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Thread: What's the downside of an Desmodromic valve system?

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    awdrifter's Avatar
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    What's the downside of an Desmodromic valve system?

    I was watching the Ducati motrcycle video, and just thought of this question. What's the downside of an Desmodromic valve system? Why isn't it used on sports cars? For example, if the S2000 used this system it might've been able to reve to 10000rpm or higher. The new M3's engine have a very complicated VANOS valvetrain, but it still uses conventional spings to close the valves. Even the expensive supercars like the Ferrari Enzo and the Porsche Carrera GT doesn't use this system. Why? What's keeping car manufacturers from using it? Is it because of patents? Thanks.
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    Brazilian Consultant Redliner's Avatar
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    Anyone else with better technical knowledge please correct me, but as far as I can remember, there are a few drawbacks:
    Bulky
    Noisy
    Maintenance - I heard something about regularly adjusting the belts etc or it obliterates the valvetrain
    Higher mechanical complexity and more moving parts

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    The desmodromic valve system was invented at a time when poor metalurgy resulted in marginal valve spring performance. These days it really is not an issue with modern materials and production techniques.

    There are plenty of engines that can rev to 11,000rpm or higher using conventional valve springs. In engines of 2000cc capacity or larger, reciprocating mass and piston speeds are a large factor in limiting rpm.

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    rides with Rebecca Black.. in the back thedguy's Avatar
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    Quote Originally Posted by Redliner View Post
    Anyone else with better technical knowledge please correct me, but as far as I can remember, there are a few drawbacks:
    Bulky
    Noisy
    Maintenance - I heard something about regularly adjusting the belts etc or it obliterates the valvetrain
    Higher mechanical complexity and more moving parts
    Bulky and overly complicated. You've got to have a cam to open and close the valves, which tends to result in a lot of rocker arms to operate just 2 valves.

    Also for a long time people had trouble getting a quality seal out of the system. The solution is the simplest, use a small soft spring to pull it completely shut (which no one wanted to do on account that a desmodronic valvetrains entire point is to not have springs). I'd wager the extra cams ground into the shaft also makes it hard to implement variable valve timing systems like audi, ferrari, and honda use.

    Lastly modern supercars don't use it because they can do just fine with conventional systems. Yamaha has a production bike engine that will run upto 16,500 rpms with springs and normal cams.
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    awdrifter's Avatar
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    Thanks for the replies. So there's no advantage for Ducati to use the Desmodromic valve system?
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    Quote Originally Posted by awdrifter View Post
    Thanks for the replies. So there's no advantage for Ducati to use the Desmodromic valve system?
    Well there is less energy required to operate a desmodromic valve system as there are no springs that need to be compressed. However if you take in all the detracting factors mentioned above you can see why most manufacturers don't bother.

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    Courteous urinator klutch's Avatar
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    Check out Ducati's service interval. Then look at how much a Ducati certified shop charges for valve servicing. Then you'll understand.

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    Brazilian Consultant Redliner's Avatar
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    Quote Originally Posted by thedguy View Post
    Lastly modern supercars don't use it because they can do just fine with conventional systems. Yamaha has a production bike engine that will run upto 16,500 rpms with springs and normal cams.
    You are right, but you are comparing a 600cc-4 cylinder engine (Yamaha) to a 1098cc, 2 cylinder engine.
    You have almost double the displacement with half the cylinders. Maybe the desmo system it is still valid for big 2-cyllinder engines.
    Entirely my opinion, so I might be wrong, I have no sources and you are free to disagree :
    When Ducati first introduced it, it was indeed better since they had large(ish) engines with 2 cylinders, and relatively to an engine with the same capacity but with more cylinders it had bigger valves and etc, and we know bigger valves etc have greater inertia and therefore was harder to make it rev (and produce power), so they introduced this system to minimize these shortcomings of using a big twin engine.
    Maybe with todays technology it isn't needed anymore and is just a part of the Ducati experience, like Porsche and the rear engine. Maybe it is really better when you have big 2 cylinder and useless for smaller 4 cylinder.
    I personally don't care too much. I like Ducatis and not because they have a desmodromic valve system.

