Quote:

does not seem they lost hp going to the 270
Frank


It's hard to misread Triumph's published specs: America max HP is rated at 54hp@6750rpm, Bonneville max at 66hp@7200rpm. It also implies the 360 is designed to rev higher. There are a lot of us America/Speedmaster owners doing aftermarket work to achieve that extra 12hp! Of course, we won't really know until some money-boy drops a stock 360 into an America and dynos it...
Anyway, This quote from T595.com fleshes out this discussion:

"The reason for the 270 degree crank is that, being 90 degrees apart, one piston is always at maximum velocity down (or up) the bore at the exact same time as the other piston is at minimum velocity at TDC or BDC.

This means that the potential energy in the high speed piston is released as kinetic energy against the drag of the stopped piston and eases the piston through this period and assists with accelerating the piston back towards its maximum velocity.

It is supposed to be smoother that a 360 degree crank with its primary balance forces acting together whilst not suffering the high frequency secondary vibrations.

I think it was Phil Irvine (assisted with the design of the Vinnie twins) that actually calculated that 72 degrees difference was the best angle to have between parallel bore motors for this exact reason."

And there you have it.