Bad distributor vacuum advance

[Huicho417]

Just found out the vacuum advance is bad on my distributor. My initial timing is spot on at 21 mercury inches of vacuum. Idles great and even runs good. It is not affecting my low end power but not sure on the high end. It does feel like its not all there on the highway past 60. Will replacing this part help at all? Thank you in advance!

 

[AR67GTX]

Not sure I understand yours post but if the vacuum advance is hooked up to a ported advance outlet on the carb - which most of them are - your vacuum advance does not function at idle.

 

[Huicho417]

Yes it is connected to the carburetor, and this is why it is not an issue at idle. However once rpm's go up the distributor uses mechanical and vacuum advance to add total timing. The reason I know the the vacuum advance is bad is because I tested it and it would not hold any vacuum on its own. My question is how much does it affect the engine at higher speeds when i am not in idle. Would it make it run lean and therefore loose horsepower?

 

[diesel_lv]

If it's not working, the only thing it will effect is gas mileage. You won't notice any performance difference.

 

[AR67GTX]

OK - my misunderstanding. It won’t lean your engine out under power since as you feed it more and more throttle, manifold vacuum drops to zero anyway. It also won’t hurt it at part throttle although your fuel mileage may drop some and under some circumstances your engine temp might increase slightly.

 

[68 Sport Satellite]

If it's not working, the only thing it will effect is gas mileage. You won't notice any performance difference.

I've had it happen if I'm already on the edge of total advance without it detonating, if I add vacuum advance it will ping when I put my foot into it. I understand that the Vac advance can controls the rate of vac advance, not the total added vac advance, correct?

I've also read from others that adding vac advance can give better throttle response.

 

[R/T Boy 67]

I would fix it. It will be more efficient at cruise.

 

[Huicho417]

Thank you for the responses to everyone. I plan on replacing that part this week and will post the results. What is everyone setting their initial timing at? I am running an LA 318 with Edelbrock intake and four bbl carb. For now I have 16 degrees BTDC and seems to run good.

 

[turbine68rt]

Thank you for the responses to everyone. I plan on replacing that part this week and will post the results. What is everyone setting their initial timing at? I am running an LA 318 with Edelbrock intake and four bbl carb. For now I have 16 degrees BTDC and seems to run good.

That depends on how much mechanical advance your particular distributor provides. If your stop plate allows for 30° mechanical that may be a bit much combined with the 16° initial. If you have 20° mechanical, great. Shoot for about 34-36° all-in.

 

[AR67GTX]

I understand that the Vac advance can controls the rate of vac advance, not the total added vac advance, correct?

I've also read from others that adding vac advance can give better throttle response.

Yes, sort of. Depending on how the canister is designed, it will control at what vacuum level it starts to pull advance in. Different canisters are calibrated to pull in different amounts of advance based on the engine characteristics. The rate of vacuum advance is not usually adjustable, but there are adjustable vacuum cans that the level of engine vacuum at which it starts operating can be adjusted. This is usually by turning a screw that tightens or loosens a spring that adds or lessens resistance to the diaphragm inside. There would be some change in the rate between the can set to activate at say, 16” vacuum, versus if it’s backed off to allow an engine with a big cam to activate it at say 8” vacuum. And I’m most accustomed to tuning engines using full time vacuum advance (pulled from below the throttle blades) where as most Mopars use ported or venturi (above the throttle blades) vacuum advance. I may not have thought through all the characteristics of ported advance.

Full vacuum advance can help idle quality to some extent, especially in an engine with a distributor set for a lot of centrifugal advance like 30 or 32 deg. That leaves base timing at around 4 - 6 degrees typically which leaves the plugs firing late, poor burn, fuel in the exhaust, etc. Full vacuum advance can easily pick the base and vacuum advance up to as much as 20 degrees which makes for a much happier engine at idle, cooler engine temps and better fuel economy.

But, in a performance engine where the centrifugal advance is reduced to around 18 to 20 degrees and base timing is set at 16 - 18 degrees, full time vacuum advance is less important and can actually be too much advance causing the plugs to fire too far in advance of TDC at light throttle causing a continuous light miss feel in the engine.

 

[68 Sport Satellite]

Yes, sort of. Depending on how the canister is designed, it will control at what vacuum level it starts to pull advance in. Different canisters are calibrated to pull in different amounts of advance based on the engine characteristics. The rate of vacuum advance is not usually adjustable, but there are adjustable vacuum cans that the level of engine vacuum at which it starts operating can be adjusted. This is usually by turning a screw that tightens or loosens a spring that adds or lessens resistance to the diaphragm inside. There would be some change in the rate between the can set to activate at say, 16” vacuum, versus if it’s backed off to allow an engine with a big cam to activate it at say 8” vacuum. And I’m most accustomed to tuning engines using full time vacuum advance (pulled from below the throttle blades) where as most Mopars use ported or venturi (above the throttle blades) vacuum advance. I may not have thought through all the characteristics of ported advance.

Full vacuum advance can help idle quality to some extent, especially in an engine with a distributor set for a lot of centrifugal advance like 30 or 32 deg. That leaves base timing at around 4 - 6 degrees typically which leaves the plugs firing late, poor burn, fuel in the exhaust, etc. Full vacuum advance can easily pick the base and vacuum advance up to as much as 20 degrees which makes for a much happier engine at idle, cooler engine temps and better fuel economy.

But, in a performance engine where the centrifugal advance is reduced to around 18 to 20 degrees and base timing is set at 16 - 18 degrees, full time vacuum advance is less important and can actually be too much advance causing the plugs to fire too far in advance of TDC at light throttle causing a continuous light miss feel in the engine.

