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Tractor Talk Discussion Board |
Re: a little tune-up idea
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Posted by john d on May 23, 1999 at 14:50:32 from (206.141.74.34):
In Reply to: a little tune-up idea posted by RKS on May 21, 1999 at 20:12:36:
You're right-on about the engine running cooler, more efficiently, and smoother in most cases with the timing slightly advanced. However.......I'm gonna have to agree with Nellie in the post below about the octane ratings on the gasoline, and take exception to a couple of points in your post regarding combustion. "Detonation" is not really what we want out of gasoline. We want it to burn, smoothly and very rapidly, but BURN, not detonate. High octane gasoline becomes necessary when the design (such as high compression or combustion chamber shape) or operation (such as lugging under heavy load) of an engine causes the gasoline to detonate in the combustion chamber instead of burn. It sometimes also helps prevent "dieseling" or "running-on" when the ignition is turned off. "Dieseling" in a gasoline engine is usually caused by something in the combustion chamber being too hot, and ignitiing the air-fuel mixture without a spark. Common culprits are carbon deposits, a spark plug with too high a heat range, or an exhaust valve that is'nt allowed time enough to cool off when shutting down the engine after a heavy load. This same set of culprits can cause "pre-ignition" in an engine, which is the ignition of the air-fuel mixture BEFORE a spark occurs. This condition, if allowed to continue, is deadly to the internal parts of an engine, especially the top of an aluminum piston. The greatest benefit of high-octane fuel comes in combatting the condition known as "detonation" which which is the explosion (not burning) of part of the air-fuel mixture in the combustion chamber. This occurs when the flame front advancing through the combustion chamber causes combustion pressures to rise very rapidly in the side of the combustion chamber opposite from the spark plug. Under these conditions, the last part of the mixture doesn't burn, it becomes unstable and EXPLODES because of the extreme pressure, causing a shock wave that reverberates throughout the combustion chamber. That "knocking" you hear under those conditions isn't just the valves clattering a little, it's the sound of a high-pressure detonation that is threatening to knock a hole in the top of the piston, or push the top ring down where the 2nd one is! Gasoline does NOT like to be squeezed extremely tightly and burned suddenly. Tractor engines that operate under light loads will generally benefit from advancing the timing a little bit. The 1-3 degrees you mention shouldn't hurt anything, and I'm in agreement with your other post where you say most of these old tractors probably aren't timed where they should be! But push the timing just a little too far and lug it under a heavy load for a while, and serious problems will develop. A tractor engine such as an old Farmall has a long stroke, relatively low compression, and is very rugged, so you can probably advance the spark quite a bit before having trouble. That does not mean, however, that modern gasolines burn too slowly and therefore engine timing must be advanced to compensate. At the speeds these old things run, and with the long stroke to maximize expansion, about the only time the gasoline will burn too slowly is when the engine is extremely cold. This becomes evident when an engine backfires through the carburetor in very cold weather because there is still some residual flame in the combustion chamber when the intake valve opens.
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