The speedoes normally read 2 to 3 MPH slower than actual.
I received a ticket for 38 MPH from a following motorcycle cop (I'd seen him) when my speedo reckoned I was doing exactly 30 MPH in the 30 MPH limit.
It's probally because some are sprocketed down for sidecars. Changing the 18 tooth front sprocket fo a 15 or 16 tooth for a sidecar, on a gearbox driven speedo, will have a huge effect on the speedo. I've always had the ones with under reading clocks. I've had mates ride alongside on various Japanese bikes over the years and comment on the speedo underreading. The trip meter also always measures distances less than mates bikes on long trips, etc.
Which manual are you reading?? I've have two of the various workshop manuals they produced including the early version.
The Type 638 has a 29 - 30 Pounds per Foot torque reading, but this is a higher revving engine. The unrestricted 1986 version made 34 BHP and had a reline at 7,000 RPM. I've had two of these. A friend still has two in his garage. Later post 1988 version had restrictors fitted in the carb manifold, exhaust baffles (a large choker collar) and in the airbox, to comply with Dutch and Austrian 350cc motorcycle power limit rules.
These only make 23 - 25 BHP and do just 80 MPH grudgingly. I know someone who imported a Austrian RD 350 YPVS that was restricted in a similar was to 27 BHP. It took a lot of money to unrestrict it. We are so lucky here at the moment!!
The CZ 250 was rated at 28.5 Pounds per Foot, exactly the same as the GT380M, another bike I've had here.
As (Torque x RPM) / 5,252 = BHP (I've operated a proper torque measuring dynomometer!!), generally high revving engines will have lower torque rating than a lower revving engine of the same cubic capacity. It's a fact of engine physics.
The Japanese went for low torque high BHP engines and JAWA, CZ and a few others went for high torque low BHP engines. MZ and a few others went for the middle ground.
The old RD250 put out less than 20 Pounds per Foot torque.
As BHP, like Wattage, is a calculated resultant number from two measured figures, it can be totally meaningless when quoted out of context.
25 BHP on a 125 will feel totally different to 25 BHP on a 250, the 125 having less obviously less torque than the 250 in order to make 20 BHP.
Basically BHP is a calculated measure of crank innertia, which is what does the work. HP is calculated from Wattage created from a generator and works out at a higher figure than BHP. This is Watt's Horsepower.
Then there is Power Starken (PS) which works out to be about 1 PS = 0.86 BHP. So a bike quote at 100 PS is actually 0.86 BHP and 1 HP is about 0.94 BHP.
The Dynos that folks can go and have their bike tested on measure HP and not BHP, uless they can give a torque reading as well. If it is calculating through current produced only, then it will be measuring Wat's Horsepower only (HP).
It is the torque that accellerates a bike and it is this that the rider feels. High torque engines will always out accellerate high horsepower bikes of the same cubic capacity.
Having tuned strokers for drag racing and road for so long, I generally just ignore horsepower quotes and focus on how much torque I have to work with.
My old Suzuki GT 550 B drag bike used to shatter the cluctch basket on runs. I warunning Nitro Toluene diluted with petrol, but the torque still destroyed the transmission from time to time. That had frightening amounts of torque and delivery.
The world's current fastest accellerating production bike, the Feuling W3C, 2.5 Litre has 216 Pound per Foot torque and puts out 217 BHP at the rear wheel. Best top
speed, down an airstrip with the rider strapped on, was 228 MPH and still going.
The bike is a naked super cruiser, like a V-Rod done properly. Needless to say, being a fan of high torque for their size engines, I want one!!
Go type in Feuling W3C and see for yourself.... its fantastic.
