67hp is what Honda claimed for their newly introduced CB750 OHC 4. It's quite possible at some stage Honda actually achieved that power on their dyno. Rear wheel dyno graphs of CB750s sold to the public proved to be shy of that output even when converting for the losses through the drive train. One CB750 tested in the US delivered 54rwhp @ 8,000rpm indicating around 61.5hp at the crankshaft.
What interests me is the fact that the BSA factory chose not to machine A65 inlet ports to the actual design drawing, ever, and when Umbeslade Hall technicians actually machined the inlet ports to the design drawings their motor made 8hp more on their dyno than production line units. This was an oil in frame photo shoot Firebird built in 1970. Not only was the engine more powerful it was also more responsive. The only difference was the machining of the cylinder head.
I'm curious what the result would be to machining the A65 inlet ports to one of the most efficient two valve port shapes known today. To do this I've enlarged the port slightly to 32mm and used a 42mm inlet valve from MAP Cycles. I'll also use slightly O/Size exhaust valves as well, though its probably unnecessary. So the head is ported and on my somewhat home made flow bench is indicating an excellent gain in flow 160cfm @ 28"W @.385"of lift compared to the factory heads 109cfm.
Fitting a round-slide carb restricts flow especially at 30-32mm bore. A 32mm Mikuni with long bellmouth reduced flow to 135.2cfm, with the bellmouth removed it was better at 139.17cfm, blending the carb intake enabled 145.5cfm.
Using Wallace Racing's on line hp calculator which bases its figures on intake port flow a 40cubic inch twin flowing 109cfm can produce 52hp at the crank (similar to BSAs claimed output) or 45.7rwhp approx. though like the XS650 Yamaha stock A65s are usually around 43-44rwhp. Calculating for 145cfm gives a estimate of 69hp at the crank, if it could achieve that it would translate to about 60.72rwhp.
So I'm interested to see if an otherwise stock A65 fitted with this head and 32mm carbs could equal or better the CB750s hp claim.
The modified carb:
The new port shape:
Not the actual head but one just like it.