Quote:
Originally Posted by sendler
So using your logic a 650cc engine running at 6hp will always be more fuel efficient than a 125cc engine running at 6hp. But in reality this is not the case due to the increased heat losses in the larger combustion chamber and the increased pumping losses due to the throttle being closed more with the bigger engine. The first thing for you to learn about here is the concept of Brake Specific Fuel Consumption versus rpm and load.
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https://en.wikipedia.org/wiki/Brake_...el_consumption
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Not really true!
Each engine has a specific range where they run most efficient.
The larger the engine, the higher the MPH is that they run more efficiently.
If I where to gear a 650cc engine the same I would gear a 110cc engine, their most efficient range would be (my guestimate) at 3000RPM, about 60 to 80MPH. This would cause lugging of the engine.
But suppose your main goal is to continuously go at 60MPH, you would not choose a 110cc, not even a 250cc to be your main engine; but rather something like a 500cc.
The larger combustion chamber actually is much more efficient than a smaller chamber, as MORE of the heat is converted to motion, not less.
The cylinder walls of a small engine drain more of that energy than on a bigger engine.
*Edit: I just read jkv357's reply, and it's the same as what I wrote.
I'm not sure if you ought to ride a constant 50-55MPH, if a 110cc would have better MPG than a 250cc, but the 110cc should run at a much higher RPM to achieve this, and the life of the engine would be shortened, so a 250cc would make more sense. Larger cc engines might lug at too low revs.
I find at 50MPH, a 250cc should work just fine between 2500 and 3500RPM. A 350cc engine would work well between 2250RPM and 2750RPM. A 650cc engine would need to run at 1500RPM to have similar MPG, but that would lug the engine.