Thanks for that great explanation.
I have a mild 451 big block. Max timing without detonation at WOT is 32 total. I run 15 deg intial, 32 total and the motor is happy, but with no Vac adv. I want to add Vac Adv. I may need to back off my initial, adjust my mech to get back my 32 total and this could allow me to add Vac Adv without detonation issues. I've always been told to set initial first, then total, then vac adv, but I think I need to look at it all together like I think you are saying.

By the way, I have aluminum Eddy RPM heads but here in California we have high ethanol content in our 91 octane fuel. If we had 93 the motor would probably tolerate 34-36 total.

 

[diesel_lv]

Thanks for that great explanation.
I have a mild 451 big block. Max timing without detonation at WOT is 32 total. I run 15 deg intial, 32 total and the motor is happy, but with no Vac adv. I want to add Vac Adv. I may need to back off my initial, adjust my mech to get back my 32 total and this could allow me to add Vac Adv without detonation issues. I've always been told to set initial first, then total, then vac adv, but I think I need to look at it all together like I think you are saying.

By the way, I have aluminum Eddy RPM heads but here in California we have high ethanol content in our 91 octane fuel. If we had 93 the motor would probably tolerate 34-36 total.

That is not how it works. Max timing of 32 w/out detonation is at full throttle. While cruising down the highway, you are not at max throttle and very little load on the engine. The vacuum will be high. Find out what your vacuum is While cruising at 60, then set the vacuum cannister to start pulling, adding timing, a couple inches of vacuum below that. Now, when you stab the throttle, vacuum will drop below that and your timing will retard back down to 32, avoiding detonation. The original Mopar distributors w the heaviest shape on the end of the cannister were adjustable using an Allen wren inside. The difference is how much timing each cannister adds. Used to be stamped on the arm. Reference, my 496 likes 30 max w crappy 91 octane. While cruising, I pull 16-17 inches of vacuum. I have my vacuum cannister set to add timing around 14 inches of vacuum. While cruising, no load on engine, I'm around 40° timing. When I nail the throttle, vacuum drops below 14 immediately and timing retards back to 30°. No detonation. But better economy While cruising.

 

[68 Sport Satellite]

That is not how it works. Max timing of 32 w/out detonation is at full throttle. While cruising down the highway, you are not at max throttle and very little load on the engine. The vacuum will be high. Find out what your vacuum is While cruising at 60, then set the vacuum cannister to start pulling, adding timing, a couple inches of vacuum below that. Now, when you stab the throttle, vacuum will drop below that and your timing will retard back down to 32, avoiding detonation. The original Mopar distributors w the heaviest shape on the end of the cannister were adjustable using an Allen wren inside. The difference is how much timing each cannister adds. Used to be stamped on the arm. Reference, my 496 likes 30 max w crappy 91 octane. While cruising, I pull 16-17 inches of vacuum. I have my vacuum cannister set to add timing around 14 inches of vacuum. While cruising, no load on engine, I'm around 40° timing. When I nail the throttle, vacuum drops below 14 immediately and timing retards back to 30°. No detonation. But better economy While cruising.

Thanks for clearing that up.
I'm not happy to hear your 496 can only tolerate a max of 30 deg total, but I am happy to find out I'm not the only one with timing below 32! I thought something was wrong - just our crappy 91 I guess.

So why is it when I hook up the vac advance that I get pinging, but without it I don't? Maybe when I stab it, on the way to full WOT it's pinging for a moment as the pedal travels down? Not sure how to remedy this and use vac advance.

 

[diesel_lv]

Thanks for clearing that up.
I'm not happy to hear your 496 can only tolerate a max of 30 deg total, but I am happy to find out I'm not the only one with timing below 32! I thought something was wrong - just our crappy 91 I guess.

So why is it when I hook up the vac advance that I get pinging, but without it I don't? Maybe when I stab it, on the way to full WOT it's pinging for a moment as the pedal travels down? Not sure how to remedy this and use vac advance.

On a stock engine, the stock vacuum advance distributor is great. Once modified, the vacuum at which the advance starts to come in, the total amount of vacuum advance etc all need to be adjusted. Hypothetical: you vacuum advance starts to come in at 10", then when you are cruising down highway at 16" and stab it, it takes a few for the vacuum to drop below 10". If you have it adjusted to start adding at 14", then it will take less time. Also the rod that is attached can be different and add more or less vacuum advance. Once you start modifying, you open up so many other things.
As for my engine not liking more than 30°, 10.5:1 in hot, 120°+, summers on 91 octane. In temps around 80ish in the dark, daylight on the hood or darkness make a difference, it can tolerate more timing. But I drive it year round and don't ever race it, not even light to light, so 30° works great for me.

 

[RemCharger]

Also, the factory mechanical curve is a long drawn out affair.. one of the first things corrected when doing a performance build. So now, when cruising the highway , you're already at your max timing at 25-3000 rpms. Add the vacuum advance to that and you've got a hell of a lot of advance going on.
But with lots of time , patience, and engineering I suppose you could make it work.

 

[diesel_lv]

I got tired of disassembling my distributor to adjust the springs for the right curve. Even 2 of the heaviest springs brought my timing in too soon for mechanical advance. Finally bit the bullet and got a Progression Ignition Distributor. Completely programmable via Bluetooth from my phone. Now my mechanical advance comes in exactly when I need it, my vacuum advance comes in and how much I need it. Pricey, but for me worth it.