`62 FI Idle Vacuum

[Michael I.]

What should the typical vacuum signal be at idle in a 1962 360 hp?

[Duke W.]

Re: `62 FI Idle Vacuum

Idle vacuum varies with idle speed, total idle spark advance, and valve lash on mechanical lifter cams. That's why I always have to ask what idle speed is when someone asks if XX" is in the ballpark for their engine. For a given configuration, vacuum readings without the corresponding engine speed are meaningless because the higher the idle speed the higher the vacuum and vice versa.

High overlap cams have a "lope" at idle so both idle speed and vacuum vary as you take the reading. In these cases, try to eyeball the average as best as you can. Some like a real lopey idle, but I prefer it to be just slightly "nervous", so I use higher than GM recommended idle speeds for better idle quality. I think the GM recommendation for Duntov-cam engines is 700, which IMO is a joke. It takes 850-900 to get acceptable idle quality.

So if it's a real Duntov cam with the no-vacuum-advance dual point distributor with about 10-12 degrees initial advance, it should be near 12" Hg with average idle speed somewhere in the 800-900 range. It's best to use an electric test tach to read idle speed as the in-car cable tachs are not always that accurate at the low end.

The LT-1 cam has about the same effective overlap as the Duntov, so idle speed and vacuum are about the same. The higher overlap 30-30 cam is about 10" @ 900, and the lower overlap L-79 and L-46 cams are about 14" @ 750. Base cams are typically 18" @ 500 idling in neutral, but lower idling in Drive at 450-500 with automatics.

Emission controlled engines have higher recommended idle speeds than their non-emission controlled antecedents.

What do you measure?

Duke

[Michael I.]

Re: `62 FI Idle Vacuum

Hi Duke,
We just put the FI unit back on the car today. It fired right up, but seemed to be very rich at idle and we couldn't coax the idle speed much above 600 rpm. At that speed, vacuum was registering about 8" Hg. The problem turned out to be retarded spark timing. Once that was dialed in, idle speed came up to about 800 rpm and vacuum to about 12" Hg. We reset the idle mixture and speed screws as recommended in the ST-12 and then tweaked from there for best idle quality. So far, so good; the engine starts immediately. Thanks for you help. Mike