Vacuum Gauge Reading for '66 L 72

[John L.]

Would like to share the vacuum gauge reading I took on my L-72, and get your feedback, as for background information:
1) engine was rebuilt years ago with a Sealed Power factory type cam similar to the original 243 cam, 2) before taking the reading, drove the car about 1 hr, 3) I am running the vacuum advance line off the rear port of the intake manifold so getting full vacuum 4) I teed the vacuum gauge into the advance line, 4) took the reading at 1M RPM


The reading was a steady 18in and when I "kicked the throttle" it bounced below a reading of 5in and back up to 22in. After reviewing posts in the archives, I was expecting to see a reading in the 14/15in range but assume the reading of 18in had to do with the RPM idle speed of 1M? thanks

[Duke W.]

Re: Vacuum Gauge Reading for '66 L 72

Idle vacuum is a function of valve overlap, total idle spark advance, and idle speed. The higher the overlap for any given idle speed the lower the vacuum, and for any camshaft, higher idle speed will increase vacuum, and for a given idle speed full time vacuum advance will also increase vacuum compared to ported vacuum advance.

14" @ 900 is "typical" for L72 based on reports from others, but some may be with the OE ported vacuum advance.

Once you get above about 15" vacuum flow past the throttle valves is sonic and choked and the idle should be smooth. This is called the critical pressure ratio - absolute manifold pressure (atmospheric minus manifold vacuum) divided by atmospheric pressure equals 0.528 or less.

My basic guideline is to set idle speed "as low as possible commensurate with acceptable idle quality". That's subjective and will vary from owner to owner. So try out this "rule" and see how low you can get it to idle. You might find something in the range of 750-850 is okay and it will have a little lope once the pressure ratio gets above critical because flow past the throttle valves is subsonic and varies. That's what causes "idle lope".

Once you have decided on an acceptable idle speed, go through the idle speed/mixture adjustment procedure to get it as good as possible.

Duke

[Michael J.]

Re: Vacuum Gauge Reading for '66 L 72

I would emphasize Duke's recommendation on idle mixture adjustment, it can be critical. I have used this procedure for many years setting my Holleys, which always have come in poor adjustment for my altitude (7000') so have had to do it frequently. Good luck.

http://www.superchevy.com/how-to/45638/

[Duke W.]

Re: Vacuum Gauge Reading for '66 L 72

I have one disagreement with the article. It says to have the transmission in Neutral or Park. On manual transmissions Neutral is correct, but the idle speed/mixture adjustment on automatics should be set in Drive.

Be sure to set the parking brake firmly and chock at least two wheels!

Duke

[Michael J.]