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Re: top speed

When in the army I had 2 WM20 to look after. Running flat out side by side one was always quite a bit faster, of course that was the one I used
The most I could get on the speedo was just over 65 MPH on the straight These bikes were a lot younger then and speedos are often a bit fast so I think the low 60s is a reasonably top speed unless specially tuned Bryon

Re: top speed

Ian Wright
Designed top speed of the M20 was 58mph...There are some that, for whatever reason, will do more than that...

I treat stories of unusually high speeds with caution...My 720cc 'M20' has more power throughout the range than standard but on an M21 carb, an 1 1/16" inlet port and raised gearing it will still only top out around 75 or so..(on a level road)...Ian
Top speeds will vary on a lot of things.
When new the top speed was limited by the low quality fuel the bikes ran on and the drag from period oils.
Modern fuels have a lot higher energy content so the potential for more speed.
Then where were a lot of other things like the 276 carburettor which is nowhere as efficient as a monoblock or concentric.
Valve faces & seats will also make a difference as will the care most modern owners take fetteling their bikes.

I got massive differences in my M20 from changing the carb, several times.
I know of one chromed monster that happily will do 120 kph all day & all night, he rides with some HD owners and has little trouble keeping up with them on the flat or twisties but long climbs do take their toll.

However speed costs money and side valves are not the most efficient engines so if you go running with OHV twins, expect to use a lot more fuel.
I regularly rode with a friend on his 70 firebird.
My 4 gallon tank gave slightly less range than his 2 gallon one did if we were riding at expressway speeds ( 110- 120 kph )
On the backroads & twistise where we run in the 60 to 80 kph we had about the same range.

And as already mentioned push it hard and it will complain.
Doing expressway runs I generally fill with the bike running because if I stop the engine it won't start again till it cools down and the iron head & barrel standing in still air takes a looooog time to cool down.

Mine is happiest in the 80 to 100 kph range but most frugal in the 60 to 80 kph range

AS was written on the walls of most performance engineering shops

"Speed costs money How fast can you afford to go ?"

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

Re: top speed

['Modern fuels have a lot higher energy content so the potential for more speed..']

This is a common misconception..

The performance of an engine comes not from the 'energy content' of a fuel but from the engines capacity to take advantage of a fuel with a higher anti knock value...

A tuned Triumph for example, didn't use '100 octane' fuel to produce more power but to allow it to run higher compression ratios etc. in a given cylinder head design without pre combustion (pinking). THAT was what produced the extra power...not the fuel itself..

If an inherent 'energy content' was the relevant factor then it would simply be a case of using a higher octane fuel and any bike would go faster...Not the case unfortunately...

Putting 100 octane fuel in an M20 would merely be wasting money on fuel to no advantage...

By design the engine is incapable of any substantial benefit from a fuel change alone...It is possible a minor improvement may have occurred after the change from wartime 'pool petrol' IF the design had a little more capacity to gain from it but I consider that is very much a debatable point. Nevertheless, the M20 was designed before the war and thus before the use of low octane 'pool' petrol so there may be a little benefit there...

Turning to the carb that is only a fuel delivery system designed to provide the correct fuel/air mixture to the engine under varied conditions...Amal carbs for road use are all based on essentially the same design and the metering system (and tuning procedures) are virtually identical in all versions...

The major design change between the different versions was primarily to move the float chamber to a position that provided a more constant supply of fuel/air under a wider range of conditions. A secondary improvement was improved flow capacity in some later versions. Simpler manufacture, a reduced propensity to leak and cheaper manufacturing cost also played a part..

However, getting back to the M20, or any other engine, improved flow in the carburettor won't make one jot of difference to an engine design that is already functioning at the limit of its port/cam design as it is 'port restricted' in that case..i.e. that is the limiting part of the design in respect of gas flow...

Again, my experience indicates the M20 has no serious inherent potential 'locked up' in its port/cam design that is only being limited by the design or function of the carb.
In fact, physical alterations to the engine specification are needed to unlock any sort of meaningful power increase from what is essentially a glorified stationary engine...
I'd love to get some of these standard 'rockets' on the dyno to determine what is really going on...:laughing: ..Ian

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

Re: top speed

Thank you, I got the picture. I did 90 km/u with a 19T engine- and gearbox sprocket :sweat_smile:

This was measured by my smartphone with GPS. Because the Indian speedo is about 20 km/u too fast at any speed.

