1965 375HP Timing

[John Lokay]

Re: 1965 375HP Timing

Duke,
I set the timing on my L-72 by bringing RPM down to 600 and then set the initial at 10 degrees with the vacuum line plugged. How does that sound? Should I give it more initial advance, say 12 degrees?

[Doug Flaten]

Re: 1965 375HP Timing - Thanks

Just wanted to thank you and others for the in depth and technical explanations on many topics. Saturday I spent all day recurving an HEI distributor on a '66 Pontiac GTO clone I bought last year. It is a Heinz 57 of parts, and all I know is it pinged like crazy if I advanced the timing at all and it did not have any power. I never had messed with the distributor curves before, but thanks to the knowledge gleaned from this site, I felt I knew what the problem was. The HEI distributor had too much vacuum advance, so at WOT, I lost most of the advance and the car was sluggish. After 5 trial and errors combinations and charting the advance at each rpm, I limited the vacuum advance and added in some more initial and centrifugal advance. It is a much faster car with only a slight part throttle pinging that would likely go away if I could find some gas with a little more Octane than 91. So basically, had I asked someone what the factory settings were, it would have been meaningless on this modified vehicle. I was able to apply what I learned from this site.

[Duke W.]

Re: 1965 375HP Timing - Thanks

Since HEIs didn't go into production until '75 most are calibrated for emissions and typically have lazy (slow) centrifugal and a vacuum cans with too much advance. This can lead to detonation under part throttle light to moderate acceleration.

There are some HEI vacuum cans that are closer is spec to sixties vintage single point vacuum cans, and I have a list of most of them with specs and NAPA/Echlin part numbers. The list includes both single point and HEI cans and is real useful to those of you who are doing igntion map tuning.

I-mail me if you want a copy.

Duke

[Duke W.]

Re: 1965 375HP Timing

According to the L-72 centrifugual specs published in Corvette News (0@900), 600 should be okay for setting initial timing because the centrifugal doesn't start until a little over 900. Also, when checking initial timing it's a good idea to slowly raise the revs with the light on the tab to ensure that the centrifugal starts above the speed that you set initial timing. If the timing advances immediately, the centrifugal may have started to deploy at or below the speed that you set timing. This is an issue with aggressive centrifugal curves.

The total centrifugal spec for the L-72 is 30@5000, so ten degrees initial should yield 40 total at WOT and high revs, which is probably about right for a closed chamber head BB. The specified initial timing range is 8-14, so you can experiment within this range to see what yields the best detonation free performance.

It's tough to tell a SOTP difference of two degrees, but two degrees can sometimes make a difference between detonation and no detonation depending on what fuel octane you are running.

Duke

[Jim T.]

Re: 1965 375HP Timing

Duke keep up the excellent work.

[Larry S.]

Re: 1965 375HP Timing

What did I miss did every one erase there post ? on this Or is it just a lost thread ?

[John H.]

Re: 1965 375HP Timing

Larry -

Re-set your "preferences" to a longer time period and you'll find the entire thread.