It's the cylinder pressures and temperatures in the later HIGH compression IH gas engines. That is, any post-1958 tractor starting with the 460/560 series.
The old pre-1958 engines are LOW compression engines, and run fine on 87 octane. Higher octane is a waste of money on these.
Lean, rich doesn't make a lick of difference. The low-octane fuel will still go *BANG* too soon in the high compression engines. It's the fuel, not the mixture.
Octane doesn't make any difference in modern cars because there is a computer, a bunch of sensors, and a bunch of controls that change timing, fuel mixture, etc. to compensate. These old engines have one "sweet spot" where they work. Performance and longevity fall off drastically when you start messing with that sweet spot.
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Today's Featured Article - Tractor Profile: Allis-Chalmers WD45 (Gas Version) - by Staff. Beginning production in 1953, the WD45 had the same general look as the WD tractor which was produced earlier - but was much more powerful. This was due to the new 4-cylinder "Power Crater" gasoline engine with a 4 x 4-1/2 inch bore and stroke. Some people feel this was one of the most significant tractors offered by Allis-Chalmers in those days. Nothing in its weight or power class could match its performance. In 1954 they began offering the WD45 with the new "Snap-Coupler&quo
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