1966STER

As you know Holley specifies that the proper power valve number is determined by dividing half the vacuum at idle.

My question is; should this be based on the port where the vacuum advance hose is attached at the base of the carburetor (ported vacuum) or should it be based on the vacuum hose going to the choke pull off which I assume is true manifold vacuum?


Application 1966 L72.

Thank you,
Ralph

JohnZ

It's based on the reading at a full manifold vacuum source, like the choke pull-off diaphragm.

1966STER

Thank you John

Follow up question please: Typically what is the vacuum increase that one would expect to see at the choke pull off vs. the ported nipple, is it around 50% or something less??

tbarb

The ported nipple is a hole drilled into the carburetor throttle body just above the throttle blade. Depending on the throttle blade position at idle (how far the idle screw is turned) that ported hole may see none or only a few inches of vacuum. As the throttle blade is opened, the ported hole is exposed to full engine vacuum.

The holes under the throttle blade see full engine vacuum all the time, this is where the choke pull off and vacuum distributor advance on C2 era cars draw there vacuum from.

danrduc

If that is where C2 vettes draw their vacuum from, why does my 65 with 2818 carb have the vacuum tube on the side of the front metering block and have ported vacuum?

Frankie the Fink

If its above the throttle plates its ported...below then its full vacuum. Hollley has some excellent youtube videos on power valves and other tuning items. Its also on their web site.

tbarb

Have you had a vacuum gauge on the nipple to see if it's ported vacuum.

JohnZ

Some later replacement 2818's had a different primary metering block which made that vacuum port a "ported vacuum" source; on originals, it was a full manifold vacuum source.

danrduc

Thanks, I was thinking this was a later service replacement carb. It has the 2818-1 but not date code showing. It is sitting on the correct manifold though.
The carb base plate has no vacuum connections, should I remove the single 1/4" pipe plug from the manifold and install a hose barb to use for the vacuum advance? Will that be a benefit for an improved advance curve, or is the ported vacuum better?
I am getting about 56' total advance at present, about 16-18 initial, up to 36' centrifical, and towards 56' total vacuum all in. Is that just a bit to much, should I back the vacuum down a few degrees?

Frankie the Fink

You should put a vac gauge on the nipple as tbarb suggested before you go moving things around to see what you have.

56* total + vac advance is pushing it; however you realize you can't measure that on a car sitting still ? Right ? Vac advance is load based on cruise vacuum not RPM. However, if you have no pinging or "trailer hitching" when driving then its prob fine..

65tripleblack

Sure you can!

I'm no expert on big blocks, but I don't think they need as much spark advance as smallblocks because the spark plug is more centrally located in the porcupine heads.

If smallblock ideal WOT spark advance is 36-38 BTDC, then BBC is more like 32-34 BTDC, add in about 17 degrees vac advance, and total cruise (that is, if L72 centrifugal is all in at cruise) should be about 49-51 BTDC.

Frankie the Fink

my 396ci Chevelle was quite happy at 32* initial + centrifugal. I don't see that you measured vac advance above tho.

65tripleblack

With manifold vacuum, it should be all-in @ idle.

Frankie the Fink

Yes - that's the way to select an initial parameter for the vac advance can. I just didn't want anybody to think they can measure vac advance on top of 'all-in' initial and mechanical by revving the motor in their garage -- I prob could have phrased the issue more clearly:
https://www.corvetteforum.com/forums...post1582053859

JohnZ

Yes - there was a production fitting for that - Paragon #1361 or #755.

65tripleblack

If you leave your vacuum advance hose connected to manifold vacuum and rev the engine to the all-in centrifugal advance RPM, then you will measure initial + centrifugal + vacuum advance. For example: 13 + 24 + 17 = 54.

If the engine is free revved (no load, in neutral) the vacuum advance diaphragm is fully deployed.