Has anyone heard of an experimental aircraft using a japanese
4-cylinder motorcycle engine? The advantages seem obvious when
compared to other non-rotary auto conversion choices. Modern 1000cc
engines make 180hp, are liquid-cooled, lightweight, and inexpensive.
If such an engine was de-tuned to 100-120hp by lowering the redline
(say from 13k to 9-10k), it might give close to a 1:1 power:weight
ratio with high reliability.

I can't be the first person to think of this. If it hasn't been done
before, why?

-Chris Wood

Probably a cooling issue. Motor cycle engines, like car engines, are
light duty. Several people have tried air cooled BMW engines for the past twenty years but they never caught on.

http://www.kawasaki.com/Products/product-specifications.aspx?id=345
Engine Type      Four-stroke, liquid-cooled, DOHC, four valve per cylinder, inline-four with VVT
Displacement     1,352 cc

Maximum Torque     100.3 LbFt @ 6,200 rpm
Compression Ratio     10.7:1

Curb Weight     670.3 lbs. 679.1 lbs. (ABS model)

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No mention is made of the red line or the HP surprisingly.
At 670 pounds for the whole bike the engine block could easily
weigh 200 pounds or more.

100 ft pounds of torque at 6200 RPM is 119 HP. About the same power to weight ratio as a Continental O-200 direct drive aircraft engine. Of course you will still need a 20 to 30 pound gear box. Motor cycles are designed for side ways mounting... chain drive so it is no easy matter to design a gear box that will take the power out inline or turn the drive 90 degrees for a propeller.

That is also half the displacement of a Mazda 13B rotary engine with about the same block weight but the rotary will easily put out 200 HP in side port form and 250 HP in p-port form.

These engines in new shape are also not cheap.

Paul Lamar