Retrofitting ignition vacuum advance

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Retrofitting ignition vacuum advance

Postby Seff » Tue May 09, 2017 12:47 pm


I drive my jeep many miles, and given that I'm not made of money and my jeep isn't a "day-one" correct jeep by a long shot, I thought I'd experiment with creating a vacuum advance mechanism for it.

"Why vacuum advance, young man?"
Short answer? Because it's more efficient, thus improving fuel economy. Long answer? Here.

"How did you do it?"
Easy. I took the distributor hold-down (which is 3 mm thick) and made a two-piece (each 1.5 mm) hold-down where the lower part holds the distributor and the upper part bolts down to the block. Then I attach one end of the vacuum canister to the upper part and the older end to the lower part - meaning that the vacuum canister turns the entire distributor. Noting the travel of the canister at full vacuum (4 mm) I determined how many degrees of crank rotation I would turn the distributor at a given offset from the centerline of the distributor. I chose to go with 22 degrees of advance, owing to my knowledge of other ignition systems.

Why? Amount of optimal advance is determined by the size of the combustion chamber (and the flathead chamber is large and long, meaning there is a long burn time for the mixture), combined with the compression and the density and whether the mixture is lean or rich. I drew from my V8 the conclusion that a big chamber can tolerate upwards of 50 degrees of advance, vacuum and total combined. The jeep has 17 degrees of mechanical advance built in and an initial setting of 5, which I have set to 7. 7+17+22=46, a conservative number. But 22 degrees of advance is a lot of "travel", which might make the response sluggish.

"So does it work?"
Oh yes it works. Care must be taken to keep it well-lubricated, otherwise I suspect that the mechanism would bind. The canister turns the distributor whenever the engine produces vacuum, which is the desired effect.

I have considered installing a form of return spring to make sure the advance returns to zero fast enough, but so far the jeep is very drivable as long as it is warm. At cold the engine tends to need a bit of throttle, otherwise it stalls. No pinging, even going up hill, though.

I will keep you updated as I get a reliable picture of the fuel economy increase (if any). Before the modification, it ran about 14 MPG mixed driving.

"So what does it look like?"

Visible is of course the vacuum canister, the hole for the hold-down bolt, and the arm that turns the distributor "clamp"
vacadv1.jpg (144.58 KiB) Viewed 155 times

Visible is the distributor "clamp" that is actuated by the vacuum canister to turn the distributor around its own axis
vacadv2.jpg (137.95 KiB) Viewed 155 times

Enjoy, gents. :)
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Re: Retrofitting ignition vacuum advance

Postby artificer » Tue May 09, 2017 4:59 pm

Forget all that mumbo jumbo on race cars, hot rods & V8 engines in that oft quoted superchevy link.

This might help G503 folk in understanding what is really going on in a considerably less confusing more simplistic format.

With distributors & very low compression engines of the 40's - 50's era vacuum for advance was ALWAYS ported above the throttle plate so:

1. There is no advance, either mechanical or vacuum, @ idle speed.

2. On acceleration the throttle plate just opening exposes both the progression port [secondary idle port] & the vacuum advance line to the vacuum advance diaphragm to relatively high manifold vacuum thus advancing ignition timing.
This is important as, @ this point, the fuel/air mixture is somewhat lean, takes longer to burn & the spark needs to happen earlier so:

As the engine speeds up so does air speed through the carburettor throat & a venturi point [vacuum/low pressure] is established near the main discharge tube in the carburettor throat.
We are now getting more fuel added to the larger volume of air rushing in to equalize any vacuum the engine is trying to create. Because of pistons moving down on intake stroke.
A richer mixture take less time to burn so we need to retard ignition otherwise over advanced engine knock would be apparent, so:

3. At 1/2 throttle there is a combination of now lower vacuum advance & the just getting going mechanical advance [neither anything like full].

And lastly:

4. At full throttle no/light load there is maximum mechanical advance & less to no vacuum advance.
John GIBBINS Member Institute of Automotive Mechanical Engineers [Ret], ASE Master Medium/Heavy Truck & Auto Technician USA -2002 Licensed Motor Mech NSW MVIC 49593 Current 2015
Understand how system parts interact with one another. GOOD parts can then be established & the NOT GOOD problem/s part/s isolated for repair or replacement.
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Re: Retrofitting ignition vacuum advance

Postby Seff » Tue May 09, 2017 10:23 pm

Why are we discussing this? Last time we got nowhere.

Let me ask in some other way: What happens when I connect the vacuum advance to full manifold vacuum that you think is so bad? I don't want you to preach the way you have learned it; I want you to compose an argument for why high vacuum at idle is bad. Because that's the only difference between a ported and a manifold vacuum source.

As for point two, I think you're confusing density and rich/lean. They're not the same thing. You can have a rich mixture that's very low density, and you can have a lean mixture that's very dense - or rich and dense and lean and less dense. If you've ever driven a car with a vacuum gauge attached, you'll notice that even when the throttle is more open at higher RPMs, it maintains high vacuum when it maintains speed. Vacuum is not something that drops as RPMs rise - vacuum is something that drops as LOAD increases.

For point three: ½ throttle has nothing to do with mechanical advance - only RPMs affect mechanical advance. Maintaining speed at a given throttle will always produce vacuum, because you're throttling the engine.

For point four: At "full throttle no/light load" mechanical advance is still controlled by RPMs, so if you go to full throttle from idle the mechanical will of course not me advanced until you reach the RPMs necessary to advance it. Given that you say no/light load, you will experience HIGH vacuum and thus a high degree of vacuum advance (once the vacuum normalizes). The caveat here is that you practically never drive your jeep at full throttle with no/light load, because then it'd keep revving. Let me know if I need to explain why this is.
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Re: Retrofitting ignition vacuum advance

Postby Seff » Sun May 14, 2017 9:09 am

You can reply whenever you feel like it, I'm anxious to hear your reply.

While we wait, I can tell you that first test run of 165 miles gave me a mileage bump to 17.17 MPG.
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