Posted by Brian in Oregon on Monday, February 12, 2001 at 1:57AM :
In Reply to: Prone to pre-ignition and detonation ? posted by R Davis on Monday, February 12, 2001 at 0:11AM :
I'm comparatively speaking about the differences between an OHV and an L-head.
A stock L-head usually has little problem in this respect, but under high compression, the build up of carbon near the valves can act as a secondary ignition source, causing an undesired second flame front that collides with the one fired by the spark plug. This can put a severe shock on the piston, rod and crank. Or it could set off the mixture well before the spark plug does, putting strain on the piston, rod and crank by trying to push it backwards on the compression stroke. Switching to a higher octane fuel is only a partial solution.
There have been aftermarket heads for some flatheads that helped get around this problem. The Ford V-8 flathead had many aftermarket heads for it. The most successful converted the flathead to an overhead valve.
L-heads also leave a bit more exhaust in the fresh mixture because the large chamber area around the valves is not entirely purged. This contaminates the fresh mixture, particularly if there is oil or excess partially burned fuel.
BTW milling a flathead a lot to raise compression has the undesired effect of reducing the cross section that the air/fuel or exhaust must pass from the valves to the cylinder.
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