There are a couple of possible factors to consider here.
One factor is the relative mechanical advantage differences between
gears. Obviously, first gear has the greatest mechanical advantage and
fourth gear the least. Although it depends on acceleration, slope, and
how much you lug the engine, there is certainly less of a tendency for
a clutch to slip in the lower gears under normal driving conditions.
Another possible contributing factor: Many clutches (I'm not
specifically familiar with the M151 clutch) have built-in centrifugal
weights on the clutch fingers that are designed to augment the pressure
plate springs and apply extra pressure at higher RPM, the idea being
that this type of clutch requires less pedal effort, and when you're
pouring on the coal, the weights will apply the added pressure you need
to keep the clutch from slipping under heavy loading. In first and
second gear (depending on how you drive), there is a tendency to run
the engine at a higher RPM than in third and fourth gears, thus
applying more pressure to the clutch.
Have you ever smelled the clutch burning? If it's slipping a lot, there
is usually a tell-tale odor...at least for a while. However, after a
clutch disc is well "fried," it doesn't emit as much odor, since most
of the organic/volatile matter has already been burned off or
carbonized.
If you continue to drive the vehicle in this condition (assuming that
the clutch is actually slipping), it may damage the flywheel beyond
repair, making the fix more expensive.
Regards,
Alan
(Salt Lake City, Utah)
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