engine science of the 30s & 70s for HT motors


Well-Known Member
Sep 30, 2012
back in the 30s Vincent wrote an article about rod length and engine power in which he noted that the sooner a rod comes square to the crank, the greater compression power available to drive the vehicle

triumph motors made use of this in the late 70s (early 80s???) by shortening their rods and barrels 1/2 inch with no other changes to get extra torque

I note now, that the PK80 motors (unlike the A80 & GT5) now have the 40mm stroke, but with the 38mm rod - low end torque & hill climbing are noticeably better.

in the 70s, dyno testing became popular and folks learned a lot - specifically, I see talk here about porting & polishing, while we learned in those days that porting alone gave greater performance and polishing tended to disrupt air flow (it seems that if walls are too smooth, they generate eddy currents in the flow)