1965 L79 Distributor Vacuum Advance Change

[Duke W.]

Re: 1965 L79 Distributor Vacuum Advance Change

Duke,
In mapping your advance curve on a stock L79 distributor, you'd have to rev it past 5000 to see what you've really got since stock centrifugal doesn't come all the way in until 5100. Is there a danger of damaging engine when you rev one way up with no load?

No, but why bother. If the springs are still OE install some lighter springs, and then set total WOT advance.

Duke

[William F.]

Re: 1965 L79 Distributor Vacuum Advance Change

Did GM correct the lazy L79 advance curve in it's last years or with the later 350 hp 350 cu in version?

[Donald L.]

Re: 1965 L79 Distributor Vacuum Advance Change

65 L79 used 1087 distributor whose maximum mech advance was 26* @ 4100 RPM

66 L79 used distributor 1156 whose maximum mech advance was 26* @ 5100 RPM

67 L79 was a 1196 distributor which was the same mech advance as the 66 model

[William F.]

Re: 1965 L79 Distributor Vacuum Advance Change

Any idea why they changed max advance up to 5100 in '66 and '67 from the more rapid curve on the first-'65 L79?

[Gary B.]

Re: 1965 L79 Distributor Vacuum Advance Change

William,

Excellent question. And what the different between '65 and '66 L79s in order for the change from 4100 rpm to 5100 rpm to happen? Only different distributor springs?

Gary

[Duke W.]

Re: 1965 L79 Distributor Vacuum Advance Change

OE spark advance maps were conservative to guard against detonation under the wide variety of operating conditions, which is why significant performance/fuel economy gains can usually be had by optimizing them for the more narrow operating environments of vintage cars.

Exhaust emission controlled engine spark advance maps were designed for emission control, and as a result were usually even more conservative than pre-exhaust emission controlled engines.

In either case, there are no magic formulas to meet design requirements, which is why spark advance maps changed from year to year on many engines such as the '62-'67 327/300, so it was a matter of experimentation and adding a safety factor, which is exactly what we do today.

Based on experience I provide recommended starting points for most vintage engines in my tuning seminar.

Duke

[Gene M.]

Re: 1965 L79 Distributor Vacuum Advance Change

On the ‘68 model L79 engine I can tell you GM totally changed the shape of the advance weights from the typical shape. I’m not sure what the springs were compared to previous years. The overall weight of the advance weights seems to be a bit heavier. But the “finger” on the weight controlling advance rate is totally different.

If one changes to the older style weight they fit sloppy, loose. So I returned the original weights with softer springs. Seems to perform good.