Normal vacuum readings for L79 327cid '66 with 4160 Holley

[Kevin B.]

Can anyone tell me what the normal / good vacuum readings are for an '66 L79 (327cid) w/Holley 4160 and stock intake manifold?

Or can someone tell me of an archive that has such info? I have the '66 Chassis Service Manual but could not find any reference to the vacuum at idle.
I just rebuilt the carb and am trying to adjust the idle mixture screws. I have an old vacuum gauge that says readings less than 20" indicate late ignition timing. I am currently drawing around 17". When I rev the motor the vacuum jumps up over the high side of the normal range. Readings are pretty steady at ~17" at idle. No coughs or hiccups noted and all else seems OK. Running with point and condenser, stock wiring and coil, R45s plugs.

- Kevin

[Michael H.]

Re: Normal vacuum readings for L79 327cid '66 with 4160 Holley

Kevin,

My 67 reads around 16 to 17" and I have everything stock just like your car, your good to go.............

Mike

[Duke W.]

Re: Normal vacuum readings for L79 327cid '66 with 4160 Holley

I have an old vacuum gauge that says readings less than 20" indicate late ignition timing. I am currently drawing around 17".
- Kevin

That's a really old vacuum gage before the era of high overlap cams that pull less vacuum at idle than low overlap cams. Heck, even the base engilne cam is a little less than that @ 500 in neutral.

GM never specified manifold vacuum at idle speed back in the day, but typical idle vacuum for most OE engine configurations are well known.

BTW reporting manifold vacuum without saying the specific idle speed is MEANINGLESS!

So...?

Duke

[Timothy B.]

Re: Normal vacuum readings for L79 327cid '66 with 4160 Holley

Kevin,

The L-79 that I have worked on will idle at approx 750RPM and hold approx. 14" vacuum. Keep in mind the higher the engine idles the higher the vacuum reading will be.

Use the vacuum gauge as a reference and adjust the emulsion screws to provide the highest vacuum reading at a set idle speed (750-800 RPM) If the engine speeds up when you adjust the screws slow it back to the idle speed and repeat. Both sides should be approx the same turns out and they should fall between 3/4 turn out to 2 turns out. Set the ignition timing at 10*BTDC first.

[Kevin B.]

Re: Normal vacuum readings for L79 327cid '66 with 4160 Holley

Using the dash's tach my rpm's are reading ~650-750.

[Mike T.]

Re: Normal vacuum readings for L79 327cid '66 with 4160 Holley

Kevin - Your readings sound pretty normal and I'll take a guess you're either at sea level or not too far above it. My 66 L79 used to read about 16.5/17 at 750 rpm and that's when we lived in Sonoma County, California, which is only a hundred or so feet above sea level. Now that we've moved to the 5K+ foot elevation of Prescott AZ, the reading is a couple inches less.