The image below was taken from a 1955 BSA publication showing power graphs for various models...Unfortunately a few pages are missing and they include the M21 chart...
Carb jet size refers to a monobloc carb and it's interesting that a lower than usual fuel consumption figure is indicated...The figures would be from a 'brake test' on a dyno so perhaps the fuel consumption wasn't that realistic, but that's just a guess...Note how peak 'power' and peak torque are approx. 1000rpm apart and how quickly both drop off as the revs increase....So, no point thrashing it!

Careful with your throttle hand!:laughing: ....Ian

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

So the M20 does really make 13HP. I always thought BSA exaggerated the power just like every other company.

The chart is a great find as it makes decisions as to how to gear the bike easier.

It would be nice to see the same chart for for a 720 cc M20.

The trick to a happy life with anuy dingle cylinder is to find the "sweet Spot" where vibrations cancellthemselves out best and gear it so you are in the sweet spot at the speeds you spend most time in.
The only thing the diagram is really good for is showing weather you should change down or not
By the diagran you would have the bike geared for running around the 3500 to 4000 rpm mark
The logic of this is as the engine slows as would happen going up a hill for instance the torque increases thus allowing a clean ascent
If you gear for running at peak torque ( 3250 rpm ) then as the engine slows it looses torque so is unable to recover speed forcing a change down .
It is the torque that propells you forward the Hp is basically the torque x the momentum of the engine

The publication covers all the 1955 models...The A7, A7SS and the M21 are the missing models....Looking at the Bantams output it's a wonder they could move at all...:laughing:...
Bikes considered fairly capable tourers and sports bikes, such as the A10 and Gold Star tend to make you question whether we really need the 130-170bhp being sold to us today as something we should aspire to...Ian

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

The trend for unnecessary horsepower is alarming.
I am looking for a reliable bike to go touring on next year when I retire and am having to fight off a friend with a very good BMW who is trying to practically give it away. I just don't need or feel happy about having 120+bhp when I only need 50 at the most.

If anyone has a good Honda plastic maggot (EU registered, too much hassle post-Brexit to export to Germany!, I might be interested :relaxed: ( I do have to sell a couple of bikes to make space in the garage first though :white_frowning_face: )

email (option): petercomley@web.de

I'd seriously consider the Royal Enfield Interceptor...There are low mileage examples about at reasonable prices....I have two friends who have them and they love them...Ian

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

I have been thinking along those lines as well. I'll have to organise a test ride.

email (option): petercomley@web.de

A BMW with 100 HP is a good thing when you are going up a mountain pass at 8,000 feet with the wife and all you camping gear on the bike. It also helps when you know that you still have 200 miles to go today to get to you campsite before dark.

I have ridden my 1962 Velocette with the wife and gear for 1000 miles in week and the same on a modern BMW. On the BMW you get to your destination a lot faster and have time to stop and see the sights. But we still do most of our touring on a old bike.

FWIW
I have done a couple of trial runs for an Iron Bum Award ( 1000 miles in 24 hrs )( google it )
This is mainly to stir up the "HE MEN" who brag about this feat of riding endurance mounted on massive bikes on easy interstates .
To date the best I have done on the M20 is 28 hours & you know every minute of them for the following week.
The B40 GA will just scrapes in at 24 hours and is a little less stressful on the body but I would only need a small amount of roadworks or a couple of extra toilet stops to end that attempt
THe A 65 eats it at 20 hours and I could have done it ever quicker if I wanted to be a little generous with the speed limits or rode more expressways
In theory it should be doable on an M20 but I am yet to find a route that I can maintain high enough speeds on unless I ride the expressway Sydney Brisbane and that is a dangerous ride in the dark on a vintage motorcycle if you can not maintin expressway speeds going up hill , then there is lighting, or lack of enough lumens to overcome the 2000 watt shadow of myself from the drongo behind .

I appreciate that 100 Hp would be a distinct advantage 2 up with gear and even more so at altitude but when riding around OZ 2 up on the B40 , B50, A50 A65 there was rarely a time when extra horses were desperately needed , handy yes but vital no .
SO I really can not see the need for the sorts of Hp considered vital now days particularly when we have a flat out maximum speed here of 110 kph

If I can get my image host to work with the forum I will pop up the 1934 data sheet
Oddly enough the W ( 500cc ) engine curve is almost identical to the M20 curve Ian posted
13.2 Hp @ 4600
M ( 600c ) is 16.2 Hp at 3800rpm
Unortunately no torque figures

Perhaps the big difference in operating environments that makes more HP an advantage is motorways...Mile after mile at a high speed isn't done easily on a lower powered bike and is impossible in a country with no motorway speed limit like Germany...A pal of mine drove for nearly two hours at 130mph on his Rocket three whilst in that country...

However, get off the motorways onto the smaller roads and think more of 'touring' rather than covering ground at high speed in a short time and the whole arguement for high horsepower drops away in my opinion...A BMW R100 RT, for example, has a top speed of approximately 130mph and a power output of 70bhp...I would think that plenty to cover most requirements in the real world....Of course if you have the time you can travel with any amount of power, or rather the lack of it...There are multiple examples of people who have gone touring on 'tiddlers'...I worked with someone who toured Europe on a Honda 90 and in the UK I've done an 'end to end' on my M20 which we all know isn't blessed with a lot of power...

As far as sports bikes go it's all marketing hype that plays to peoples egos...The current crop of bikes have vastly more power and speed than the average rider can cope with or use with any degree of safety on a public road, but it does impress your like minded mates....Ian

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

The old adage goes something like " its more fun to ride a slow bike fast then a fast bike slow" ... or something like that ;)

email (option): wadeschields@mindspring.com

So here is the 1934 data

For the sake ofa fair comparison.
THE 1934 W 500 CC is 85 x 88 agains the M20 which is 83 x 94
So the shorter stroke will tend to peak at higher revs

The 1934 M 600cc is 85 x 105 agains the M21 which is 82 x 112
So again the shorter stroke will peak at higher revs


Higher revs should translate to slightly higher Hp
And shorter strokes should translate to slightly lower peak torque

But the curve for the M21 should be slightly to the left of the curve for the M-34 in the diagram above

For What it is worth .


I have some other data sheets somewhere but I don't think any others are for pre WWII bikes but if they are I will put them up