We have made small diameter crank pulleys in the past that were designed to
reduce the water pump and generator speeds by about 1/3. We have never
found any advantage in using a narrow fan belt or in installing a "damper".
The engineers that we have consulted with in the past have voiced the
opinion that a 6 cylinder or a v-8 crank with the rod throws at angles
relative to one another that are not 180 degrees develops torsional
vibrations that require a damper, but a crank with throws at 180 degrees
like our "tractor" engine does not. They have said that a damper on our
engines is a waste of time and just extra weight and inertia. Consequently
we have never installed dampers unless the owner of the engine insisted.
With a good balance job and rods and pistons as light as we can get them, we
have had no failures that could be attributed to torsional vibration as long
as the revs were kept under 7300 rpm.
The way we have made crank pulleys is to start with a standard hub and
cut it down and redrill the holes for the pulley to the smallest diameter
possible. Then we machined a new pulley to match the new bolt circle. The
belt groove was then machined to match the standard generator pulley
profile, but with the inside diameter just larger than the new bolt circle
that attaches it to the modified hub. This procedure has worked very well
for us.
In back to back engine dyno testing , at 6,000 rpm on an engine making
165 hp, slowing down the water pump by 1/3 was worth about 6 hp at the
flywheel. The top water hose from the thermostat housing was partly a clear
glass tube. With the normal water pump speed there are a lot of small water
bubbles flowing away from the engine. These are caused by cavitation on the
back side of the water pump impeller. Slowing down the pump dramatically
reduced the number of air bubbles. This would tend to imply that the water
pump is operating more efficiently at the slower speed.
The main reason that we did not go to the narrow belt was that we just
did not want to go to the trouble of having to change to narrow pullies on
the generator and water pump. The wide belts have been very reliable and
operate at a very low tension setting so do not put to much side load on the
water pump or generator bearings.
Greg Solow
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fot@autox.team.net
http://www.fot-racing.com
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