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Engine torque?

I have seen it listed that the M20 and M21 make 13 and 15 HP. I assume these are numbers provided by BSA and may be not even close to the truth. So has any one had a bike on a Dyno? At what RPM is max power made and how much? And even more important to me is at what RPM is max torque made and how much?

Re: Engine torque?

BSA didn't quote torque figures, just BHP...Engines (when tested) were tested on a dyno at the factory so outputs are probably reasonably accurate...

I wouldn't imagine anyone has paid to put a standard M20 or M21 on a dyno..I've done it with tuned B33s to assess the gains made but at £30 an hour or more that rules out doing it just out of curiosity...

Why do the peak torque figures interest you?....Ian

email (option): ian@wright52.plus.com

Re: Engine torque?

As BSA was gearing the bike to ridden at 40 MPH and I am setting up the bike for touring on modern roads, I am wondering at what RPM the bike makes max torque. Maybe a waste of time and will stick with factory gearing or maybe not.


Bike is a 1950 M21.

Re: Engine torque?

I've had my M21 on a dyno for giggles at a club dyno day, produced a blistering 11 hp at the wheel.

Re: Engine torque?

bruce
As BSA was gearing the bike to ridden at 40 MPH and I am setting up the bike for touring on modern roads, I am wondering at what RPM the bike makes max torque. Maybe a waste of time and will stick with factory gearing or maybe not.


Bike is a 1950 M21.


Usually you would fit a tacho ( a cheap mower one will work fine ) and find the rev band where the motor runs with the least amount of vibrations then gear the bike to have the middle of this "Sweet spot" at the speed you will be riding at.

The actual revs will vary from bike to bike depending upon how well it is balanced.
If you intend covering a lot of miles, the absence of excessive vibrations more than makes up for having to do a few extra gear changes to get up hills.

email (option): bsansw1@tpg.com.au

Re: Engine torque?

Actually guys, BSA (or was it M.O.D. ?)did publish M20 torque figures….. see the WM20 Technical Handbook page 14:
Max 13 BHP at 4,200 rpm and max 220 lb.in torque (which is 18+ lb.ft) at 3,250 rpm.

In metric units that’s just shy of 10kW and 25NM.

Other BSA publications credit the M21 engine with 15 BHP (11+ kW) at 4,000 rpm. I know of no published torque figures for the M21 but it’s reasonable to suggest that the 20% larger engine produced 15-20% more torque (i.e. 21-22 lb.ft) at a lower rpm such as 3,000-3,100 rpm.

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