John

email (option): john.kater@chello.nl

Re: top speed

John that is 56 MPH to us. Sounds fare to me. Ron

email (option): ronpier@talk21.com

Re: top speed

Please Ian ,
I would have expected better from you.
Extrapolating to the ridiculous does not make a good discussion.
While all of the Amal carbs used by BSA are all venturi - moving slide type carbs there is a world of difference in the quality of the flow & mixing from a type 276 through to a Concentric.
The latter having a much more consistent fuel flow at all openings and maintaining a substantially better air:fuel ratio through out the entire operating range of the carb
You are quite correct that the side bangers are low reving stored momentum type engines, just lacking the appropriate 1/2 ton flywheel found on their fixed speed stationary cousins and as such do have a lot of design limitations, but even they do benefit from a charge that burns at the correct rate & extinguishes before the piston reaches the bottom of the power stroke as is the case even with the garbage "fuel" we get now days let alone real petrol as was available from the 60's through to the 90's.

However even our designed innefficient engines can extract more power from better fuels.
While they will not extract anywhere near as much of the extra available energy as a modern OHV design will, burning a better fuel will release more power, and if you have ever run a engine on power kerosene you would know that.
And yes there are limits and the benefits of newer fuels in our bikes will not be as apparent as using the same fuel in a more efficient engine you will get SOME benefit from better fuels.

Then there is the oil.
Modern low ash oils do not build up on the piston crown , head & valve stems so again adding that little bit more, not world land speed record stuff , but a little better than was available in 1942.
And of course modern oils put less load on the oil pump ( another little bit more ) and have a lot less drag , again a little bit more and every little bit more that the engine does not consume is a little bit more the bike has to push it forward.

The last period road test of an M20 I have seen had it with a top speed of 63 mph.

And yes the ability of the engine to rev is limited by the side valve design & valve timing but there are a lot of little things that all add up, right down to computer controlled rebores that are actually round and properly squarred to the crank and the biggie for me was getting a magneto that actually fires 92% ( this was measured by comparing an induction tacho with an optical one ) the first one only managed 70%, Silver spiral wound HT leads also helped a lot

Then we have better gear box oils with a lot less drag than SAE 30 engine oil
A friend had a beetle he raced moto cross and was unbeatable.
He ran 10Wt engine oil in the sump and dextron in the box.

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

Re: top speed

Meanwhile back in the real world...

A standard B31 produces about 17BHP and will turn in 70-75mph in todays conditions..(on modern fuels and oils). Not vastly different in fact from the original figures...

The M20 produced 13BHP and had a designed top speed of 58mph..again results that a standard spec. bike will more or less produce on todays fuels/oils etc. with a degree of variation.. The M21, with a bigger port and carb and the 100cc capacity boost made 15BHP in standard spec. and its performance today will fall between the M20 and the B31 as expected...

My 720, which is an entirely standard M21 apart from the capacity boost and a slight increase in compression ratio has a top speed, perhaps not unsurprisingly, more or less the same as the B31...although delivered in a very different way...

The fact is if you want to increase the performance in anything other than a marginal way something more than different oil and a more efficient carb is required...
As I stated in my post the carb is just a fuel delivery system and whilst later versions of the Amal may produce better fuel atomisation than earlier versions in some conditions the (possible) increases in burn efficiency won't be delivering sufficient extra power to do anything than slightly improve low and mid range performance...at best.

Top speed, which is governed by wind resistance not power to weight ratio, requires substantial increases in power output to achieve any substantive improvement and that simply isn't going to happen with an M20 in standard form apart from a later carb, some lead free fuel and lighter gearbox oil.....

If the machine has been substantially altered then that's another story but that was not the question in the original post.

Top speed can be expected to conform pretty much to the original figures with, as I noted, some bikes achieving slightly better results for a whole host of reasons...

Finally, coming back to fuel it does not in itself produce extra power unless the engine has been running on a fuel that does not allow it to reach its full potential, a point I made note of in my post...or it has been modified to take advantage of the fuels properties....

If for example you took an M20 running on modern fuel (which we are assuming is better than the original fuel) and accept that the fuel is allowing the engine to produce its best output within the design limits and the octane rating of the fuel was then raised from that octane rating to a higher one it would make no difference to the power output at all....

I accept entirely that there will always be variations between individual machines and modern conditions can, and will, affect the figures achieved but those effects will only produce performance variations within a very limited envelope in my opinion and claiming anything more is just wishful thinking, self delusion or faulty speedo readings....

Of course I'll accept any dyno reading that proves otherwise but I get the feeling it isn't going to happen...Ian

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

Re: top speed

My (dutch) manual mentioned 33 km/1000 rpm. So with a 4200 rpm engine the bike should be running much faster than 90 km/u.

John

email (option): john.kater@chello.nl

Re: top speed

That would be about 84mph. Maybe on a rolling road with no wind resistance. The faster you go the greater the wind resistance. Ron

email (option): ronpier@talk21.com

Re: top speed

Or flat out down the side of Mount Everest...:100: ...:beginner: :laughing: ...Ian

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

Re: top speed

What I need for my M20 is one of these in Khaki Gas Proof No.3...Ian:yum:

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

Re: top speed

"What is the top speed?"

