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Lower Crankcase shape.

Old 12-27-2010, 08:13 AM
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Default RE: Lower Crankcase shape.

So is that transition or "low end power"? The difference between a prop and wheeled vehicle is how long it takes to reach high RPM. If the engine can't transition from part throttle to full throttle it will make poor power at the low RPM until the mixture improves to the point it can. I don't think carb issues are in question here. Fuel mixture issues are a separate problem from how much power an engine makes at a given RPM and what happens when that power becomes available. In a car when the engine is at low RPM and more power becomes available it takes time to reach high RPM. This is felt in the seat of your pants as acceleration. If the engine makes good "low end power" acceleration is quick. This is easily seen on a dyno plot as well. In an airplane, the RPM response to increased manifold pressure is virtually instantaneous, assuming fuel mixture is such that this can occur.
Old 12-27-2010, 08:46 AM
pe reivers
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Default RE: Lower Crankcase shape.

Had not seen your reply Greg. So this answer is directed at Hobsy.

There IS a difference.
The load on a propeller at idle is practically NIL. During transition, the engine has no time to heat up. It uses the spare fuel in the crankcase plus what the carb supplies to overcome the fuel shortage during acceleration.
Low end torque is with the engine loaded at full throttle without rpm change (i.e. by a large prop). Not all engines are alike when treated like that.
Torque and power graphs are drawn with full throttle operation results, the engine gradually slowed down by adding brake power, or gradually accelerating by driving a spinning mass. Transition data do not enter that picture. There should be no throttle management during those tests.
The latter dyno test is a bit like accelerating a car, albeit without the added air resistance that a car, or in general a vehicle, would impose on an engine. These dyno tests normally start their readings, when transition from idle to full throttle operation is finished. Data accumulated during the transition however also can be very valuable.
For instance, I saw MVVS dyno tests, done to record spool-up with different air entry bells on the carb. Also interesting to see was that with throttle closing, a very slight power rise could be observed as the valve traveled from fully open to partly closed. So the walbro should not be completely open if one seeks best power!

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