B26 bushing

[Carmelo P.]

1967 L79
Replacing a 236-16 vac with a B26 .
The 236 bushing was in found pieces.
Does the B26 also need a bushing on the pin riding in the slot ? Thanks...

[Harry S.]

Carmelo, my understanding is that the bushing was actually a spacer. The spacer(s) came in different thicknesses depending on the vacuum can. The spacer controlled how much vacuum was actually generated by a particular can. So, I believe the answer to your question is yes. I'm sure others will chime in.

[Duke W.]

The specs for the B26 are start at 6", 16 deg. @ 12", and I don't think they have a bushing. Bushings on OE VACs were there to limit total advance, and I don't recall that OE 236 16 VACs from service parts have a bushing, but I can't be sure.

The only way to tell how much advance is provided by any VAC is to test on the car. This is done by starting with a high idle speed where the centrifugal has clearly started with the VAC hose disconnected and pluged. Then, while watching the timing notch retard while reducing idle speed, determine the engine speed that centrifugal is fully retracted. If necessary, install stiff centrifugal springs for this test or temporarily tie up the centrifugal advance mechanism with rubber bands if you can't verify that centrifugal is fully retracted before the engine stalls due to to low engine speed, and set initial timing in the range for the specific engine configuration.

Once you can get the engine to idle at a speed that you know for sure is below the start speed of centrifugal advance and you know initial timing, connect the signal hose to the VAC. This will increase idle speed, so lower it down to below the speed that centrifugal starts, measure total idle advance and subtract initial. The result is the amount added by the vacuum advance, assuming it meets The Two-Inch Rule.

Duke

[Owen L.]

Would it also work to hook up a mity-vac to the vac-can while the engine is idling and pull vacuum to whatever in.hg you're testing for? For this method, I don't think dropping to zero mechanical advance would be necessary to get vac-advance readings. (yes?)

[Duke W.]

The trouble is that as you pump down the VAC it will add advance, which will increase engine speed, likely to the point where centrifugal advance is added.

IIRC the '67 L-79 centrifugal is start @ 900, 30 @ 5100, but check the AMA specs and CSM to be sure. The other issue is that it may not be OE as it was a common modification to quicken even back in the sixties, so you need to verify.

If it is OE then set initial timing with the VAC hose disconnected and plugged at 750, and the engine should idle stably if it has the OE cam. Then connect the signal hose to the VAC and reduce engine speed back to 750. You are now reading total idle advance. Subtract initial and you have the amount added by the VAC. You could also use the vacuum pump to determine not only added advance, but also the amount of vacuum required to bring it all in.

I always recommend buying VACs locally and checking that the part pulls the pin to the limit at spec plus or minus 1" Hg. If not, don't accept the part.

I'm assuming you have an OE cam, and if so it should idle at 750 @ 14-15" inches, so a 12" VAC meets the Two-Inch Rule. I also recommend you install lighter centrifugal springs to bring all 30 degrees in earlier - say 3000 or so.

I never understood why the L-79 spark advance map was set up with with laziest centrifugal curve from the base engine and the most aggressive VAC from the 30-30 cam engines. Given idle behavior a 12" VAC passes The Two-Inch Rule, and if you have 93 PON fuel available. even with the original CR, it should tolerate the very aggressive centrifugal of the 30-30 cam engines, which starts at 700 and should bring in all 30 degrees by about 3000, and 6-10 initial gets total WOT advance into the optimum max power ranger of 36-40.

Dukef

[Owen L.]

The trouble is that as you pump down the VAC it will add advance, which will increase engine speed, likely to the point where centrifugal advance is added.
Duke

<forehead slap>Ah, yes, the idle will rise with any timing advance so it becomes a problem in keeping the two separated in the readings. </forehead slap>

[Carmelo P.]

I'll try the B26 without the bushing first and see how that works out.
Distributor and 236-16 vac advance appear original...
Thanks for the responses...

[Rocco S.]

Carmelo & Duke Williams,

I have a 1967 L79 Stingray too. My car has TI and A/C. I posted awhile back inquiring about a recommendation for a replacement VAC and Duke recommended the B26. I ordered one from my local NAPA store and brought a Mityvac to check it. I did not meet the specifications. In the meantime I got busy with other projects and have not time to return to this yet.

I have always wondered what the original VAC Delco # was for my distributor. I also have been searching for specifications of various NOS VAC units I have. Original units are marked with the last 3 digits of the Delco part number and the maximum advance. It seems nobody has that info.

