10 replies to this topic

[WPT]

A calculation was made to determine the frictional loss between the rocker arm(s) and cam lobe(s) for a desmo and a valve springer (for one valve only) using identical lift curves. A 1970's era Ducati 750 V-Twin was used as a model (Ducati made both versions of this engine). Calculations for three crankshaft speeds were made: low (3000rpm), medium (5500rpm), and high (8500rpm). The results in horsepower:

3000 5500 8500

Desmo 0.040 0.269 0.895

V.S. 0.568 0.960 1.236


The coefficient of friction used was 0.05 and was a best guess. In addition, for the desmo the peak loads were less and the torque reversal frenquency was twice that of the V.S. WPT

[Cappy]

I'd like to see what happens at higher engine speeds. If you extrapolate the data, the Desmo would exceed the valve springer (spring?) at higher speeds.

By the way, what's a Desmo?

[marion5drsn]

In the upper right hand part of this column is a block entitled “search”, go there and type in the word “DESMODROMIC”. You will find enough information to keep you busy for about a week or so.
Even to the extent the reason the word exists (It is Greek.).
Yours, M.L. Anderson

[WPT]

Sorry about the confused post. That is not the way I typed it. The RPM figures used were "real" for the engine I used as a model. At higher RPM the desmo would not pass the V.S. because the V.S. would require higher rate valve springs to reach that higher RPM without valve float. For this study the V.S. begins valve float at 9000rpm. WPT

[ZoRG]

What is the limitations in a desmo system rpm wise?

Any idea how a desmo will fair against a pneumatic system?

Cheers

[WPT]

Don't know what the RPM limit is for a desmo system. It would depend upon many factors. At high RPM would expect the pneumatic system to do better than the V.S. because of the varible rate nature of the "spring". Whether this would be better than the desmo system, I don't know. At low and medium RPM think the desmo system would be better. WPT

[TFBundy]

Many years ago I had a Ducati 250 valve spring single and I remember being told there was virtually no measurable power difference [about 28 bhp IIRC] from the Desmodromic version. Top end reliability was a different issue.

[JwS]

The rpm limits depend on the design of the desmo valvetrain, on the Ducati Pantah engine the rocker arms will be in danger of fractureing at around 10,000 rpm. The Desmoquattro 916 engine will go a bit higher reliably, I think 12-14 K, depending on valve size, the new 999 motor is a bit better.
Though there is a reputation for higher maintainence requirements, the reality is not so bad, the Pantah only needs a shim or two every 10K (or more) and the new ones are as good or better. and they are easier to work on than 4valve shim under bucket motors in my opinion.
Jws
Ok, more info than required, but I love my Duc..

[WPT]

Ducati uses desmo valve system in their current MotoGP bike (a 90 degree V-4), which one would think has a rev limit much above 14,000rpm. Not suprised that that at peak hp RPM not much difference between the two. But at lower RPM the advantage is to the desmo system. Also to be considered is the lower peak loading in the desmo drive system that could result in lighter componets (less weight). WPT

[JwS]

You're right, I think they are getting up to 18,000 in the Motogp engine.
As I understand it the standard valve train has more drag at low speed due to excess spring drag (unless you have pneumatic springs!) But as speeds rise the loadings on the desmo to accel and decel the valve rise and thus the drag until it basically matches (or exceeds??) the spring setup. Remember that the spring gives back much of it's energy when closing!
JwS

[WPT]

The "drag" is caused by the friction. If friction is zero, then there won't be any losses in either system. The energy taken out of the V.S. system to open the valve against the valve spring is returned when the valve closes. In the desmo system the energy goes into increasing the kinetic energy of the various rocker arms, and valve. This energy is retuned to the system when the various kinectic energies return to zero. WPT