Hats off to The Duke

[Joel T.]

Duke;

I followed your advice from the thread "Question on 65' 396 dying and restart" and replaced the vacuum can on my '65 L78. The can that was in there was a 355... In testing it with the MightVac, it began to pull in at 7" and would not reach full advance until about 15" when you could hear it bottom out. I had an extra B28 can and installed it... The B28 provides full advance at 8" and given the lower idle vacuum my car produces it is no wonder the idle was erratic at times.

I have not had time to take it out for a long ride but I can tell you that the idle is rock solid and the low speed driveability is much better... no stumble at all...

At your suggestion, I made this same swap a while back on my other car; a 1963 SWC 340HP.. which had similar problems...

When in doubt... follow that 2" rule!

Thanks again Duke!

Joel

[Duke W.]

Re: Hats off to The Duke

Most guys don't understand how critical the VAC is to good idle, low speed engine operation, and coolant temperature control, and Chevrolet didn't always follow the Two-Inch Rule. I have no idea why. They should have figured it out... should have known better.

The L-78 and '63 L-76 are two examples. I figured out the problem on my SWC and installed a 236 (modern B28 is equivalent) VAC circa 1965, and it solved the idle stability problem, but many have not figured it out yet despite numerous Web posts and articles about the spark advance map.

Since the L-78 VAC has the same specs as the '63 L-76 I figured the OE VAC was the 201, but both the 201 and 355 are within production tolerance of each other.

The L-78 should produce enough idle vacuum for a 12" B20/26 VAC to work okay. The 8" B28 is more aggressive than needed, but if there's no part throttle or transient detonation then all is well.

Duke

[Daniel K.]

Re: Hats off to The Duke

Joel and Duke,

Since I'm in the throws of replacing the TI system on my L78, (pick-up coil, harness, and amplifier) I might as well as replace the vac advance can. Is the appropriate vacuum advance can available from GM or will I need to go to one of the on-line retailers?

Also, my 340Hp SWC has always run well and the L78 has been problematic since purchased 6 years ago.

As always, your help is appreciated.

Dan K.

[Duke W.]

Re: Hats off to The Duke

GM hasn't offered VACs with the OE stamped numbers for nearly 20 years. Replacement VACs are manufactured by Standard Motor Products regardless of the name on the box and are IDed by B-number.

The best fit for the L-78 is the 12" B20 or B26. This is sold at NAPA under the VC1765 part number, but the same part is available in other brands. You can search this site to find them.

The 327/340 should have the 8" B28, which is NAPA VC1810. They are getting hard to find. GMpartsdirect.com might have them under D1312C, but I'm not sure if that's the correct current Delco part number.

Maybe you can trade Joel a B20/26 for his B28.

The L-78 should idle in neutral at 900@14", so a 12" VAC meets the Two-Inch Rule. The 327/340 at 900@12", so it needs a 10" VAC but the next increment is the 8" B28, so that's the one.

There is one of three VACs that I have identified that are functionally correct for any vintage single point/TI equipped OE engine/transmission combination with full time or converted from ported to full time vacuum advance. The 15" B22, 12" B20 or B26, and 8" B28. I have previously listed the applications, so selecting a functionally correct VAC should a no-brainer.

What's stamped on the currently installed VACs - both engines?

Duke

[John H.]

Re: Hats off to The Duke

The 327/340 should have the 8" B28, which is NAPA VC1820. They are getting hard to find. GMpartsdirect.com might have them under D1312C, but I'm not sure if that's the correct current Delco part number.

Duke -

I'm sure you meant VC-1810 for the B28 VAC. The GM part number is 88924985, and the AC-Delco number is D1312C; I get them from GMPartsDirect.com for about six bucks each.

[Duke W.]

Re: Hats off to The Duke

Thanks for the corrrection. I should know better. (I edited my post to correct the part number.)

I have a local chapter buddy that ordered I think three of the D1312C and a couple of them didn't meet spec.

Always check that the plunger starts and stops at the proper vacuum reading before you install, then check that it adds the correct advance once on the engine.

Duke