B28 VAC (vacuum advance control)

[Gary S.]

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.

good suggestion, Mark, I do like to experiment a bit...but I don't believe my old-fashioned 67 L36 has a ported vacuum source

[Duke W.]

If the engine pulls 15" at 800 and you're okay with that idle speed (don't want to pursue a lower idle speed) then a 12" B26 meets the Two-Inch Rule - NAPA VC-1765, and any other parts store should be able to cross reference this to whatever "brand" they sell.

If the engine has a tendency to stall on a quick stop and you don't have time to downshift you may be pushing the clutch in too early or too late.

Early closed chamber big blocks don't seem to like as much low rev advance at either light or full load than small blocks. The last '66 L-36 I set up we thought about limiting vacuum advance to 12 degrees, but ended up replacing the OE 30 degree cam assembly with a 32 degree (OE on '67 L-36) and only ran 4-6 degrees initial to maximize low speed smoothness, so 36-38 total WOT advance. I think it has the gold springs from the Mr. Gasket 928G spring kit. I don't recall when all centrifugal is in with the gold springs (probably about 3500), but OE is 5000, which is really lazy.

All indications were that the cam is OE equivalent. Idle is 600 @ 15" with just a hint of lope, and with 16 max. vacuum advance total idle advance is 20-22 (don't think the centrifugal starts as low as 600), which seems low, but like I said early big blocks with that funky "dual quench" combustion chamber don't seem to like as much low rev advance as small blocks even though they like about the same high rev total WOT advance.

I've not worked on a later open chamber big block, but I'm sure they like and can tolerate a lot more low rev advance, so an optimum spark advance map for an open chamber big block is not optimum for a closed chamber version and vice versa due to the substantial difference in combustion chamber shapes.

Duke

[Gary 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

Rocco, as I seem to be starting a small vac can collection (possibly safer than bitcoin but less upside potential), I also got a Napa VC1765 B26 today in addition to the Standard VC181 mentioned earlier. They both test out the same, firmly pegged at 14.5". Like you, local store didn't have any to choose from, so had to order. If you want one of the two, PM me, or search along with me to find a B28. I'm waiting to get back the RockAuto replacement and the Carquest/Advance version.

Gary

[Gary S.]

If the engine pulls 15" at 800 and you're okay with that idle speed (don't want to pursue a lower idle speed) then a 12" B26 meets the Two-Inch Rule - NAPA VC-1765, and any other parts store should be able to cross reference this to whatever "brand" they sell.

If the engine has a tendency to stall on a quick stop and you don't have time to downshift you may be pushing the clutch in too early or too late.

Early closed chamber big blocks don't seem to like as much low rev advance at either light or full load than small blocks. The last '66 L-36 I set up we thought about limiting vacuum advance to 12 degrees, but ended up replacing the OE 30 degree cam assembly with a 32 degree (OE on '67 L-36) and only ran 4-6 degrees initial to maximize low speed smoothness, so 36-38 total WOT advance. I think it has the gold springs from the Mr. Gasket 928G spring kit. I don't recall when all centrifugal is in with the gold springs (probably about 3500), but OE is 5000, which is really lazy.

All indications were that the cam is OE equivalent. Idle is 600 @ 15" with just a hint of lope, and with 16 max. vacuum advance total idle advance is 20-22 (don't think the centrifugal starts as low as 600), which seems low, but like I said early big blocks with that funky "dual quench" combustion chamber don't seem to like as much low rev advance as small blocks even though they like about the same high rev total WOT advance.

I've not worked on a later open chamber big block, but I'm sure they like and can tolerate a lot more low rev advance, so an optimum spark advance map for an open chamber big block is not optimum for a closed chamber version and vice versa due to the substantial difference in combustion chamber shapes.

Duke

Thanks Duke. I'd prefer a lower idle speed. It runs a decent idle at 700 which gets even smoother with even more advance than I currently have it, but I don't want to push my total >38 WOT or >54 cruise.

The stall occurs even if the clutch has been in several seconds, braking gently to 3mph, then brake a little harder to a full stop in the last couple yards. If I have the idle up to 900, the rpm dips to 600 but the engine can pull itself out of the hole.

There is no stumble off idle, at least the way I drive; I haven't tried to make it stumble.

As mentioned to Rocco, the currently available B26's that I have need 15".

I suspect my distributor cam may not be stock as it is 36deg and requires the black soft springs to be all in at 3500.


Interesting what you say about the initial on the 66. Those settings you used are close to where I'm at now with an 800 idle speed (10 deg initial, 26 with the vac. Vacuum is 16" at 800. Total 38deg)

So maybe if I go through the trouble of limiting the centrifugal to the Lars formula (which I took from a discussion about a 68 L36 with closed-chamber heads), I might end up unhappy at low rpms?



Do you know if the 66 had stock CR or not? Mine is appx 9.2:1, dynamic calculates to 8 on the UEM piston online calculator that adds a 15 deg fudge to .050" intake close time.

Gary

[Duke W.]

The engine had been rebuilt back in the eighties... OE cam, but not sure of the CR. Detonation was not an issue with 91 PON California premium. If your static CR is only 9.2:1 detonation should not be an issue. Low speed/load advance on closed chamber big blocks seems to be limited more by driveability issues ("trailer hitching") rather than detonation if CR is no more than about 10:1.

Duke

[Gary S.]

Update:
The replacement WVE 4V1053 I got back from RockAuto in exchange for the imposter had correct B28 stamping and reasonably within specs; starts pulling at 5"Hg and all in at 9.5".

Likewise, a CAC2094 ordered from CarQuest/Advance had the B28 stamp and same performance. That one shipped directly from WVE but was in a CarQuest box.