I’m replacing a defective vacuum advance on my 69 427 390 HP (L36). The one that is on the distributor now is marked MS 360 12. Is this an original part number or a service replacement.
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Jeff------
GM number 1115360, stamped 360, is the original vacuum control used for the distributor originally installed for your L-36 application. It's impossible to say whether the vacuum control you described is actually the original unit installed on your distributor because the 1115360 was available in SERVICE from GM for many years and, thus, could be a replacement. Regardless, it's the correct vacuum control, originally installed or not.In Appreciation of John Hinckley -
There is no longer an exact replacement for the MS 360 12. In case you are not aware, 360 is the last three digits of the GM long part number and 12 is the maximum crankshaft advance, which is provided at 12 or more inches Hg manifold vacuum.
SMP used to provide an exact replacement, but it was discontinued at least 15 years ago. The closest is now the B26, NAPA VC-1765, which can be cross referenced to other "brands" like BWD, Standard, Delco, and most others.
This part starts to add advance at the same 6" Hg as the 360 12, but max advance is 16 @ 12" rather than 12 @ 12". This might cause some driveability issues since in my experience early closed chamber big blocks don't like as much low speed, low load advance as small blocks with similar valve overlap. Go ahead and install it and see what happens.
To install with the distributor in the engine, pump down the can as far as possible or manually force the pin to the limit. This will expose the forward mounting screw. Then remove the screws and carefully wiggle the pin out of the breaker plate. Pump down the new VAC to pull the pin to max advance and install.
Duke
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Thank you gentleman, very informative. I have the NAPA replacement on the way and it will be fun to check the advance at full idle vacuum. When the original was working I would get 12 degrees right on the nose at idle vacuum. I guess if I get 16 from the NAPA replacement and it becomes a problem I could fudge the initial back 2 degrees to split the difference. Also, I assume there is probably an adjustable advance available for this distributor. Either way, it’s going to be way better than the “no advance” I’m getting now😀. It failed slowly so the diagnosis was not apparent. The car would run and idle well, then it wouldn’t, with no rhyme or reason. I assume the diaphragm was slowly tearing, and would sometimes pull some advance and then other times not. I have the distributor out so replacement will be easy.Comment
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Duke,
For the L36 engine, the factory spec (factory service manual) for vacuum advance says 0@7”. So I assume the advance starts around 8”. The VC 1765 is in at 6”, whereas the VC 680 (B1) starts at 8”. Both provide a max advance of 16 deg, though the VC 680 requires a higher vacuum to reach max. However, the L36 should make plenty of vacuum to be 2” above that. It seems the VC 680 would be closer to the stock spec, but I assume there are reasons you prefer the 1765?Comment
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Zero at whatever inches Hg means it starts at or slightly above this, like a few tenths above, certainly no more than one inch. I don't have any '66 or '67 service manuals, but according to the Vehicle Information Kits from gmheritage.com the '66 L-36 VAC is start at 6", 15 @ 12" max and '67 is start at 7", 12 @ 12" max. I believe the '66 L-36 VAC ID is MS 360 12. I'm not sure of the ID number for the '67.
The VC 680, is a boat anchor, repeat BOAT ANCHOR! I don't recall the GM number but is was a totally inappropriate substitute as part of a parts consolidation effort decades ago and the aftermarket followed suit. It may not provide full advance until as much as 19" so It won't even pass The Two-Inch Rule for a base small block. NEVER order a VAC based on what some catalog says. Since a L-36 with OE equivalent cam and manual transmission has typical idle behavior in neutral of 600 @ 15", the VAC must provide full advance at no more than 13".
There are currently only three replacement VACs that are a reasonable fit for any OE small or big block from the era that is either OE full time vacuum advance or converted from ported that is typical of all emission controlled engines, but also includes '63 L-84, and all L-72, '67 L-71, and all '66 and '67 engines in cars delivered in California With K-19 Air Injection Reactor.
One of the following three available VACs, big or small blocks, single point or TI ignition will the least aggressive to meet the Two-Inch Rule for any OE cam or any aftermarket cam except radical "racing cams" that have no business in a road engine.
B-22: start @ 8", 16 @ 15"
B-26: start @ 6", 16 @ 12"
B-28: start @ 4" 16 @ 8"
These aren't real precision devices, so figure plus or minus 1 to 2" Hg and +/- 2 degrees max advance from spec, and in recent years many have been reported as significantly out of spec, which is why I recommend buying locally and bringing a vacuum pump to check that it's reasonably within spec before you accept the part and pay.
The Two-Inch Rule says the VAC should provide full vacuum advance at no less than 2" less than idle vacuum. You might have to think about this a little, and here are some examples of typical idle vacuum for OE engines with manual transmissions and no AC.
18" @ 500, base small blocks
15" @ 600, L-36/68
14-15" @ 750, L-79
14" @ 900 L-72/71
12" @ 900 small blocks with Duntov or LT-1 cam
10" @ 900 small blocks with 30-30 cam.
CAVEATS: If an automatic transmission the idle speed/mixture should be set up in Drive (set parking brake securely and chock wheels), and if equipped with AC the compressor should be engaged, either transmission type. Measure and document vacuum. It will be lower than the same engine at the same speed with a manual transmission in neutral, and it may be best to use a higher idle speed. The AC compressor puts a considerable load on the engine at idle, so set idle speed as low as you find acceptable, and then with the compressor off the engine will likely idle 100-200 RPM higher. And if you drive to higher altitude idle speed and vacuum will fall and require adjustment. Always measure your idle vacuum for whatever configuration you have and use the Two-Inch Rule to choose the least aggressive from the three listed, which is the "best fit".
So, if anyone is game use The Two-Inch Rule to pick the best fit VAC from the three choices for any or all of the above cases with manual trans and no AC, and I'll let you know if you're correct or not.
Finally, OE specs are a good place to start, but who knows what camshaft or spark advance map your "original appearing" engine might have. Over the decades most were modified, so you have to measure the spark advance map and idle vacuum to understand YOUR engine. The above listed idle behaviors are typical. My recommendation is to set idle speed "as low as possible commensurate with acceptable idle quality". This is subjective, so it's a matter of personal choice. Especially with medium to high overlap cam engines some guys like a low, lumpy, idle speed where the engine feels like it'a about ready to stall. I prefer a higher idle speed that still has some lope, but isn't lumpy and rough. A higher idle speed also helps get the car rolling easier from a dead stop, especially with taller axle ratios. Your car, your choice!
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