Variable Vacuum Advance Cans

[Mark E.]

A question for Duke, Joe, Stuart and the other ignition timing experts: What do you think of the aftermarket variable vacuum advance units that are available? One brand is Accel:
http://shop.accel-ignition.com/ignit...points-v8.html

I installed one years ago in a 72 Chevelle 350 with good results. I was able to dial in a vacuum advance curve that worked well for the engine. The link has a document showing how the curve varies as the can is adjusted.

I know this isn't for judging, but it worked well for me in the past. It may be a good alternative to searching for those hard to find canisters; plus it enables fine tuning of the vacuum advance.

[Duke W.]

Re: Variable Vacuum Advance Cans

I've never recommended them. In fact, I've probably recommended avoiding them. Of the OE replacement VACs, all of which are manufactured by Standard Motor Products regardless of the brand name on the box, there is only one of three that is correct for any OE engine/transmission combination that is either OE full time manifold vacuum signal or converted from ported - IDs B22, B26, and B28.

I have listed specs for each and the applicable engine/trans combinations that each is suitable for many times. A couple of OE full time advance applications may not really be "OE equivalent" because the OE spec was poorly thought out - basically screwed up ('63 L-76 and '65 L-78 come to mind), and converting ported applications to full time will usually require a different than OE spec VAC; and you can always use the Two-Inch Rule if you think you have a non-OE cam.

The "adjustability" of these "adjustable VACs" is one dimensional - maybe 1.5 dimensional. You can adjust the total amount of vacuum advance, which also alters the start point, but the maximum is always all in at about 10", which is too aggressive for many OE engines.

Plus they look hokey and cost three times as much as a SMP OE replacement. The B28 is apparently no longer being manufactured and is getting hard to find, but they still appear to be available in the Airtex brand while stocks lasts, so if you have a mechanical lifter small block you should think about buying a spare.

I really can't think of anything good to say about them.

Duke

[Frank D.]

Re: Variable Vacuum Advance Cans

I replaced the dual point (no vac advance) dizzy on my '61 270hp car with a Pertronix billet distributor and added an adjustable can (the one mentioned above).....it worked out fine. I don't know that it looks all that bad. I dialed in my curve right where I wanted it.
That was IIRC about 6 years ago on a car that is driven often and hard - haven't even thought about it since.

With the tremec 5-spd OD tranny and vac advance I can get 22 MPG if I keep my foot out of the dual quads.

Here is the spec curve on the device:

[Duke W.]

Re: Variable Vacuum Advance Cans

So what are the current spark advance map specs for your 283/270?

Duke

[Frank D.]

Re: Variable Vacuum Advance Cans

I don't have a SUN machine so I did the job with a dial back timing light...

12* initial (vac plugged) @850 RPM
36* (vac plugged) "all in" @2500 RPM
52* (vac operating)....of course this is an estimate as I can't simulate cruise loading....I know many prefer 48* but I was able to crank in a little more for my particular engine.

Perhaps I could have gone with a fixed can; the adjustable worked fine and the car runs superbly.

[Duke W.]

Re: Variable Vacuum Advance Cans

That looks close, but I recommend at least 38 deg. total WOT advance, so you should try bumping the initial another couple of degrees or even up to four. The actual true CR of Flint-built 270s is only about 9:1 and many will operate detonation-free on 87 PON fuel.

Maximum cruise advance can be measured by revving the engine to a few hundred revs above the point of maximum centrifugal with the VAC connected and reading with a dial back light.

The B28 VAC is necessary to meet the Two-Inch Rule for all SB OE mechanical lifter cams with full time vacuum advance.

Since the adjustable is all in by about 10" it will work okay for Duntov and LT-1 cams, but doesn't pass the Two-Inch Rule with the 30-30 cam.
Duke

[Frank D.]

Re: Variable Vacuum Advance Cans

I'll tinker with it a bit then when I get time... Although the lil ole 283 sure runs like a scalded dog as is...