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    The biggest disadvantage to a desmodromic valve system is having to explain to people what it is when you tell them your engine's got one.
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    OT:wtf is a pneumatic valve?

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    A pneumatic valve system works using conventional camshafts to open the valves but uses compressed nitrogen to close the valves instead of springs.

    Modern F1 engines use complex electronic/hydraulic valve operation that completely eliminates conventional camshafts.

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    I thought ducati used that system to ensure a smoother power delivery through the rev range so you could power out of corners earlier

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    Brazilian Consultant Redliner's Avatar
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    Quote Originally Posted by ruuman View Post
    I thought ducati used that system to ensure a smoother power delivery through the rev range so you could power out of corners earlier
    Not exactly. Ducatis have a smoother power delivery because they have 2 cylinder engines.

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    Quote Originally Posted by Redliner View Post
    Not exactly. Ducatis have a smoother power delivery because they have 2 cylinder engines.
    And because they use a non-symmetrical firing pattern, which allows riders to get the power down earlier.

    The downsides the the Desmo system? Service life, complexity, cost. My Yamaha R6 didn't have to go into the shop every 6000 miles for valve adjustments like the Monster did. Service on a desmoquattro engine can run you over $1,500.

    Hearing a v-twin wind to 10,000rpm is quite lovely though.

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    awdrifter's Avatar
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    Damn, that's really expensive to service it. How does the Ducati's 2 cylinder egnine compared to the 1L 4 cylinder engines in other motorcycles like the CBR1000RR? Do they make more power and torque?
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    Brazilian Consultant Redliner's Avatar
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    Ducati 1098 R (the top of the line): 180 hp@ 9750 rpm/134 N·m@ 7750 rpm (Despite the name, it is actually 1198cubic centimeters)
    Ducati 1098 (the more "mundane" version, actual 1098cc engine): 160 hp@ 9750 rpm/122.6 N·m@ 8000 rpm
    Honda Fireblade: 178hp@12000rpm/113Nm@8500
    They produce a little less power (usually) but the torque comes on a lower rev range and adding to that the fact they are narrower and have a great handling, on a track they're usually faster.

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    The main advantage is that Ducatti's system prevents valve float at high RPMs. Sometimes a spring has trouble closing the valve at high RPMs, hence the piston crashes with the valve, not good.

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    I think one of the reasons that Ducati is keeping it, is that they very one of the first (if not the first, not sure on that one though) companies to use desmodromic valve systems and have ever since. So I think that there is some kind of tradition and pride involved as well.
    Should that be the case than I can't say that I'm against. Not many manufacturers dare to do that anymore. And I guess the results prove the mright as well. They make very nice bikes. Both for the road and the track.
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    Brazilian Consultant Redliner's Avatar
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    Quote Originally Posted by Svempa View Post
    I think one of the reasons that Ducati is keeping it, is that they very one of the first (if not the first, not sure on that one though) companies to use desmodromic valve systems and have ever since. So I think that there is some kind of tradition and pride involved as well.
    Should that be the case than I can't say that I'm against. Not many manufacturers dare to do that anymore. And I guess the results prove the mright as well. They make very nice bikes. Both for the road and the track.
    I think it was Mercedes who first made an engine with a desmodromic system...and I agree, after all those years developing the system, they proved it works.

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    awdrifter's Avatar
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    Quote Originally Posted by Redliner View Post
    Ducati 1098 R (the top of the line): 180 hp@ 9750 rpm/134 N·m@ 7750 rpm (Despite the name, it is actually 1198cubic centimeters)
    Ducati 1098 (the more "mundane" version, actual 1098cc engine): 160 hp@ 9750 rpm/122.6 N·m@ 8000 rpm
    Honda Fireblade: 178hp@12000rpm/113Nm@8500
    They produce a little less power (usually) but the torque comes on a lower rev range and adding to that the fact they are narrower and have a great handling, on a track they're usually faster.
    A 9750rpm redline doesn't sound that high. The F20C can rev to 9000rpm with 2L. I guess like Swempa said the main reason is probably tradition then.
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