I was at Corvettes at Carlisle on Friday and found and purchased a Delco Remy Service binder filled with bulletins with test procedures and specifications for Distributors, Starters, Alternators and Generators dating from about 1979. In the distributor section there were 2 bulletins with tables showing VAC's in numerical order and complete specifications. I believe this to be very valuable info for anyone who has NOS VAC's or want to search for one with specific values.

I have attached the VAC tables for interested parties. Buy the way, the 1967 TI distributor, #1111157 was originally fitted with Delco Remy VAC CA to Reissue Black Plates Starts at 5-7 in Hg, ends 11-13 in Hg and 16 drgrees max. Note that the tables list distributor degrees which need to be doubled for crank degrees.

I hope the tables are attached. If not send me a PM and I will email to anyone interested.

Regards, Rocco
Delco Remy Vacuum Advance Specification Tables.pdf

[Carmelo P.]

Thanks Rocco for the info...

[Mark F.]

...I was at Corvettes at Carlisle on Friday and found and purchased a Delco Remy Service binder filled with bulletins with test procedures and specifications for Distributors, Starters, Alternators and Generators dating from about 1979. In the distributor section there were 2 bulletins with tables showing VAC's in numerical order and complete specifications. I believe this to be very valuable info for anyone who has NOS VAC's or want to search for one with specific values.

I have attached the VAC tables for interested parties. Buy the way, the 1967 TI distributor, #1111157 was originally fitted with Delco Remy VAC CA to Reissue Black Plates Starts at 5-7 in Hg, ends 11-13 in Hg and 16 drgrees max. Note that the tables list distributor degrees which need to be doubled for crank degrees. I hope the tables are attached. If not send me a PM and I will email to anyone interested. Regards, Rocco [ATTACH]n1582359[/ATTACH]

Hi Rocco,

3 questions:

1. The Table is dated 2-15-65. Is it valid for your '67 ? (I have a '67, too).
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2. What is the significance of the link to "CA to Reissue Black Plates" ?

3. How was Carlisle - I haven't been there in years, but it was huge when I was going every year from about 1998 thru 2008 (?)

[Rocco S.]

Mark,

Thanks for your three questions.

1. If you scroll down in the attachment you will see a few pages from another bulletin, dated 1977. In the 1977 table you will also see the VAC# 115355 specifications. So it was still a good number in 1977. The L79 engine was made from 1965 thru 1968. I believe this VAC was also used in many other Chevy distributors. I also have an extensive list of distributor #'s in numerical order showing both the centrifugal advance curves and the original VAC. If you need any specs for a specific distributor # let me know. Note that my L79 has TI. A standard, points distributor will have a different # and possibly a different VAC.

2. The link you refer to was a typo. I have no idea how that got inserted. Please disregard it.

3. This year was only my 2nd time to Corvettes at Carlisle. My old home was too far to make a day trip. Since I recently moved, its only a 2 hour ride. But I did notice there were lots of empty vendor spaces and talking with others, the gathering is not as good as in the past. I will say that prices for parts was very high. Not many people buying things.

[Mark F.]

Good deal, Rocco and thanks !

Yeah, sorry I didn't scroll down far enough...I'm astounded at how many VACs there are ! YIKES !

As for Carlisle...that's what I've heard (it ain't what it used to be)...
Anyhow, I think I went @ least 3 times as a vendor selling old parts I had...did very well getting rid of stuff and made a few bucks - which sometimes I spent on other things before I could get out of there

[Duke W.]

The 1116236 VAC was used on the '64-'65 365/375 HP engines and all L-79s, '65-'68. This very aggressive VAC that provides all 16 degrees of advance at only 8" Hg was necessary on the 365/375 engines because the very long duration, very high overlap 30-30 cam yielded poor idle quality. About the lowest that is acceptable is 900, and at that idle speed these engines only pull about 10", so a 8" VAC is necessary to pass the Two-Inch Rule.'

It is much more aggressive than necessary for the L-79 since L-79 typically pulls 14-15" at 750, so a 12" VAC is just right. For the four years that the L-79 was in production each year had what is essentially the same centrifugal specs as the 250/300 HP engines, which was the laziest of all Corvette 327s. So the L-79 combines the laziest centrifugal with the most aggressive VAC, which doesn't make sense. I figure the L-79 calibration group must have been different than for the other engines, and they didn't talk to each other.

