Desmodromic 4 valve Manx

For sale, any takers ?

Claimed 75 PS and 126 kg weight. See at

http://m.carandclassic.co.uk/car/C854690

Thanks for that link.

Interesting, hey what.
Wonder what year it is ?

And how successful it is - didn't we hear that the 4 valve manxs in the IoM
didn't actually show in the results ??
10,000 rpms and all that.... (not desmo though).

BTW, Nortons experimented with desmo in 1959, not 1949 as mentioned there.

I was offered a 4 valve G50 which had been fitted with a Chev conrod and a belt drive to the cam. What would you do with it - boat anchor ?

It would be eligible to run in the IoM .....

Seriously impressive engineering!

Do we think the 75 PS is correct? With that power and only 250lbs that thing should be unbeatable !

Not sure what class it would be allowed in though.

Not quite what at first it appears to be.......

The engine is 96 bore by 85.6 stroke for 619.5 cc, also the engine power is estimated, not measured (so it could be more).
Crankshaft is a pressed up assembly with a Titanium conrod, and the desmodromics were lifted from a Ducati engine, with the drive converted to chain after the original vertical shaft gave trouble.

It's still a very commendable work, and apparently was done over a considerable period of time, and from a write up that accompanies another web site in which this machine is featured, it is stated that all machining was done with manual operated equipment, even the cylinder head which started as a 26 kg block of aluminium !

Aw gee, thats letting the cat out of the bag !

I've got a Duc 2 valve desmo head that would bolt onto a Norton. (ie aircooled)
Although its a bit finless at the moment, so not terribly useful....

And, if the 4 valve dohc Molnar Manx will safely rev to 10,000 rpms anyway,
the desmo bit doesn't gain much ?

Rohan
while a Molnar 4 valve Manx may run up to 10000 rpm, that is way past it's maximum power point. The Molnar engine starts to seriously run out of breath if any attempt os made to push it beyond 8500 rpm, and at that engine speed the mean piston speed is close to 4000 ft/min. The Desmo 4 valve Manx with a stroke of 85.6 mm, reaches the 4000 ft/min mean piston speed just before 7200 rpm.

The estimated output of 75 PS is not clarified as to whether it is expected to be at the crankshaft, or at the rear wheel.

Snotzo said:

The Desmo 4 valve Manx with a stroke of 85.6 mm,

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In that case, we need to ask what is the stroke of the Molnar Manx ?

The stroke of a standard (factory) late short stroke Manx is 86 mm,
which is only a hairs width different to the desmo !!

??

BTW, Joe Craig, many years ago (prewar), pontificated on that 4000 ft/min number.
I think in front of a meeting of the SAE (Society of Automotive Engineers).
Even then, race Nortons were comfortably and reliably exceeding 4000 ft/min. (100 mm stroke then)
His comment was something along the lines of if we ignore the theory, all will be well.
(This was a VERY well engineered race engine by then though, everything tested to the point of destruction)

Rohan said:

Snotzo said:

The Desmo 4 valve Manx with a stroke of 85.6 mm,

Click to expand...

In that case, we need to ask what is the stroke of the Molnar Manx ?

The stroke of a standard (factory) late short stroke Manx is 86 mm,
which is only a hairs width different to the desmo !!

Click to expand...

The 4 valve Molnar Manx has a 95mm bore with a 70mm stroke

With these engines based on an elderly design, the 4000 ft/min is still a good yardstick to apply for long term use with a good expectancy of reliability. Modern race engines are now able to easily exceed 5000 ft/min mean piston speed, but with such engines every means possible is used to minimise frictional losses, crankcase windage etc.

As a point of interest, the 2 valve Summerfield Manx engine is of 92mm bore and 75 mm stroke.

Snotzo said:

With these engines based on an elderly design, the 4000 ft/min is still a good yardstick

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Well, yes. As Joe Craig pointed out though, and as mentioned, race Nortons could already comfortably exceed 4000 ft/min back in the 1930s !! Even as a longstroke design. Not by a huge amount though.
As his talk to the SAE demonstrated, that 4000 ft/min yardstick wasn't an immutable law.

But, as more than a few privateers found out, the factory bikes seemed more bulletproof than most.
In the 1950s, it was strongly rumoured that titanium rods had something to do with this.
When a Ti rod cost more than yer average house. As did a customer manx then too.

The Vanwall race cars - sort of 4 manx top ends on a crankcase - were advertised as having Ti rods,
so this would seem to be a fair indication. Never officially confirmed for Manxs...

Joe Craig gave the mean piston speed of his 1938 TT winning Manx as 3700 ft/min, claiming it was an 'excessively high mean piston speed'. As the bore and stroke of those engines was 79 x 100, this equates to an engine speed of 6350 rpm. Here Craig would be using Frederick Lanchester's 'corrected' mean piston speed, which was the mean piston speed calculated by multiplying twice the stroke in feet, by the rpm, further adjusted by dividing the result by the square root of the stroke/bore ratio.
This gives a considerably more accurate representation of the stresses in the engine. The resulting values are higher than the uncorrected values where the engine is over square, and lower when under square. Thus the corrected value for the 1938 Manx was 3703 ft/min at 6350 rpm, and the uncorrected value was 4167 ft/min.

