Ducati Monster Forum

As a general question, non Monster related (ST3 could be in the future if I don't stumble upon an ST2 beforehand) -
what's the deal with the 3 valve motor, how can two intake valves be such a great idea when still only a single
exhaust valve?
Is engine geometry such that there's more room for a larger exhaust valve?

I kinda get that more intake area and lower intertia with 2 smaller intake valves, gotta have someplace for that which
was in-took to be out-taken.

Education please?

maybe, this can help

http://en.wikipedia.org/wiki/Multi-valve (http://en.wikipedia.org/wiki/Multi-valve)

I forget the exact figures (and am too lazy to go find my automotive engineering texts in the archives), but in general any internal combustion engine only needs to have X percentage of the intake flow capacity on the exhaust side.

I personally feel that a 3-valve is a good compromise between 2- and 4-valve engines. You get some of the bottom end grunt of the 2V and some of the top end of the 4V instead of just one or the other.

I totally agree with you speedbag... In 4V engines, let it be cars or motorcycles, you need to have the engine on the high end of rpms to have something..OK except some European manufacturers that have found how to have a flat from the bottom torque curve on their engines...

And, actually, why did Borno Panigale stop manufacturing the 3V engines?

intake:  low pressure, high density
exhaust:  high pressure, low density

need less volumetric flow for exhaust.  multivalve intake allows higher (re: more efficient) intake flow with two low mass valves rather than a single high mass valve.  (smaller valves can open and close faster)

once the mixture has burned, it is now a low density gas but flows quickly due to high pressure -- temperature + exhaust stroke.

that's why exhaust valves are always smaller diameter (re: lower volumetric flow) than intakes.  bigger valve = more flow, but also means heavier valve as the stem also increases in diameter to support a heaver head.  divying it up into two smaller valves achieves the same effect with a lighter valve train.

DP Termi kitted ST3's are stout - like 110 HP and boat loads of torque.  It's a really great motorcycle.

And anybody who doesn't think a 4v puts out good low end power hasn't twisted the throttle on a S4R/RS Quattro or Testa.  Keep some weight on those hands and feet and lean forward or all you'll see is sky. [thumbsup]

LA

Thanks All -

I had looked at that wiki entry, it didn't explain the relationship between one exhaust and
two intake valves other than size, not sure I still get that.

Two smaller better than one bigger, I get that.

Numerous dynamics involved that I don't understand.

For a given area of intake, smaller area exhaust, which is hotter, exhaust must flow much faster. Similar forces at work
on both intake and exhaust side, right (not an internal combustion expert I admit - just play one on the internet).

Sufficient to say the 3V breathes better than the 2V, is simpler but doesn't breathe as well as the 4V? That would probably satisfy my curiosity.

Anyway, proof is in the pudding, good words about the 3V motors are assuring.

yes, the desmotre is a nice motor.. the only problem i see with it?  there were only a few thousand made -- i think maybe 6000 total based on my estimates so it's not like you're going to find tons of spares on ebay.

i've toyed with the idea of picking up an ST3 and turning it into a monster, but .... engine parts...

kind of a pregnant dog to do adjusts on, moreso than 2Vs, but not that much less than 4Vs.

I have to throw this out there... Anyone know if a 3 valve head would fit the DS motor?  Bolt pattern?  Cooling would be an issue of course with no water circulating about.  Just curious....

rofl, yes 50% harder..

no, i would not say it is a pregnant dog.  it is more like doing a valve adjust on a 2v rather than a 4v.  it's a single cam head and i believe a single lobe actuates the intake valves.   i don't remember if the open/close arms are joined or not. 

LT Snyder lumps the 3v in with the 2v in his book.

not sure.  the desmotre bottom end is nearly identical to the ST2 watercooled 2v.  actually i think except for the heads it is identical.

have to have the radiator, etc.

One additional item is that smaller valves don't need to lift as far. A valve will reach about 90% of it's max flow when it's open by an amount equal to 2/10th its diameter.

So one 44mm valve will be flowing pretty efficiently once it's lifted 8.8 mm, but two 31mm valves (same area as the 44mm valve) will achieve the same flow at 6.2 mm of lift.

DesmoTre rockers are individual, so 3 openers and 3 closers per head.
7mm valve stems.

Speeddog how many cam lobes?

From the Parts Catalogue, it appears there are 6 lobes.

Also appears to be plain bearing cams like the DS1k.

Never had my hands on a DesmoTre, so I'm depending on Ducati's info....

putting 6 lobes for 3 valves blows my mind.

i guess it is cnc nowadays so who cares, but sheesh..  seems they could have designed a single rocker/lobe setup.

Speeddog is correct. I still have mine apart doing the valves.
It's much easier than a 4V. same as a 2V just one more due to the single cam.
Plus the cover gaskets are reusable for some reason.
This bike flys, in comfort. Even 2up with luggage.  [evil]

IIRC, the ST3 head has the same bolt pattern as the DS1000 motor (same bore and stroke too)