66 SB vacuum advance question

[Bill Richards]

Re: 66 SB vacuum advance question

Duke, with vac gauge hooked up per your instructions, I`m pulling 17" off the throttle plate and about 16" off the power brake tee, at 500 RPM in drive.
Bill

[Joe C.]

It's Impossible.....

to hook up a vacuum gauge in series because the Bourdon Tube gauge only has one port. A series connection would require two ports......inlet and outlet.

Joe

[Dick G.]

Re: 66 SB vacuum advance question

On my original 65 350HP engine I have a 67 1111196 9 L 7 distributor. Can: MS 236 16. The same can is on my 60 (The Monatana Legend) distributor which is from a 65 corvette. 1111969 5B1. I would think these cans will work on my 65 350HP engine, right? Thanks DG

[Duke W.]

Re: 66 SB vacuum advance question

I don't understand what you mean by "off the throttle plate". Tee the vacuum gage into the vacuum advance signal line. THAT'S manifold vacuum and what the vacuum can "sees". Teeing into any other vacuum system should yield the same reading. If less, there may be a leak in that system.

Since the B1 can requires 16-18" to pull to the limit you have no margin, which is why the B1 can is not suitable for your engine/trans.

You need to install one of the 12" cans I referenced earlier.

What is the idle in Drive spec for '66.

For '67 it is 500 in Drive. Earlier year 300HP/PG are speced at 450-475 in Drive. If you can achieve a lower idle speed than 500 with acceptable idle quality, then use a lower speed, but since vacuum falls with idle speed you will not be able to do this until you install a proper OE replacement 12" can.

Duke

[Duke W.]

Re: It's Impossible.....

This has happened more times than you can imagine.

Some guys just disconect the signal hose from the vacuum can and connect it to the gage, which is a "series" connection, but now there's no vacuum advance and the vacuum readings are not valid for the normal engine configuration with vacuum advance.

That's why I have to explain the "parallel" connection with the tee harness.

Duke

[Duke W.]

Re: 66 SB vacuum advance question

For the umpteenth time here is "the rule" for selecting a vacuum can.

The vacuum can should provide full vacuum advance at no less than 2" less than typical idle vacuum with the proper total idle timing, which is in the range of mid-20s to low 30s. High overlap cams should have total idle timing in the upper part of this range and lower part for low overlap cams.

Total idle timing is the sum of initial, plus full vacuum advance, plus maybe a couple of degrees of centrifugal if the curve starts at lower than idle speed, which is the case on some SHP engines. Check the specs!!!

If the engine has no vacuum can or it is dead, tie up the centrifugal with a rubber band and increase the timing to the above range. As you do this engine revs will increase, so use the idle speed screw to reduce revs to your target idle speed.

Now measure manifold vacuum and use "the rule" to select a vacuum can.

L-79s all have 8" cans (1116236 OE p/n, ID "236 16"), but since they typically pull 14-15" at 750 idle in neutral with total idle timing in the above specified range, a 12" can complies with "the rule" and I consider it to be optional.

A 8" can will work on an engine that pulls much more than 8" idle vacuum, but it may be "too aggresive" and cause part throttle detonation. In this case select another vacuum can using "the rule". There are three vacuum cans that will suit virtually all pre-emission Corvette engines - 8", 12", and 15".

Since you provided no data on the '60 engine suggest you run the test and use "the rule" to select a proper vacuum can.

I listed NAPA/Echlin part numbers for 8, 12, and 15" cans in a recent discussion of this issue within the last couple of months, so if anyone is still confused suggest you search.

This subject has been hashed out and explained a zillion times.

Duke

[Gerard F.]

Just a note on 4 ATDC initial timing

I found that with the 194 distributor and rotor contact lined up with the dimple on the lower gear, it's not possible to go below 6 BTDC as the vacuum can will be up against the coil. However with the dimple and rotor contact reversed, it is possible.

Just wondering if the 117 distributor accounted for this with a different orientation of the dimple, different lower gear, or reverse position of the dimple and rotor contact?

Jerry Fuccillo
#42179

[Dick G.]

Re: 66 SB vacuum advance question

Duke the engine in the 60 is immaterial. I only wanted to know the specs on the can and if it will work on my 65 350HP car. This 60 hasn't run since 74 and will be totally stripped and experience a total body off. Wrong power train front to back, etc.. Where does one get the specs for these cans? Duke, Thanks for you're reply. DG

[Duke W.]

Re: Just a note on 4 ATDC initial timing

You should be able to reduce the timing below 6 BTDC with the dimple pointed in the "normal" direction - same as the rotor tip. If not, your wires many not be indexed properly on the cap. Check your CSM for proper indexing of #1.

I don't know if 4 ATC could be achieved with the dimple in the "normal" condition. There may be some applications where the dimple needs to point opposite the rotor tip.

The 117 drawing has the answer if anyone has it or can get it.

I'm assuming that your car is not K-19 if it has the 194 distributor.

Duke

[Duke W.]

Re: 66 SB vacuum advance question

OE L-79 had an 8" VAC, and a 12" should also work okay as long as the idle speed is set high enough to achieve 14". The NAPA/Echlin replacement for the OE 1116236 8" VAC is VC1810 and the ID is "B28".

You can cross reference this part number to Standard, Niehoff, and BWD on their web sites. All are manufactured by Standard since they bought Dana Controls a couple of years ago. The different "brands" are now just different marketing channels.

Delco also buys from Standard, so their VACs will not have the original last three digits of the part number and max crank advance. They will have the same ID numbers as all the other replacement brands.

Duke