Let's talk spark advance again.

If you did not know by know, I like to research things, this week it is spark advance.

Spark advance is used so that the flame front reaches the piston when it is typically about 17 degrees ATDC. I would imagine the bore/stroke ratio's would affect this some. The total distance the flame front has to travel is from the spark plug to the top of the piston. The flame front will travel faster on higher compression engines, or in a more rich mixture. This explains why a high compression engine needs less timing advance, I would assume the rich mixture travels faster because it is more dense.

I am going to measure the distance on my tractor from the spark plug to the top of the piston when the crank is at 17 ATDC just for learning. I should be able to back calculate the flame speed to get rough timing numbers.

I was surprised by the amount of total advance car engines have at idle. In the old days, the initial timing was usually set with the vacuum advance shut off. This value would be 8-12 BTDC, then would advance 10-15 degrees under vacuum. I remember them running OK, then picking up an sounding really good when the vacuum was plugged in. The total advance would be between 20 and 30 degrees at idle.

I was also surprised with the total advance at full throttle and light loading.

The extra advance, within reason, will allow for a leaner mixture.

I would think that any engine with varying loads would run better with vacuum advance, especially off idle torque. I am lucky, that I can program in these settings without having to change springs.

I have noticed that my MH44 EFI engine runs much better with the timing advanced to 12 @ idle to 30@ 1500. I have them the same for any vacuum right now, and it did ping under load, so I will assume I can run a little more advance during light loads, but need to back it off several degrees when loaded.

Here is a pretty good link http://www.megamanual.com/begintuning.htm

Please comment, everyone has good ideas.

I believe typical flame front speeds are about 45-50 inches per second

A guy brought me an Oliver 1650Lp that had a vacuum advance. First time I had seen that on a tractor. Seems like a good idea for chores and putting around. I didn't measure the amount but the vacuum advance was not very much.

Related to this discussion is combustion chamber turbulence. Later model heads are much better than earlier in this respect. Tightening the quench clearance on older motors helps. How tight depends on RPM and how brave you are.

The tightest one I built was a 350 with .028 between the piston and the head. True 9.5:1 compression and flat top pistons, iron heads, Quadrajet. Sitting in a 3800lb Camaro it could deliver 25mpg at highway speed with 23 being more common. No expensive parts in it.

That's some interesting stuff!

Thinking about the dimensions of your engine vs. an automotive engine...

Smaller bore, longer stroke, lower RPM power range, low overlap cam, all add up to less advance being needed.

Another factor is rod length. The longer the rod, the longer the TDC dwell range. Takes more degrees of rotation before significant movement of the piston, gives more time for the flame to spread. So would the 17* still apply?

My insight:
The peak combustion chamber pressure is not related to the distance to the piston. It is the moment that 90 or so % of the fuel air is combusting. The flame is going to progress to the farthest location in the chamber and be quenched against the cool iron of the head and piston. (comparative coolness)
The peak pressure should come between 10 an 12 degrees after TDC. (a variable)
Maximum advance under full power is the specific degree of advance that just begins to cause detonation.
Detonation is the increase in pressure and temperature in the cylinder as the fuel burns that causes the remaining unburned fuel to explode through its volume rather than burn.
Piston pin offset, compression ratio, chamber shape (including squish areas and surface areas of the crown) fuel octane, and temperature all are variables in the equasion. Jim

An unobstructed combustion chamber requires less advance. Spark advance in a JD 60 with the sparkplug on the block is much different than a Chev LS engine.
Ideal spark advance gives the highest mean average combustion pressure vs mean average compression pressure WITHOUT detonation.

DavidG,

My brother had an old Indiana MC. The right hand was the throttle, left hand spark advance. All I can tell you is better retard the spark when you kick start it or it will kick you back.

I'm thinking some old cars and tractors were like the Indiana and had manual spark advance. Back then you adjusted them for max power. geo

Take all this info and toss it out the window. Hook it to a dyno and set the timing advance for max. HP !

Unless you have a way to measure any change I feel it is best to stay with factory specs . Or as you say check with a dyno to see what you are doing.