Spark Advance Positions....

Nabco

Nabco

Senior Member
Based on what I've read on FB, the operators manual, other web sites and YouTube videos I "think" I have a good understanding of where the spark advance should be set based on conditions.....thought I would post it up here to see if I'm on the right track and maybe it will help some other newbies too.

Starting - always fully retarded
After engine starts, during warm-up - about half way
Driving in traffic - about half way
Cruising speed level roads - fully advanced or close to that
Climbing a hill - retarded (not sure how much retarded)

If there are any other conditions/positions that I've missed it would be great to know. I also know each car will be a little different, just looking for a general rule of thumb.

Thanks

Nick
 
well if you think how a "modern" distributor works the higher RPM the more advanced it goes(to a point). So starting its fully retarded and a good idle its down a bit and faster you go the more advance you give. If it starts bogging on hills retard it some as the rpm is falling.
 
With a higher compression head, less advance than a standard head. The higher compression heads,5.5:1 and higher, do not go over 28 to 30* advance.
 
I like to think of when I was a kid and riding a bike... When I had a lot of speed and momentum, I could really push hard at the bottom of my pedal strokes... when I had less speed and momentum and in a lower gear, I pushed hard when not at the bottom of the stroke... I see timing of the same... if the piston is pushed at TOD, the car needs speed and momentum.. and if not the push need to happen less so it's easier...
 
I read somewhere , I'm not sure if it was in a book or online but it said advance one notch every 7mph. It well may have been on here in the archives. I try to read a lot of the old threads.
 
I've wondered if it can be related to the speed of your vacuum wipers? Slow wiper-less advance. Fast wiper-more advance. Or is that just a crazy-weirdo way to look at it? :)
 
Art Newland said:
I've wondered if it can be related to the speed of your vacuum wipers? Slow wiper-less advance. Fast wiper-more advance. Or is that just a crazy-weirdo way to look at it? :)
Click to expand...

That's pretty much a crazy weirdo way to look at it ;)

Sea Slugs nailed it. Advance on a Model A is purely related to engine RPM. It's like the centrifugal or mechanical advance on a more modern distributor ignition. As RPM increases...advance should increase (to a point).

When you floor it at 45 MPH, vacuum drops to near zero, wipers stop working, but the engine still needs a lot of advance. When you floor it in high gear at 5 MPH, wipers stop working, but the engine needs little if any advance.

More modern cars that are equipped with vacuum advance in addition to mechanical advance provide even more advance under high vacuum conditions (ie. light throttle cruise) primarily to give higher MPG.
 
At any given speed there will be a point that when you lower the lever there will be no discernible change in engine RPM. That it the location for that speed.
 
Art Newland said:
I knew it! I'm crazy. And weird.
Click to expand...

No comment ... but

My take on it is that a vacuum advance is, in reality, a way of retarding the spark when you floor it to stop pinging.

vacuum+mech+operation.png
 
Barber31 said:
I read somewhere , I'm not sure if it was in a book or online but it said advance one notch every 7mph. It well may have been on here in the archives. I try to read a lot of the old threads.
Click to expand...

My advance is so worn there is no click, just smooth.
As for using the windshield wiper speed, I have an electric on it now, so...just saying.
 
burner31 said:
My advance is so worn there is no click, just smooth.
As for using the windshield wiper speed, I have an electric on it now, so...just saying.
Click to expand...

At a given speed lower the lever until there is no discernible change in engine RPM and then a tad up. That it the correct timing for that speed.
 
updraught said:
No comment ... but

My take on it is that a vacuum advance is, in reality, a way of retarding the spark when you floor it to stop pinging.

vacuum+mech+operation.png
Click to expand...

That is true for manifold vacuum, but some vacuum advances take the vacuum from the venturi, which would increase as you open the throttle, and have no vacuum at idle.
 
updraught said:
No comment ... but

My take on it is that a vacuum advance is, in reality, a way of retarding the spark when you floor it to stop pinging.
Click to expand...


Vacuum essentially goes to zero when the engine is pulling hard...like full throttle acceleration. A vacuum advance would not be operating in these cases. A vacuum advance only works under light throttle conditions when there is engine vacuum to operate it. Its main purpose is to add about 15 degrees of advance over the centrifugal (mechanical) advance to increase fuel mileage at cruise.

Vacuum advance mechanisms use either full manifold vacuum or ported vacuum. The only difference is that ported vacuum systems only see vacuum when the throttle plate is cracked open and the vacuum "port" is uncovered. Otherwise both systems are seeing the same amount of vacuum at the same engine conditions.

A moot point obviously with our Model A's :)
 
Vacuum advance, once again my take on it, is to fine tune the mechanical advance. In fact it can be done away with even with quite high compression.

So what you are doing with the lever on a Model A is to try to emulate the smooth operation of the mechanical advance found in later distributors.
As Bill said, it needs to be matched to the speed.

I have no idea why Henry thought a bunch of levers was cheaper and easier to operate by millions of people (who most probably had no idea what they were doing), than to add a couple of weights and springs the distributor.

Here is a nice picture of the later mechanical advance.

cetrif+2.png
 
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