The original figures were taken on a Heenan and Froude type dyno, probably at the crank. It isn't unreasonable to subtract 10% for gearbox losses, so I'd expect a really good Shadow to show 50 on a rolling road. The club Shadow when first built showed 37 I think, boosted to the mid-forties by some tweaking.
Tom, I recall reading in MPH that the club did do some dyno work when designing the current version of the spiral baffle muffler. The bike used was said to be a somewhat below average performer, according to observations from riding. The horsepower figures were quite low, mid thirties, but then the testers only ran the RPM up to something like 3200 RPM for some reason. A full RPM test up to 5500 RPM would have provided more info. Torque was very high at the tested speed, especially with the spiral baffle muffler installed. Torque was in the 65 foot pound range at about 3200 RPM, which explains why Vincents hold 70 MPH in top gear so easily, even when loaded right down on a fairly steep hill. It also explains in part why they are so pleasant to ride, the high torque at low RPM means you are not struggling to find the power band.
I know of one local bike which has been dynoed at 65 RWHP, (John McDougall's Shadow) but it is quite modified.
Those power characteristics are typical of a low compression , small port , mild cam , big displacement motor.
I reckon 70-75 bhp should be easily achievable with higher compression and friskier cam , some port work & larger carbs . Still with a good torque band though.
How about 115+ HP at the rear wheel? Steve Hamel is getting that much from a modified 1000cc Vin on petrol, good for 155+ MPH at Bonneville. You can see what he did to the ports here. This is the sweetest sounding twin I've ever heard.
Paul Packman's Rapide, well put together and moderately tweaked is, if I remember correctly, giving over 60 BHP possibly 65. It is quite the most desirable Vin I know of and has sensible but unobtrusive mods done by Paul, a highly knowledgeable engineer.
He has done it this way so that he can keep up with his son on a Ducati ! Pictures of the machine distinctive with homemade 2ls front brakes is on www.thevincent.co.uk.
My Shadow shortly after running in after an engine rebuild was put on John Renwick's brake and Lambda equipment. He did an excellent and highly accurate job, but it was found to deliver a measly 46bhp. it goes ok though...strange things, horses, everyone seems to have a different breed.
Torque is the important thing for how any vehicle feels to drive or ride. I've never seen a torque curve for Vincent but I suspect it would be high and wide relative to the BHP curve.
Thats why Ducati's have been so dominant in World Super bike & Moto GP.
If you have the dyno graphs I would be very interested to see them ?