Then there are the 250/300 HP engines. Beginning in '62 the 201 15 VAC was a pretty good choice with 15 degrees at 15.5" and with a manual transmission, a 500 idle in neutral yields about 18". But with only 24 centrifugal even at the lazy 4600+ at least 12 degrees initial is required to get total WOT advance into the bottom of the 36-40 ideal range, and that much advance can cause detonation in the lower rev range with the OE compression ratio.

The 250/300 HP engines have short duration, low overlap cams. The L-79 cam and more so the mechanical lifter cams have long duration and high overlap. The longer duration cams have late closing inlet valves which lowers the dynamic compression ratio, so they can tolerate more low rev advance. Also, high overlap causes lots of exhaust gas dilution which slows the rate of combustion propagation especially at low speed and light load, so you need to light the fire earlier, and they like much more advance in this low speed/load regime.

Back to the 250/300 HP engines. The '62 201 15 VAC carried over through '64, but centrifugal got a little more aggressive with 26 @ 4100. Then they screwed the pooch in '65 by installing a 22 @ 12" VAC (1116238). This caused driveability problems due to excessive low speed /load advance. A TSB was written on the problem. It's on the Web - probably on this site and the CF.

By '66 they almost got it right. Centrifugal was increased to 30 @ 5000 and max vacuum advance was 15 @ 12. The centrifugal was still super lazy, but that's a two minute fix by installing lighter springs. So now only 6-10 initial is needed to obtain the 36-40 optimum for best power, advance but it doesn't provide so much at low speed advance that it might detonate under high load.

Also note that full vacuum advance, 15, is in at 12" rather than 15" as on the '62 to '64 setup. Why is this?

Powerglide! PG should idle in Drive at about the same speed as a manual in neutral, but because there is load on the engine, manifold vacuum is below what a manual in neutral will pull at the same speed, and if total idle advance is not held steady, idle speed will fall and lead to engine stalling. That was the problem with my SWC's 340 HP engine. It would not idle without stalling for more than about 30 seconds unless I blipped the throttle. The "experts" I talked to said: "It's a racing engine, they all do that". It took me a couple of years for figure out that a 15.5" VAC was not functionally correct for an engine that only idled at 12", and by that time the 365/375 HP VAC, weights and springs were available from service parts so I installed them, which not only solved the idle stability problem, but noticeably improved low end torque due to the much more aggressive centrifugal advance.

I was also pleasantly surprised that the engine did not detonate since the Duntov cam inlet valve closes earlier than the 30-30 cam. I only needed Chevron "Blue Pump" premium, not "White Pump", even with up to 40 degrees total WOT advance all in at 2350. I didn't find out until I pulled the engine down for inspection (and found a badly cracked connecting rod) in the mid-seventies that the engine was built with double head gaskets. I later found the TSB on the subject, and because the decks were a bit high actual CR was not much more than 10.5:1.

It's nice to have the historical data on original VACs for our Corvette engines, but GM hasn't made those parts for decades. Also, due to some ill-conceived parts consolidation efforts over the years, VACs, like the B1 that are called out in parts manuals for base cam engines may require up to 19" to pull to the limit, so they won't even pass the Two-Inch Rule for a base cam engine.

There are currently only THREE replacement VACs that meet the Two-Inch Rule for all OE engines, or for that matter aftermarket cams as long as they are reasonable for a road engine.

As I previously stated GM has not manufactured these parts for decades... back to the early nineties when they began shedding all the parts divisions. Standard Motor Products (SMP) took up the task until recently. The 8" B28 was the first to go as usage is very low - only the 365/375 HP engines and L-79 that were exclusive options for Corvette. Finding an original MS 236 16 that actually works and is only lightly corroded is going to be tough, and expensive if you find one. Considering the VAC is only worth 3 points in Flight judging, there are likely more cost effective ways to avoid deductions. The following three VACs will be a best fit based on the Two-Inch Rule for the listed applications, along with the OE part number that it matches most closely in specs. These parts fit all single point and TI distributors

15" B22 (1116201) all base engines or optional engines with base engine camshafts and manual transmissions, NAPA VC-1802

12" B26 (1115355) all base engines or optional engines with base engine camshafts and auto trans, L-79, L-82 (man. trans.), all big block possibly with the exception of big blocks with AT, NAPA VC-1765

8" B28 (1116236)all SB with mechanical lifter cams, possible big block with AT, WVE 4V1053 (formerly Airtex 4V1053)

I always recommend verifying idle vacuum with an accurate vacuum gage. You may think you have an OE cam, but maybe you don't. If the car has a manual transmission set idle speed as low as you find acceptable in neutral. For automatics this MUST be done idling in Drive. In either case, if equipped with AC the compressor must be engaged.