The Commando engine in 73 x 89 format, has at 6800 rpm an uncorrected mps of 3971 ft/min, and a corrected value of 3597 ft/min.
Dominators with 66 x 72.6 bore and stroke should be within 4000 ft/min, both corrected and uncorrected, at 8300 rpm, but as with the 750 Commando at 6800 rpm, who needs more for normal use (is there such a thing as a 'normal' Commando rider ?)

Todays top race engines can easily surpass a 4000 ft/min mps, whether uncorrected or corrected, but the most outstanding one in motorcycle racing that is available to the man in the street, if he wishes to purchase one, is the GM speedway engine. Turning out peak bhp at 10500 rpm, this is a 90 x 78 bore and stroke single, that has a mps uncorrected of 5408, and corrected 5791, but I have personally measured the rpm of a former world champion's engine when practicing before the start of a season, and with
data logging equipment installed, was astonished to see the engine speed at the start gate at 13200 rpm before the gate opened, a corrected mps of 7251 ft/min. Small wonder a few rods let go on the start line!

A check on many of today's engines will reveal some surprising results when calculating mean piston speeds. machines such as the Yamaha R1 or the BMW S1000RR are way over the 4000 figure, both corrected and uncorrected.

Back in 1965, Phil Vincent wrote two articles for the English magazine Motor Cycle, the first in issue for July 22nd, entitled 'Short and Fat' , and another in issue for September 23rd, entitled 'What - No Ceiling ?'. In the articles he discussed mean piston speeds at some length, but never mentioned Lanchester's 'corrected' version.

Snotzo said:

The Commando engine in 73 x 89 format, has at 6800 rpm an uncorrected mps of 3971 ft/min, and a corrected value of 3597 ft/min.

Click to expand...

Redline on stock street Commandos was officially 7000 rpms.
And as more than a few Combat owners can tell you,
they can indicate 8000 rpms if you linger on the gearchanges... !

Dunno what this proves, other than a stock showroom bike was pushing the boundaries ??

P.S. Where do NASCAR engines feature in this.
A safe 9000 rpm redline, from a dirty big ohv V8.
Although they do change components fairly regularly/frequently....

It is common practice to fit the main bearings into Manx crankcases using steel sleeves and they still get failures. The main advantage in using a Manx in road racing lies in it's reliability. Once there is an increase in power output, reliability must suffer. It takes a lot of money to race one of those 4 valve, large capacity Japanese dirt bike motors in a road race bikes, due to the blow-ups. That is why Sounds Of Singles failed. It is not easy to improve on a 1962 Manx engine. If there was a class for 500cc air-cooled single cylinder four-stroke engined bikes, a good Molnar Manx or Summerfield G50 would be very difficult to beat.

The suggestion about using titanium con-rods is interesting. If you have ever had a bracket come loose on a titanium bolt, the change in dimension due to hammering is astounding. Imagine what would happen to the con-rod if the roller bearing big end had too much clearance ? With a Commando plain bearings the situation might be better. I know Phil Irving suggested fabricating titanium con-rods in Tuning For Speed - perhaps he had never worked with titanium ?

acotrel said:

Phil Irving.... - perhaps he had never worked with titanium ?

Click to expand...

You say some odd things.

Have you voiced your concerns to the people who sell titanium conrods?

Triton Thrasher said:

You say some odd things.

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N'er a truer word was ever spake !!!

Even if Alan doesn't have a clue on a subject, he will venture in head first..

acotrel said:

It is common practice to fit the main bearings into Manx crankcases using steel sleeves

Click to expand...

Its not only common practice, it was factory practice - they were built like that !

Thats because MAGNESIUM cases, camboxes etc expand with heat - quite a bit.
If they weren't encased in steel top-hats, the bearings could well just fall out...

You make it sound like a biblical revelation....

When Nortons began to experiment with titanium conrods is not known, but they were used for dyno testing as evidenced in the last dyno book that was saved following the factory's close down. This was after Joe Craig had left the company, so whether he had experimented with such conrods I cannot say, but he certainly turned down Harry Weslake's 4 valve head offer -possibly because it was not his own idea ?

Re a Summerfield G50 there is no such thing. Summerfields specialise in their version of 2 valve Manx Nortons only, it's Andy Molnar who manufactures 4 valve versions of both G50 and Manx

Snotzo said:

but he certainly turned down Harry Weslake's 4 valve head offer

Click to expand...

When was this ?
How ye know such things ?
Are there details of this anywhere.

4 valve heads would have been uncommon in the Joe Craig era,
only Rudge and Excelsior (openly) used such things (all prewar.)(?).