

It's easy to check both with a Mighy Vac or equivalent vacuum pump/gage.

Duke

[Dennis D.]

Re: B28 VAC (vacuum advance control)

Manifold vacuum on my 70 goes through the TCS system. I bypassed that and use full manifold vacuum. It's there when I pull the distributor hose, so I'm assuming bad b28. Broke my vacuum pump, but the mityvac I ordered should be there when I get home.

[Gary S.]

I know this is an old thread, but the B28 problem is ongoing. Just got a WVE 4V1053 from RockAuto and it does not have the B28 stamp, rather the 20210628 others saw in 2022. Starts at 10", not pegged until 17" on my Mityvac, which has not had heavy use.
I'll contact RockAuto. Who else sells them?

[Duke W.]

That sounds like a former Standard Motor Products B1, which is a boat anchor! It won't even pass the Two-Inch Rule for a base small block that idles at 18" Hg.

...suggest you contact WVE starting with the link I provided in my original post. Forget Rock Auto. They are clueless! Part is parts! You have to contact the manufacturer and get them squared away like I though I did two years ago, but i guess it didn't stick.

You have to remember that these "Technical Service Advisors" are typically the age of our grandchildren (for those of you who have them), and they may not understand things like like the Two-Inch Rule, and they may not understand why different engine configurations may need VACs that have different start and stop specs. So be patient and try to explain to them the basics of how different engines require different spec VACs... like a wise old grandfather.

For reference the SMP B28 VAC is a replacement for the OE 1116236 whose nominal specs are start at 4" Hg with maximum crank advance of 16 degrees at 8" Hg manifold vacuum.

This VAC is necessary for OE engines that idle at 12" Hg or less manifold vacuum (or OE ported vacuum advance that are converted to full time vacuum advance). The above family includes ALL OE small block mechanical lifter camshafts, but given the age of vintage Corvettes and the tendency of cams to be replaced by aftermarket cams over the years that have higher overlap (which is a big determining factor in idle vacuum), one should ALWAYS carefully measure idle vacuum (in neutral with a manual trans and Drive with an auto, AC compressor engaged if equipped) before choosing a VAC for an engine with OE full time vacuum advance or one converted from ported to full time.

Virtually ALL OE emission controlled engines have ported vacuum advance - no vacuum advance at idle. This increases EGT to help oxidize HC and CO and is still used on modern emission controlled engines to keep the catalyst bed hot enough to oxidize HC and CO (and reduce NOx) at idle an low load operating conditions, but it deceases exhaust valve life due to higher average EGT and increases fuel consumption, especially on vintage engines.

Duke

[Gary S.]

Thanks, Duke.
I do need to deal with RockAuto, at least to get my $ back, which won't be a problem.
I'll contact WVE tomorrow.

Gary

[Gary S.]

Duke,
Talked on the phone with a guy at WVE, who was sympathetic but unaware of the issue. Mentioned Dan Shreve who you worked with in 2023. He apparently quit a couple weeks ago. We don't know if the issue is fixed at WVE or not; this could have been old stock that RockAuto got from some other warehouse.
He did say Carquest/Advance sells it as CAC2094, so I'll work with my local store, I guess.
$38 there; RockAuto was $23 + $8 shipping

Gary

[Rocco S.]

Gary,

I had a similar experience with a NAPA B26 vacuum advance that Duke recommended for my 1967 L79. I ordered one fom my local NAPA store but told the parts guy that when it arrives I want to test it with my Mighty Vac and if it doesnt meet the spec's I didn't want it. When it arrived it did not meet the specs not reaching full movement until 18 inches.

So what to do now? Use a Crane aftermarket unit that is adjustable? Problem is they look differnt than an OEM unit and will not pass judging.

Rocco

[Gary S.]

Rocco,
I also got a Standard VC181 stamped B26 from RockAuto this week. Starts at 6"Hg, stops at 14", not as bad as yours but still >12"Hg spec. Don't know how many degrees it pulls as it's just on the workbench. Maybe as I build an assortment I'll caliper measure their travels.

[Duke W.]

Duke,


He did say Carquest/Advance sells it as CAC2094, so I'll work with my local store, I guess.
$38 there; RockAuto was $23 + $8 shipping

Gary

These VACs cost about ten bucks prior to the Biden administration.

Once you have a "brand" and part number I suggest buying VACs from a local parts store and check it with a vacuum pump to ensure the start/stop points meet the nominal spec. Allow +/- 1" Hg tolerance.

