Distributor Advance Discussion

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Willysnut1959
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Joined: Sun Dec 16, 2018 11:01 am
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Years Owned: 1966 Toronado

Distributor Advance Discussion

Postby Willysnut1959 » Wed Oct 26, 2022 11:39 am

In an effort to resolve an overheating problem, I recently replaced the distributor vacuum advance canister, then ran some tests to determine the distributor advance characteristics. I did not have a tachometer, so my estimates of engine RPM is estimated.

At idle, 7.5" vacuum, 5 degree BTDC with no vacuum canister plugged

At idle, 11" vacuum, 23 degree BTDC with vacuum canister working

2500 rpm, 14" vacuum, 16 degrees BTDC with vacuum canister plugged

3500 rpm, 14" vacuum, 42 degrees BTDC with vacuum canister working

From this data, I surmise that the distributor vacuum canister is contributing 18 degree advance at idle and the centrifical advance is contributing 11 degrees. I've read that the total advance should be 52 degrees at wide open throttle and that the vacuum advance canister should be worth about 15 degrees and the initial advance is 7.5 degrees. I'm thinking that my centrifical advance is not working properly and should be providing an additional 10 degrees of advance by 3500 rpm.

Does anyone know what the total advance should be on our 425 engine?

Marty

Schurkey
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Re: Distributor Advance Discussion

Postby Schurkey » Thu Jan 19, 2023 10:22 am

The stock advance specs should be in the Genuine GM service manual for your vehicle.

The OEM vacuum advance canisters had the amount of advance stamped-into the bracket.
HEI_VacuumAdvance1asm.jpg
HEI_VacuumAdvance1asm.jpg (74.85 KiB) Viewed 2992 times


Bear in mind that the stock advance curve for the centrifugal and vacuum advance may not be optimum for actual performance. Tuning the advance--centrifugal and vacuum--can make a big improvement in power. The "usual" recipe is to add initial timing, reduce centrifugal advance but lower the RPM needed for "full" centrifugal advance, and limit the number of degrees provided by the vacuum advance.

While the exact tuning would need to be assessed for each vehicle and varies with gearing and vehicle weight among other variables, 15 degrees of initial, 17--20 degrees of centrifugal "all in" at 3200--3400, and 10--15 degrees of vacuum advance from either a manifold-vacuum nipple, or a ported/timed vacuum nipple (whichever works best) should be in the ballpark.

You'd be unlikely to see 50-something degrees of advance in real life. At full-throttle, high RPM there'd be no vacuum, therefore no vacuum advance. You'd be looking for 35-ish degrees of centrifugal advance at WFO and 3000+ rpm. Adding another 15+ degrees of vacuum advance gets you to 50+ degrees of advance, but you'd have to be at high rpm with a closed throttle (to generate vacuum) so perhaps coming down a steep mountain road.

Willysnut1959
Posts: 107
Joined: Sun Dec 16, 2018 11:01 am
TOA Membership Number: 0
Years Owned: 1966 Toronado

Re: Distributor Advance Discussion

Postby Willysnut1959 » Thu Jan 19, 2023 12:26 pm

Thanks for all of the good information. I did not find any specs in my service manual. I can now pull my distributor and see if the centrifugal advance is coming all of the way in.
Marty


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