Hi,
just curious as to typical cruising speed of these machines?
what would be the top speed u wouldn't want to push?
tnx pjw

email (option): pwegelin@moog.com

I have found that 50 mph is as fast as I want to cruise. Even since I have built my engine into 600cc and changed my engine sprocket from 19 to 21 teeth. I find 50 is my top cruising speed.....But now with less revs. Ron

email (option): ronpier@talk21.com

Ron
Would you recommend increasing the front sprocket to 21 on a standard 500cc?
Not yet at that stage of rebuild but always open to others recommendations
Pete

email (option): cruaser@aol.com

45mph. Or 40mph if you want to cut the vibes and save some fuel.

email (option): dannydefazio@sumpmagazine.com

Normally around 55mph and it's still cheaper on fuel than the

email (option): stinkypete80@hotmail.com

The WD bike has its gearing lowered for convoy/off road use by the reduction of the gearbox sprocket from the civvy 19T to 18T. Personally I wouldn't recommend gearing beyond the civvy ratios..
They suffer from limited power, so if you raise the gearing to improve cruising speeds on level ground you will pay for it on the hills (or even into a strong headwind) where it doesn't have the power to pull the taller gearing. As the M20 slows down at the sight of a hill in the distance anyway, I don't think reducing its capabilities any further is a good plan.
The best option is to enlarge the engine either to 600, 640, 712 which all require the M21 crank and a different piston (and unlinered barrel ideally).
Standard M20s seem to be happy at about 45-50 mph cruising speeds...on the flat bits..Performance will drop off in very hot weather when volumetric efficiency becomes even worse...Ian

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

Ian, can you explain that "volumetric" thing? I can't be the only one who doesn't understand it.

email (option): dannydefazio@sumpmagazine.com

About 50, possibly 55, is about right for my M20. But, 50mph has never seemed so fast!

email (option): lee@twowheelstested.co.uk

Hi Danny..It's just about heat in this context. When the air temperature is lower and engine is running at a lower temperature on a cool day the incoming petrol/air mixture is more dense so you get more in a given space (the cylinder) and hence a 'bigger bang for your buck'.
The reverse is true when running in a very hot environment.
You will see on an M20 cylinder there is a vertical gap in the cylinder fins above the tappet cover...That is there to limit heat transfer through the fins from the exhaust side to the inlet port, thus keeping that area a little cooler for the reasons outlined.
The effect seems to be most noticeable I have found on days when there is a damp, cool atmosphere..or when it's a very hot Summer.
'Volumetric efficiency' is just the fancy term for how efficiently the cylinder volume is filled,(the percentage of the static volume of the cylinder filled on the induction stroke) so the term is used when discussing the effects of gas flow in the ports as well...Ian

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

Thanks, Ian. My M20 and T140s run noticeably better on cool, damp days. But it doesn't seem to make any difference in my BMW Boxer. Which is odd, because the Germans are supposed to be the efficiency experts.

email (option): dannydefazio@sumpmagazine.com

Ian: Do you have any real life backup for this? I understand the theory but I doubt that the temperature difference will make a measurable difference in performance, even if you measure performance with instruments, not just seat-of-the-pants. A lot of things might be believed to make a difference but they only work in theory, not in real life. As an example, some time ago I looked into the possibility of an ad-hoc highway speed "supercharger" consisting of a square "cone" which faces forward and wraps around to feed the carb at the small end of the cone, so that in effect (theoretically) a lot more air enters the big opening, so that it raises the air pressure at which air enters the carb, increasing both the density and the speed with which the air enters the carb. It turned out that, compared with the pressures and speeds that are considered effective in mechanical superchargers, the increase in pressure of my "cone" was so minuscule that it would not have been measurable, something like one-seventieth of what it needed to be. It would have become effective at something like 150mph. In theory, it was a good idea but in practice it would have been a waste of time.

