Timing

[jeepguy1948]

I am curious where spark occurs when spark advance is fully retarded (lever all the way up)? Please don’t waste my time telling me why I didn’t need to know, I simply “want” to know. Thanks

[Flathead]

It should be ATDC or "after top dead center" the spark would occur after the piston starts going down on the power stroke.

[J Franklin]

I thought the points were set to open at TDC with the spark lever fully retarded. Factory default.

[Ramman]

Originally Posted by J Franklin

I thought the points were set to open at TDC with the spark lever fully retarded.

Nope. Set your points opening and rotor position with spark advance lever down 1 to 2 notches.

[alexiskai]

If you are timing your A per factory standard, then when the distributor is set to 20° retarded, which corresponds with the top of the spark control rod’s travel, ignition will occur exactly at top dead center.

[Flathead]

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Timing is measured from TDC which would be 0. Advance would be how many degrees before TDC that the spark occurs, retard would be how many degrees after TDC the spark occurs. Advance=before TDC, retard=after TDC, it is all about piston position when spark occurs. That's my story and I'm sticking to it.

[jeepguy1948]

I understand that Flathead, perhaps I need to ask my question in a different way; assume that I wanted to set the timing with a timing light (it may be unnecessary but it’s not illegal) and I have installed a pointer and degree marks on crank pulley. If, the spark advance lever is all the way up, where would I want the timing to be? TDC? 2 degrees before TDC? 2 degrees after TDC? Etc., etc..

[Ramman]

2° - 4° after top dead center. Especially if you ever plan to start it with the hand crank.

[Ed in Maine]

When I first got my 1914 Model T (no starter, just crank), I came close to selling the car. I did see the previous owner retard the spark, prime the engine with 3 cranks, full choke and ignition off. Then, turn on the ignition and crank till the engine fired. He was big guy and there was no doubt he could throw that crank from 9 o'clock to 2 o'clock faster than an electric starter! But, I was a 75 year old weakling and with his timing adjustment, that crank would come to a complete stop before I could get to 2 o'clock. While I wasn't hit by the crank, it still hurt my wrist when the crank suddenly stopped. I then took the time to see just where the piston was when the contacts closed on the timer. I used a dowel into #1 spark plug hole and I could easily see that the piston did was not at the top of the travel when the timer contact closed. I used a volt-ohm meter across the timer. I then changed the setting on the timer so that the contact closed in the timer when the piston was seen going down about 1/32 - 1/16 in. of an inch. What a difference, I had turned that Model T into a kitten. When you crank the Model T after priming, you can feel the crank suddenly ease up when the car is about to start. The engine suddenly surges ahead of the crank. There is no doubt, that the car will be running after the next crank.

With Model A, just think how easy you are making it for the starter. if the engine is slightly past TDC when the points open. You are removing all that stress on the starter gear and that long shaft at the moment of ignition. Plus, your car be running in the first turn of the crank shaft. Obviously you must initiate the spark when the piston is at or past TDC by a tiny amount. Ed

[alexiskai]

Originally Posted by jeepguy1948

I understand that Flathead, perhaps I need to ask my question in a different way; assume that I wanted to set the timing with a timing light (it may be unnecessary but it’s not illegal) and I have installed a pointer and degree marks on crank pulley. If, the spark advance lever is all the way up, where would I want the timing to be? TDC? 2 degrees before TDC? 2 degrees after TDC? Etc., etc..

The thing you're describing is called the "initial timing." Initial timing is the most retard possible in the ignition system; typically it's the setting people use when starting the car. The initial timing you choose is really dependent on what you're trying to do:
- TDC is sufficient to start with a hand crank, and it's the factory default.
- A lot of folks like being able to move the spark lever all the way down when cruising, so those people like to set spark 5-10° ATDC when fully retarded.
- Others are using a distributor with some kind of auto-advance, so those folks sometimes adjust the initial timing so that the total timing comes out to a particular degree they want.

[goodcar]

Originally Posted by Ramman

Nope. Set your points opening and rotor position with spark advance lever down 1 to 2 notches.

Yes, a 6-volt light connected from the points to ground will just begin to glow at 1-2 notches down.

[J Franklin]

Originally Posted by Ramman

Nope. Set your points opening and rotor position with spark advance lever down 1 to 2 notches.

I set my timing as the factory did. Others set timing differently. The question has been answered. Setting with the lever down some just retards the engine more while running.

[J Franklin]

Originally Posted by Flathead

Timing is measured from TDC which would be 0. Advance would be how many degrees before TDC that the spark occurs, retard would be how many degrees after TDC the spark occurs. Advance=before TDC, retard=after TDC, it is all about piston position when spark occurs. That's my story and I'm sticking to it.

Interesting theory, but running at TDC is running retarded. all notches down on the quadrant are advance.

[nkaminar]

Flathead is absolutely correct. TDC is 0 degrees, neither advance nor retarded. J. Franklin is also correct in that running an engine when the plugs fire at TDC is retarded. Why? Because it takes some time for the fuel to burn and running with the plugs firing at TDC means that the pressure is developed way after the piston starts down. So in a sense the pressure buildup is retarded.

[old31]

X2 with nk and flat.

[Bob Bidonde]

These slides give an overview of what's happening with spark advance:

[alexiskai]

Originally Posted by Bob Bidonde

These slides give an overview of what's happening with spark advance:

Just to give slightly more context to Bob's excellent infographics, the arc traveled by the hand spark control lever is obviously greater than 20° as shown in the diagram. But it's labeled as 20° because it represents 20° of rotation on the distributor upper plate. Each degree the upper plate is rotated advances the spark by two degrees as measured at the crankshaft. Therefore the total range of the hand control lever is 40° of crankshaft advance.

You'll often see discussion of "each notch on the lever equals X° of advance" – this measurement is really unique to each car and should not be taken as universal. The amount of advance corresponding to each notch varies, not just between the 28-29 and 30-31 models, but also on the advance quadrant itself. The amount you get from moving a notch at the top and bottom is different than the amount you get in the middle, due to the multiple steps in the linkage between the lever and the distributor.

[redmodelt]

I set it with the points just about to open when the pin is in the recess on the gear when using Model A front cover with the round boss.

[Flathead]

This rabbit hole is deeper than I expected. Jeepguy sorry if I misunderstood what you were looking for.

[jeepguy1948]

It’s all good boys, it’s all good.