Then use the Two-Inch Rule to pick the least aggressive of the above three VACs that meet The Rule.

As was the case when SMP made this parts, they were available in many different "brands including "Delco"... same part, just a different box and marketing channel. If you don't have a convenient NAPA store, most any other parts chain should be able to cross reference the above part numbers to whatever brand they carry.

Due to the numerous complaints of so many of these parts not meeting spec in recent years, buy locally, not online, and bring your vacuum pump to check that the part is within one inch of start and max vacuum points before you accept and pay.

To summarize, different engine configurations need different spark advance maps. At one end we have base engines like the 327/300 that have relatively short duration, low overlap camshafts with relatively early inlet valve closing and low overlap, and with a manual transmission a good starting point is:

Centrifugal: start @700-900, 30 @ 3000-3500 max, Vacuum: start @ 8", 16 @ 15" max (B22), 6-10 initial.

At the other end are the mechanical lifter engines - long duration cams, late closing inlet valve with lots of overlap, and it turns out the OE 365/375 HP OE map is about perfect, and all you have to do is go as aggressive on initial as your fuel octane will support.

Centrifugal starts at 700-900, 24 @ 2350 max, Vacuum: start @ 4", 16 @ 8" (B28). 12-16 initial (use total WOT method to set timing)

In between with have the L-79. Depending on actual CR and available fuel octane it might tolerate the 365/375 HP centrifugal, but if not, try: start at 700-900, 27 @ 2700 max, Vacuum: start @ 6", 16@ 12" max (B26), 9-13 initial.

Big blocksdue to the larger bore are more prone to detonation and they don't like as much low speed/load advance, so 30 degrees centrifugal in at 3000-3500 max with a 12" VAC that might need limiting to 12 degrees and up to ten initial or as much as it will tolerate without detonation.

The Two-Inch Rule only applies to engines with full time vacuum advance. Almost without exception all exhaust emission controlled engines (all '68-up, all 50 states) use "ported" vacuum advance by placing the vacuum advance port above the throttle blade. This increases EGT, often significantly, to aid oxidation of HC and CO, especially with air injection, and to keep the catalyst bed hot so it works with close to 100 percent efficiently at idle. Also included with the ported vacuum advance crowd are cars built to be sold in California in '66 and '67, all '66 L-72 and all '67 L-71, and the last one I can think of is '63 L-84. (They fixed it for '64.)

If you have ported vacuum advance and don't have to be field emission tested, I recommend converting to full time, and in many cases this will require a different VAC, one of the above three, to meet the Two-Inch Rule. This will improve around town fuel economy and reduce the cooling system load in stop and go driving. To check for ported vacuum simply tee a vacuum gage into the VAC signal hose, and if the reading is zero or very low, it's likely ported.

If you want to take a deeper dive into the subject of spark advance maps, check out my tuning seminar. It's in the "restoration documents" thread near the end of the sticky section. It lists suggested starting points to optimize the spark advance map for all OE Gen I small block and Mark iV big blocks.

Duke

[Duke W.]

I'm astounded at how many VACs there are ! YIKES !

That's because all GM divisions used the same family of interior parts for V8 distributors. Each had unique housings to fit their particular engine dimensions, but all the interior parts were interchangeable. The same applies to alternators and starter motors.

If you spend some time looking through the various VAC specs you will find some with virtually the same specs. It seems the divisions didn't talk to each other back in the day and each speced their own VACs, so Cadillac might spec a new one that was within production tolerance of one that was already in production for Buick.

That eventually led to consolidations that in some cases like the 1116163 (B1) substitution on many engines including base Corvette engines was a total screwup, and the aftermarket followed right along.

Duke

[Mark F.]

That's because all GM divisions used the same family of interior parts for V8 distributors. Each had unique housings to fit their particular engine dimensions, but all the interior parts were interchangeable. The same applies to alternators and starter motors.

If you spend some time looking through the various VAC specs you will find some with virtually the same specs. It seems the divisions didn't talk to each other back in the day and each speced their own VACs, so Cadillac might spec a new one that was within production tolerance of one that was already in production for Buick.

That eventually led to consolidations that in some cases like the 1116163 (B1) substitution on many engines including base Corvette engines was a total screwup, and the aftermarket followed right along. Duke

Understood and Interesting...

The 1977 pages list 241 separate VAC Units.
Presumably they were retroactive as far back as they kept manufacturing/stocking those items as available GM part numbers...service and production...dunno