Patrick: The cruising speed of an M20 is whatever you get at full throttle, as all of your cruising will be done at full throttle. You will stick the grip against the stop and keep it there your entire trip. You will not use less throttle than that except in traffic or around town. The good thing about side-valve engines is that there is really no way to over-rev except possibly (and this, too, is doubtful) in neutral. I usually get 50-53. If you ever take your M20 from where you are down toward the New York City area and you use Route 17 ("Quickway") there is a section on that road that is a significant downhill, probably about 15-20 degrees, which is about 2 miles long. On that section you will do 60+. WOW! Of course, you'll pay for it on your return trip, but the return (uphill) has a "slow vehicles lane" so you can make friends with the truckers that will also be going uphill at walking pace.

email (option): jonny.rudge@verizon.net

the air is thicker when cold ie liquid oxigen is air very cold ian what about an intercooler and water injection you could put a jerry can in the painners

Hi John..Just my own observations of riding M20s in widely varying conditions...so on that basis it is subjective, but I think noticeable.
It is interesting that early M20s didn't have the 'gap' in the fins to limit heat transfer, so BSA must have thought there were benefits in introducing the change. There is no doubt that the effect is there across the full range of operating temperatures, that is a physical fact...but it would take dyno tests under tightly controlled conditions to determine the exact extent of it.
Consequently I'm not about to make rash statements..it's just my opinion....Ian

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

In service use, there was a maximum permitted speed of 40mph. That's probably a bit slower than you'd want to ride these days and an extra tooth on the engine sprocket will make a higher speed more realistic.

I'm not an M20 owner but they run alongside 16Hs quite equally under most conditions and whilst the 16H will cruise at 50 mph or a busy 55mph, there is a very definite sweet-spot running a civilian engine sprocket at about 45mph which co-incides nicely with the 70 km/h limit that is so common over here.

An indicated 60mph is quite achievable on the 16H but it's not terribly pleasant, doesn't feel kind to the machine and more importantly brings some seriously long theoretical stopping distances into play (I use the word 'theoretical' because I'd hate to do it in an emergency from that speed with no way round the obstacle).

...and anyway, the slower you ride, the more people you can smile at !

Some time ago, on the Brit-Iron site, there was some discussion (and tests, as far as I know) about water injection by Steve Shriver, aka Hobot. During the war (number 2, not the Gulf) some famous planes like the Thunderbolt got extra ummph from that, so why not an M20

email (option): viaconsu # planet dot nl

I think London Transport experimented with water injection on buses and achieved better performance and fuel consumption, etc. But this, as far as I can remember, was offset by more hydrocarbon acids in the engine, or whatever the hell it's called - and more maintenance.

Rik's point about a "sweet spot" is what I meant about cruising my M20. That spot, on my bike, is around 40-45mph (probably depending on the air temperature or atmospheric pressure and whatever brand of petrol is in the tank).

I think the mood I'm in might also have something to do with it.

email (option): dannydefazio@sumpmagazine.com

Ten mph faster!

Quite...If you want to improve the M20s power output some work on the head and, ideally, the ports is going to be needed. There is no doubt they are 'port restricted' (and in that you can include the head, as the valve is in effect not at the end of the port). That is why bumping the capacity to over 700cc without additional modifications results in a stronger torque curve but has limited effects from cruising speeds upwards...Ian

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

Had my bike clocked at 62mph [ completely standard], saying that on our bumpy roads over here [Aussie] I find 45mph comfortable, any faster the bike is hard work

I love it when a post goes off topic: free supercharging, weather effects, water injection, airplanes, buses and all. Here’s my two cents worth.

Air temperature, pressure and humidity – in other words, atmospheric conditions - do all make a measureable difference to an engine’s performance. That’s why all respectable dyno operators apply a correction factor to their results in order to show engine output as if it were running in ‘standard’ atmospheric conditions. If they don’t, the exact same engine will give different results on different days, meaning that you can’t quantify any tuning improvements you may have made.

A little-known fact: humid air is less dense than dry air (yes, it goes against common sense). So why inject water vapour into the incoming fuel-air mixture when that would only serve to reduce the volumetric efficiency? My understanding is that it is done as an anti-knock measure, meaning that you can run with more spark advance than the engine would normally tolerate and, if that moves you closer to the ideal spark timing, get more power as a result. Or if you choose, you could use water injection to enable you to lean out the mixture and get better fuel economy. As side-valve engines are very prone to detonation it could be worthwhile experimenting with this on an M20 or 16H, assuming you’ve also fitted it with an ECU and sensors and such like !!