Simple answer........10mph slower than a WDG3L. :wink:

Re: top speed

Quite right Ferg! And the fable of the Tortoise and the Hair comes to mind.:turtle: :rabbit: Ron

email (option): ronpier@talk21.com

Re: top speed

I just sold my m20 today and im from australia.
i have read Trevors imput over time and concider him a loud mouth bragard and not the typical understatad Australian

Re: top speed

Well I wouldn't go along with that opinion! To my mind there's nothing wrong with a healthy debate or disagreement or even an argument. As long as it doesn't get rude or personal. We can all often learn things or at least make up our own minds about a subject. Ron

email (option): ronpier@talk21.com

Re: top speed

I'd agree with that...and it's often me that's having the 'discussion' with Trevor...

The key thing is, as Ron says, that the debate doesn't descend to the level of personal insults...That is something any forum can do without but which sadly, many suffer from....Ian

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

Re: top speed

Here here I've only been on this forum a couple of monthes and find it informative and entertaining it would be a very boring world if we all agreeded with each other .If we didn't challenge opinions we would all still think the world's flat ,pity it's not I might get to that elusive 60 mph😂😂

email (option): andrewbell12@hotmail.co.uk

Re: top speed

I get an indicated 64 from my M21. It's running the carb it came with, off a Huntmaster with the slide cutaway filed. It appears happy and smooth at an indicated 60 on very little throttle. After that it just makes more noise with little appreciable difference in velocity. It's my third M21 and from memory they were much the same. They all used so much oil I never changed it, just kept pouring it in. I should imagine the bore distorts with heat. Can't see how it wouldn't...
My 16H may well be close in top speed but it produces peak torque just when the engine gets vibratory so it's often happier at around 40. Some days it seems fine at 50 but I'm not sure if that's due to extraneous factors or my lack of mechanical sympathy on any given day. I'm pretty sure it runs without a head steady to stop the frame breaking....... It was rebuilt at the end of the war and electrickery and tyres aside is untouched. I did think the head gasket had gone but it turned out to be the two piece plug reverting to it's constituent parts.

email (option): j@clogmaker.co.uk

Re: top speed

Sorry that you feel I have come across that way but it is a free world and you are perfectly entitled to your opinion.
If you had frequented the All British rallies or the BSA National Rallies you would have seen my M20 running,,,, quickly.
There is a pickie of it on the members page from about 20 years ago and you will see it does not look like a WM20.
That was the way I got it.
I have done nothing intentionally to make the bike go fast, apart from changing the gearing several times in an attempt to keep the rest of the club members who ride A65'a in sight and have not made any claims that my tuning is better than anyone elses.
Apart from a new piston for the new standard liner and the inlet cam I have not done any major engine work, unless you consider a 3 angle valve seat major work.
The bike ran a lot quicker than my housemates 29 deluxe from the day I got it running and is about on par with the WD B40 except it drank about twice the fuel and accelerated slower.
The speedo might be reading a little quick but on expressways it could just keep up with the rest of the traffic when reading 120 kph and vehicles slowly pass when reading 110 kph so it would not be too far off.
Now it just might be because of the $ 80 hand turned gudgeon pin that Barry Greahame fitted for the previous owner but I doubt it.
You should also have noted that when the subject of different carbs comes up I do mention that the concentric went on as a desperate swap because I was late for a run I was leading not for the reason of making the bike go faster and the results were the observances .

You might also note when proven wrong I generally thank the person who proffered the proof because that is one of the mechanisms of learning the truth, having some one else prove that you are wrong, so from then on you will be right.

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

Re: top speed

I really enjoy the friendliness of this site with very little 'bickering' or name calling. I was once one of the founder members (member #3) of a custom bike forum which had a similar atmosphere but after a while a certain type (mainly racist Nazi sympathisers) started to appear, which was OK to start with. Nobody had to agree with them and most of the debate was reasonably civilised, until the day they arrived in force and the site became awful. I couldn't stand it and I left the site, only returning after 3 years when all the idiots had been swept out. I always felt that Politics, like religion is a personal view and does not belong on a bike site. Friendly banter, joshing and debate are OK, as stated, unless it gets personal.

email (option): stinkypete80@hotmail.com

Re: top speed

I have never seen a standard M20 that could go faster then 55 MPH. My M21 does 60 MPH.

Bruce

Re: top speed

I got an indicated 67mph out of mine recently. An Army M20 taken out to 600 with fairly standard gearing. My engine doesn't vibrate badly & smooths out a lot at full throttle. But 45-50's the best.

Re: top speed

Read my first post at the top of this topic.:laughing: :laughing:

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