Subaru RPM/cams

[Gordon Mohr]

Just a note to those who are "souping up" their 2.2 to get more
power. The high performance people use the engine blocks from the 2.2
Turbo. The Turbo crankcase has beefier main bearings and a stronger
case. Subaru wouldn't have done this if they didn't think it was
necessary on engines with over 135 hp.

With regards to cams; regrinding isn't necessarily just for more
horses. A car's cam is ground to give good throttle response and even
power over the whole speed range so it's easier to drive. This is a
compromise that robs the engine of efficiency at operating speed.
Because aircraft, boat, etc. engines are intended to run at a steady
speed with no gear box, (which is solely a device to convert high revs
to torque) they can have a cam that brings the max horsepower curve
nearer to the max torque curve. On the stock Legacy 2.2 the max torque
is at 4400 rpm and the max horsepower is at 5600 rpm. (horsepower is
torque x rpm, therefore higher rpm at a given horsepower means less
torque, or pulling power) . If you don't have an adjustable gear box,
or a constant speed propellor (same thing) a properly ground cam can
give more actual power at a reasonable propellor speed. (Prop
efficiency decreases above 1800 rpm or so) A typical aircraft engine
cruises at the max torque rpm, which gives the most efficient power for
fuel. If your propellor is set to give you 5600 rpm on takeoff, it
will be too fine at 4400 rpm. If a reground cam can give you max
horsepower for takeoff at something like 5200 rpm and cruise at a max
torque of something like 4600 rpm, there should be a similar effect to
having a constant speed prop, with better cruise speed using the same
fuel and less wear and tear on the engine.

Regards, Gordon Mohr



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