One more mystery: as Ian noted, most engines feel more sprightly when the weather is both cold and damp, and he explained the positive effects of cold incoming air. But as above, the water in the air has a negative impact on engine power, so where does the dampness fit in? I’m not too sure of my ground here but I’ll note that we humans actually sense Relative humidity (“it feels like a damp day”) but the Absolute humidity of cold air is actually quite low – warm air can carry a lot more water vapour than cold air. So on a warm damp day, the engine suffers from both effects. On a cold dry day you get maximum benefit. And on a cold damp day, it’s not anything like as wet as you think it is and the engine performs very nearly as well. In the UK we mostly get cold damp days so don’t have much of an opportunity to contrast and compare.

all these wonderful suggestions how about nitros oxcide zoooooooomm!!!! manic laughter from rider

I'll wait for a more rigid explanation. ChrisJ's reasoning based on corrections made by dyno operators does not convince me. When you multiply pressure by use of a supercharger or turbocharger you also multiply the effects of any initial pressure difference. A pressure difference of 0.5 psi in ambient pressure (between cold and warm air) becomes a difference of 4 psi at the output of the supercharger, if the supercharger is compressing the ambient air 8x (I don't know if this is what it does, I'm just using it as an example). A difference of 0.5 psi may not be significant, even if a difference of 4 psi may be. Separately, when they do comparisons of different superchargers, for example, they should be more accurate in order to compare like things, even if the difference is small. That does not mean that the difference in volumetric efficiency between warm and cold air is significant enought to be felt when you are riding your bike.

As for humid air being less dense than dry air, yes, it's correct, but this analysis doesn't go far enough. The reason for the lower density is that the molecular mass of water is less than that of air. But the total number of molecules in a given volume remains the same (other conditions being the same). Combustion in the cylinder does not depend on the mass ("weight") of the fuel-oxygen mixture but on the number of free oxygen molecules available for combustion. So the lower density of humid air does not, in itself, mean less efficient combustion.

A possible additional explanation (although I don't know enough to make a definitive conclusion) may be that air contains only 21% oxygen but every water molecule contains an oxygen atom. I don't know whether these water-oxygen atoms are available for combustion but if they are you are, in effect, getting more oxygen into the engine by replacing air with water.

When Ian puts his 700cc M20 on a dyno to measure the difference in HP between a dry, warm day and a cold, damp day, we'll all know.

email (option): jonny.rudge@verizon.net

I read somewhere that the reason damp air makes an engine run better/smoother is simply that water expands to hundreds of times its original volume. I've heard the figure 900 times, and I've heard 1700 times.

In any case, the expansion of that water (or water molecules, or whatever) acts a bit like a steam engine and simply forces the piston down a little easier, and (importantly) more evenly.

I have no idea if this is true, but it sounds like something you can believe in - and that's a rare thing these days.

email (option): dannydefazio@sumpmagazine.com

Help, we need a chemist!

This is all getting a bit above my pay grade, but the weight (ok, mass) of air drawn into the engine certainly does matter as that determines the amount of fuel which can be usefully added to it, and that in turn determines to a large extent the power output of the engine. I’m not saying that atmospheric conditions make anything like as big a difference to engine output as adding a turbo/supercharger but could vary things by 5-10%, which is enough to feel the effect from the saddle and more than enough to measure on a dyno.

As to Danny’s steam engine? Not quite convinced by that. The oxygen in air is immediately available for combustion with the fuel, whereas with water molecules you first have to split them apart to ‘get at’ the oxygen before it can take part in combustion. That splitting apart requires significant energy input, so where’s that coming from? Probably it takes some heat away from the surroundings, which makes things run cooler and supports my premise of using water as an anti-knock agent. And don’t forget that for every water molecule you add, something else has to be replaced (see JH’s prior post) so you could end up with less oxygen to burn in the first place.

At this point my brain is hurting and I’m going for a recuperative pint.

ChrisJ

email (option): pwegelin@moog.com

Whoah, boy. I'm not sure the "steam engine" premise has got anything to do with splitting H2O molecules. I thought it was something to do with the sudden expansion of water into steam. Something like that, anyway. And I'm not sure I believe it. It was just something I read. But then, I read a book about fairies once, and I've got my doubts about that too.

Meanwhile, seems like the cruising speed question has been soundly answered, and we're all right.

email (option): dannydefazio@sumpmagazine.com

Someones going to have to go to the Amazon Jungle, the Arctic and the Sahara Desert and run their M20 over a measured 1/4 mile for comparison figures...Ian

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

Somewhere in my old dusty files I might have information on the effects of temperature, humidity & altitude on engine performance

If I find 'll share, many tests were on older side valves

Job

email (option): Jonnyob1@googlemail.com

Listen chaps, all this scientific talk is missing the empirical evidence.

All motorcycles are faster on a summers evening after the pubs have chucked out. Not just in a straight line but round the bends as well. It must be the moisture in the air.

Patrick, it goes as fast as it goes. Simple