[Mark E.]

Re: Variable Vacuum Advance Cans

Since the adjustable is all in by about 10" it will work okay for Duntov and LT-1 cams, but doesn't pass the Two-Inch Rule with the 30-30 cam.
Duke

Thanks for the input, Duke. It sounds like a major concern with the variable VAC is how much vacuum is needed for it be fully advanced, regardless of its adjustment, which may be more vacuum than some camshafts provide at idle. Right?

Question about your sentence above: I thought "30-30" referred to the Duntov cam. Is there a distinction?

[Frank D.]

Re: Variable Vacuum Advance Cans

The 30-30 cam is NOT the Duntov 097 cam......that I know.
I'll have to let others cite the specific differences...

[Duke W.]

Re: Variable Vacuum Advance Cans

Thanks for the input, Duke. It sounds like a major concern with the variable VAC is how much vacuum is needed for it be fully advanced, regardless of its adjustment, which may be more vacuum than some camshafts provide at idle. Right?

Question about your sentence above: I thought "30-30" referred to the Duntov cam. Is there a distinction?

Just the opposite. For example, base engines with manual transmissions idle in neutral at about 18" @ 500, so a VAC that provides full vacuum advance at 10" is too aggressive and will have more tendency to cause detonation. The B22, which requires 15" for maximum advance is less aggressive, but still passes the Two-Inch Rule. L-79s and SHP big blocks pull about 14-15" at typical idle speeds, so the 12" B26 is spot on.

The Duntov cam (3736097) was used on all mechanical lifter engines from '57-'63. It was superceded by the 3849346 mechanical lifter camshaft for '64 and '65. It is called the "30-30" from the recommended valve clearances in owner's and service manuals, which are NOT the same as the clearance specified on the engineering drawing, but that's a whole other story, which has been discussed many times.

The 30-30 is sometimes referred to at the "Duntov 30-30", but that is a misnomer.

At about 900 idle the Duntov cam pulls about 12", but the 30-30 only 10" manifold vacuum, so the referenced adjustable VAC passes the Two-Inch Rule with the Duntov cam installed in Franks 283/270, but doesn't pass in a 30-30 cam engine. All mechanical lifter small blocks with full time vacuum advance should use the 8" B28 VAC.

Small blocks with automatic transmissions usually need a more aggressive VAC than the same engine with a manual trans because automatics pull less vacuum idling in Drive, so the 12" B26 is best for a 327/300 with Powerglide.

The best VAC for any engine with full time vacuum advance is the least aggressive VAC that passes the Two-Inch Rule.

Duke

[William B.]

Re: Variable Vacuum Advance Cans

This is for Duke. I've got a SB 327/300 powerglide with a very mild cam. It pulls 12"@650 and currently has a B26 VAC. To me this doesn't appear to meet the 2" rule. Would you go with a B28 or stay with the B26?

Ken

[Mark E.]

Re: Variable Vacuum Advance Cans

Thanks Duke!

[Duke W.]

Re: Variable Vacuum Advance Cans

This is for Duke. I've got a SB 327/300 powerglide with a very mild cam. It pulls 12"@650 and currently has a B26 VAC. To me this doesn't appear to meet the 2" rule. Would you go with a B28 or stay with the B26?

Ken

The only "very mild cam" I know of is the OE 300 HP cam, which should idle very smooth and steady at 450-500 in Drive. So you must have a higher overlap cam if it won't idle acceptably lower than 650.

You didn't say, but with an automatic transmission idle speed/vacuum must be measured and reported idling in Drive. So assuming this is what you report a 12" VAC does not meet the Two-Inch Rule, and you should try a B28, which passes the Two-Inch Rule and may improve idle quality.

Just as a reminder, high overlap cams and automatics are like mixing amonia and bleach. There are well proven ways to get "more power" from automatic backed engines with a low overlap cam than will idle just like OE and have plenty of low end torque.

Duke