Combustion timing for John T

The point at which peak mean effective pressure is achieved in the cylinder depends on four (primary) factors, the speed of the engine, the octane rating of the fuel, the temperature (at the top of the compression stroke in the cylinder), and the compression pressure. These affect the flame propagation rate, and thus the point at which spark based ignition should occur.
The near zero RPM of hand cranking, and the resulting modest compression, and ambient temp. usually need a TDC setting for zero chance of kickback. This should be the setting for all ignition systems used on hand cranked engines. It does not drive the total advance, nor does it affect the advance curve.
If modern fuel is considered, and RPM is still at 1600 to 2100, total advance must still be in the mid to high 20s. This is due to the relationship between flame propagation rates and RPM. In slow turning, low compression engines, compared to the Automotive engines of today, the advance seems insufficient, but if taken higher it will detonate.
Advance should be set to produce the first hint of detonation (knock). At that setting, the Peak pressure is going to arrive near 4 to 6 degrees ATC After TDC. This is the sweet spot to gredge power out of the combustion, without driving the rings down around the skirt, or making holes in pistons. I love it!! Jim

Gee Jimbo, youre about as techy (i.e a Nerd) as I am lol. Having started several hand crankers and HATING kick back I was pretty well aware of the TDC static/start timing. Ive still taken issue over the years (BEFORE COMPUTER TIMING) of buddies who always timed their cars or trucks too darn fast that they got a noticeable pre ignition spark knock when they floored it or were going up hills. I dont mind a tad of it but if they knock n ping too hard n too loud at the slightest hill or acceleration my intuition just tells me THATS TOO FAST...They were the same guys that when they started their cars you heard those bumping the starter and grunting sounds, again because TOO DERN FAST. They insisted that gave them more HP which I didnt n still dont agree with IF ITS TOOOOOOOO FAST.. As you know there are engineering trade offs involved which is why I PRETTY MUCH would hang my hat on what the engineers recommended for day in and out LONG TERM performance versus long engine life. If I was a puller only running short times I can see it different, Im talkin long term average use here for engine longevity.

This all started out when I saw no differecne in timing if the tractor was a 6 versus a 12 volt.

Its cool n damp here in mid Florida today but it beats whats up north I hear

John T

"Total advance must be in the mid to high 20's." Why is it then that the Farmall H calls for 35 degrees running timing with a magneto and 40 degrees with a distributor?

Why in the world would you be bothering an engineer with these statements, when the other day you agreed with your friend old that he knew way more about combustion theory than any engineer?

(quoted from post at 12:54:01 01/09/10) .....................buddies who always timed their cars or trucks too darn fast that they got a noticeable pre ignition spark knock when they floored it or were going up hills............................... were the same guys that when they started their cars you heard those bumping the starter and grunting sounds, again because TOO DERN FAST.

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Actually neither one of these indicators is a true test of timing that's "too fast" It can simply mean that because of typical "hot rodding" ---in which changes in induction, exhaust, and especially camshafts and cam timing, have "re-engineered" the engine!!!----and now that "he" has done that, the timing CURVE is all screwed up. The curve can be to short, too long, too slow, too fast, and initial timing can be in the wrong place

There have been SEVERAL friends of mine who, when "flooring" the loud pedal at MEDIUM speed got lots of knock because the advance was "too much" AT THAT MEDIUM RPM yet tests by timing ONLY at high RPM total advance showed the engine WAS making optimum top end power

The point? You can't make blanket statements, especially if the engine has been modified.

Let's not forget, either, the simple things like worn bushings CAN CHANGE THE ADVANCE CURVE because as the shaft moves in the bushing slop it CHANGES THE POINTS GAP and that changes the timing!!!

Worn pivots, springs, etc in these the mechanical advance of these old girls can render conclusions completely erroneous

I mess around on the weekends with a restored M&W P-400 dyno.

We time motors with a digital timing light with a tach built in.

When you establish a final rpm that your happy with we time it.

Clip the pick up to cylinder #1 wire and spin the motor up to full throttle. At this point watching the tach in the timing light turn the dist. tell you get max rpm.

This works great , but we get even better results doing the same thing under full load of the dyno.

Under load we have seen 4 to 8 hp gain and this is on stock motors.

We have messed around with light weight springs on the advance weights to bring in full advance real fast.

I always woundered how much actual advance we would really get with a stock dist in a Super M with those bed springs they use on the weights.

The springs are so heavy and the rpm is only 1800 so do they really open.

Keep it simple these old tractors are sloppy.


A pulling tractor is another story all together MSD/crank triggers/ high out put coils/ timing retards etc etc etc....

I am staying out of this

I think they are both great people, and great contributors to this forum. The issue is clearly respect for both Old, and John T. My statement on the timing numbers is remembered, and only that. Look up the specs for each engine, every time. The true best method of timing is to have a knock sensor on each cylinder use that data to teach a processor to make a curve for each cylinder, and keep it learning so it integrates each tank of fuel and each weather change, and each load condition. The best we can do with an old tractor is to keep it operating well to use it, or tweak the boogers out of it if pulling. Jim

Yo again Jim, on the old Deeres Im familair with, most that run on gas time indeed as you speak of in the mid twenties advance at rated RPM. Its ONLY the older tractors when they used low octane distillate or drip fuel that some ran at 35 BTDC (Mags NOT variable RPM based advance distributors) but thats ONLY THE LOW COMPRESSION USING LOW OCTANE DISTILLATES (Tractor Fuel).....

Thanks again for the technical information, always fun chattin with ya

John T

Thanks, great info 440 and fun technical discussion Ol Jim started. I hear what youre saying, but without using any timing light (they just loosened the distributor and advanced them lol) they advanced them and I can guarantee you if you heard how hard n loud they knocked, YOU WOULD SAY THEY WERE TOO DERN FAST ALSO I BET YA LOL

Take care yall n God Bless

John T

If my summer teaching schedule permits,I will be at RPRU in LaPorte, IN this summer. My folks are from that area, 30 miles or so. Jim

Let me know as the time gets closer maybe we can tip a few longnecks

John T

oh yes engineers aint they brilliant, like ford engineers when they introduced the y block from 54 to 65 after market suppliers made a fortune on just the overhead valve oiler kits alone. the problem was in the cam bearing that would spin and shut off the oil flow the problem was never fixed right up to end of manufacture there are many examples of this type of brilliance. some of the dumbest people are very highly educated. al

if you bought a fire-crater kit from your ih dealer in the sixties it would have come with a new distributor plate that was a 22 degree advance.