Power to weight is a factor here, but I think the question asked was simply why doesn’t the 1500cc bike have twice the horsepower of a 750cc motorcycle, as it’s twice as big?
The answer here is mass. A 750 V twin means one cylinder displaces 375cc. A 1500cc twin then works out to 750cc per cylinder. Yes, twice as big, and that means a piston twice as large and twice as heavy. Add in the heavier connecting rods, a heavier crank, bigger valves, and just an overall bigger motor, that extra mass will need more energy to move. If it is twice as big it will take twice the energy to move all those parts. So it shouldn’t be a surprise that the horsepower from either motor would be about the same. But the extra mass would increase torque.
Talking just V twins here, the smaller motor should be able to rev higher (higher redline). V twins are also a bit “unbalanced” as power strokes between the two angled cylinders isn’t as even as say an inline four where you can have two pistons going down as the other two are going up
Mass wise look at a 1000cc in-line four. One cylinder displaces 250cc. A 1000cc twin has 500cc displacement per cylinder. That extra mass will limit its redline. (That and of course valve size and duration)
The Yamaha R6 is an in-line four that displaces 598cc’s..... which works out to about 150cc per cylinder. This reduction in moving mass allows the bike to have a redline of 17,500 rpms.
Obviously using the technology of titanium valves, ceramic pistons, etc can raise not only redlines but overall engine performance.
Most V twin (cruisers) are as I said made to appeal to your eyes, actual horsepower isn’t their selling point. With some good engineering ($$$$) you can have a big V twin with impressive power. But again, that’s not the main selling point.
BMW makes a sport bike that at 1000cc puts out 200hp on a bike that weighs close to 400 pounds. There’s your power to weight ratio.
As I’m talking physics here, the reason the bigger (heavier) motorcycle are “smoother” to ride is directly due to that extra mass. When your tire hits a small bump, your suspension tries to absorb it by compressing. But looking at it as action-reaction, the wheel moves up, “pushing” the spring up...against the frame of the bike. If the bike is light, that “push” will bounce the bike up. If the bike is heavy, it won’t bounce it up as much. Your forward momentum on a heavy vehicle simply takes more energy to deflect than it does on a lighter vehicle. Newton’s second law, you can easily deflect a ballon but not as easy a bowling ball. This is why Cadillacs in the 60’s were considered to have the smoothest ride, they just were huge and heavy, so their momentum down the road was hard to alter.
My 600+ pound Yamaha FJR made roads I thought were just bumpy as hell on my Vulcan seem glass smooth.
If you have ever rode a Goldwing down the interstate you’ll understand.
I’m not advocating big motorcycles here. Im
Saying if you want to tour the US coast to coast, bigger is better.
If you want to tear up the twisty roads around where you live and don’t need to go 200 miles, smaller is better. I had more fun on my 400cc Yamaha RD doing that than any motorcycle I’ve owned.
But riding 500+ miles day .... the FJR was the winner
