CanadaGrant

My 69 427/390 has the original motor and is stock except for a mild roller cam. The original distributor has been converted to Pertronix from points and these are the numbers I get with my timing light.
Initial advance of 16 degrees with a mechanical or centrifugal advance of 17 degrees for a total of 33 degrees. Idle is set at 800 rpm and mechanical advance starts at 1200 rpm and is all in by 3000. The vacuum can produces another 16 degrees and at idle the vacuum is 17.5"hg.
My shop manual says that the stock mechanical advance from the factory was 24 degrees so my question is why would someone reduce it which then requires such a high initial advance? Are there any pros or cons to running it like this? The engine starts easily, runs strong and smooth with no hesitation anywhere, no pinging or knocking on 91 octane and I have no complaints. Just wondering what the pros and cons are to running such a low centrifugal advance requiring that high an initial. If I wanted to run 36 total I would have to bump up the initial to 19 degrees....
Thanks guys!

homestead

Have you tried running it at 36 total? It may like that much timing.

69427

Remember, the factory settings are for worst case applications (high ambient temperature, high barometric pressure, and possible lower octane fuel). A modest timing curve allows the engine to survive the variety of conditions that the "production fleet" of L36 engines might encounter. While the engines survive, the drivability and fuel mileage suffers for the majority of engines that aren't driven in the extreme conditions. If you're fortunate to not have to contend with those unfavorable conditions you (or a previous owner) can tune the engine with a more optimized timing curve (more low-RPM advance) to improve the idle quality and road speed fuel mileage.

CanadaGrant

Thanks guys. I'm going to bump it up a couple of degrees so I have 35 total and see how it works. I just couldn't get past the idea of running so much initial but I guess it's the total that really matters as it starts and runs really nice. I thought it might be a little slow cranking on hot starts with that much initial but it's no diff from cold.

toobroketoretire

Does it look like the stock flyweights have been removed and replaced with zinc-plated aftermarket flyweights? Perhaps a previous owner put a kit into it at some earlier time. If so it could account for the less mechanical advance. And the mechanical advance does not "require" a higher initial advance to achieve a certain total. If I remember correctly most stock distributors didn't exceed 30 degrees total but rather somewhere in the high 20's. The 36 degree figure is a desired figure for racing applications for maximum power using high octane gasoline.

69427

Yes and no. It's the total at each RPM that matters. It's pointless to set the timing at a certain number (35) during higher RPM WOT conditions, while the rest of the RPMs where the engine spends 99% of its time is ignored or only given minimal thought. Efficiency and power is obtained by getting the timing correct for all RPMs that the engine spends its time.

CanadaGrant

The weights look stock (old bare metal) and have not been welded up to reduce advance but who knows after 46 years. I would bet the springs have been changed. Centrifugal advance starts in at 1200 rpm and is all in at 3000.

Scott Liggett

First, don't assume that what a factory manual says for the timing to be gospel, or even accurate. Especially on a 50 year old distributor. The only way to know is to actually check it. Factory GM distributors were normally around 24-26 degrees advanced, but way up around 4000 to 4500 rpm fully advanced with their big heavy springs.

If you are wanting the best performance, I would suggest any one of the aftermarket distributors out there where you can choose how fast the timing curve comes in and how much total mechanical advance it will give. Just changing the springs and weights on a stock distributor will not change the total advance. Only when it will happen.

A distributor like a MSD 8360 will allow you to dial in your curve exactly like you want. Like others have said, 18 degrees initial, plus 18 degrees of mechanical for a total of 36 would be great on your 454. Big blocks like less mechanical and more initial, especially with big cams that bleed off cylinder pressure.

MelWff

This thread is from 2015

Scott Liggett

Whoopsie.