I can only compare my first std. Rapide, which was smooth up to 4000rpm and had some vibs over 5000rpm,
using MKIII cams. It reached max. 5.800 rpm at 110mph. I had the crank done by Tony Maughan, who´s
balance factor I did not know. When racing Ernst´s BL, it was different: It was dead smooth up to 6500rpm
and pulled much stronger than the Rapide, no comparison. I think, Ernst told me once, he used 55% balance
factor, but I am not sure. His BL was the only twin, which did not move backwards on the paddock stand,
when revving up to 6500rpm. My Norvin, using a D-Shadow engine with an original crank, does vibrate more
than the BL, but is also smoother, than the Rapide, using MKII cams, 8:1cr, 32mm inlets, double ignition.
My impression: For speeds over 100mph, a higher balance factor is recommendable.
Also Erich Kruse´s Comet with Ernst´s short stroke engine revs up to 8500rpm is quite smooth, but I do not
know more abt its engine. It is stronger than Ernst´s 1948 racing Comet with normal crank and max revs
up to 6200rpm. Erich´s engine feels a bit less torquey under 4500rpm, but stronger, exceeding 5000rpm, than
the older Comet. Erich uses a 5 speed box, which compensates the lower torque, compared to the 4 speed
box of the 1948 Comet racer.