There are only three that are "best fit" for any engine with a single point or TI distributor, and these are the old Standard Motor Products IDs stamped on the mounting bracket

8" B28
12" B26
15" B22

Even if you think your engine has an OE cam always verify idle vacuum after going through the idle speed mixture adjustment procedure. Select and idle speed as low as possible commensurate with acceptable idle quality. If a manual trans it should be in neutral, Drive if an automatic and if equipped with AC the compressor should be engaged.

Duke

[Gary S.]

Duke,
Well, went to Carquest/Advance store this afternoon ISO the B28. Not in stock there or the other store in town, no surprise. What was surprising is when they said all the West Coast Carquest/Advance stores are being shut down in a few weeks, and they can't even order anything!
So I'll have to go the internet order route and they will only ship to me, not the store.

[Mark E.]

Gary,

It's frustrating dealing with parts quality issues. A suggestion that may mitigate the 2" rule and your idle issues: try ported vacuum and compare results. Many engines I've worked on perform better with ported but the trick is more initial timing.

For example, my LS5 likes 12 degrees initial, 38 degrees total (26 centrifugal from the distributor) with zero vacuum advance at idle (using ported vacuum). This provides adequate advance to control exhaust heat at idle yet no bucking from too much advance that can occur using manifold vacuum.

This approach also avoids off-idle stumble that can occur using manifold vacuum. Using manifold vacuum, when the accelerator is depressed from idle ==> vacuum drops ==> VAC RETARDS timing just when the engine wants more advance ==> stumble.

To compound this, when manifold vacuum is used, the amount of initial timing the engine can handle is limited because the VAC is engaged at idle. For example, let's say the engine can handle 15 degrees of total advance at idle without bucking and the VAC provides 14 degrees of this. This limits initial timing to 1 degree BTDC. When the accelerator is depressed from idle ==>VAC retards timing from 15 to 1 degrees ==> stumble. Not a recipe for responsiveness.

With ported vacuum: 15 degrees initial at idle ==> accelerator is depressed ==> centrifugal advances timing smoothly from there ==> good throttle response.

So experiment and compare ported with manifold vacuum. The 2" rule becomes moot but you need to make sure you have maximum VAC during cruise.

You'll need different centrifugal advance curves and total advance for ported vs. manifold. Years ago, Steve Newsom in Florida 850-776-7334 rebuilt several distributors and created custom ignition curves for me for a reasonable price. He's just a great guy with a bunch of Sun machines. I don't know if he's still doing this but you might give him a shout.

[Patrick H.]

You'll need different centrifugal advance curves and total advance for ported vs. manifold. Years ago, Steve Newsom in Florida 850-776-7334 rebuilt several distributors and created custom ignition curves for me for a reasonable price. He's just a great guy with a bunch of Sun machines. I don't know if he's still doing this but you might give him a shout.

Yes, I just received one back from him this week.

[Gary S.]

Here's my PM to Mark that he's answering. we were discussing the Accel adjustable vac can, which is supposed to fully actuate at 10" Hg up to 20degrees.

I have an L36 with an Isky 265/275 cam which produces 15" vac at idle 800rpm. Been using a Pertronix adjustable vac that my builder had put in, but it's suboptimal as it requires 15"Hg to advance 16 degrees crank.

• Overall the little problem I have is, I think, a "stopping violation" of the two-inch rule; when I stop the car a bit abruptly and the rpm drops, the engine can't pull itself out of the hole due to lack of advance unless I tap the throttle right away or increase idle to, say 900rpm. Two factors, I believe:
  1) My centrifugal gives 36deg instead of spec 32--probably lost it's bushing, so initial is 2degrees to keep total at 38. Compensated partly by light springs so centrifugal starts at 500 rpm and is 10deg at the 800rpm idle I'm forced to use, but falls into the hole on abrupt stop.
  2) the 2 inch rule violation as my current Pertronix vac needs all my 15" vac to give 16degrees; lose a few more degrees when I get the stop dip.
  
  I know the answer, according to Lars' writings, is to get my centrifugal limited with a little weld and file job by somebody skilled who has a Sun machine, and I found somebody in the area, so probably will do that.
  So goal per Lars is to have initial 16-18deg, total 38deg, with a 10-12 deg vac that obeys the two-inch rule.

So the Accel adjustable can work, if I can get one that does what it is supposed to by being all in at 10"Hg, or I need to find a B28 that then needs to be modded to reduce travel.