Educate me on piston speed, bore vs stroke

[stopintime]

I'm trying to understand why Ducati seems to prioritize bore over stroke.
Read somewhere that it means lower average piston speed... True? How?

[J5]

you need to think about how far a piston travels

think of it 2 ways

2 inch bore 1 inch stroke

or

1 inch bore 2 inch stroke

now add 10,000 rpm

the 2nd scenario has to travel twice as far hence twice the speed

by having larger bore with shorter stroke you can get more rpms generally speaking

[ungeheuer]

....by having larger bore with shorter stroke you can get more rpms generally speaking

.. and have room to fit in more or bigger valves. 

[Moronic]

And makes the engine shorter - handy for big L-twins.

[Raux]

you kind of have it backwards though IMO
Ducati prioritizes the stroke more than the bore.

the strength of the rod limits your RPMs. 
Piston speed = STROKE (in inches) times RPM divided by 6
this gives you feet per minute.

you have to determine the rod strength or it's maximum feet per minute it can handle.
this will help you determine you max RPM

here's a minor example
the 696 has a stroke of 57.2mm or 2.251968504 inches
if say the theoretical max f/min of the rod is 5000 then the max rpm for the 696 would be about 13375 rpm
that's not to say it's breathing at that rpm, just the stroke helps push up its max rpm
but take the new (or old) 800 (803) with a stroke of 66mm or 2.598425197 inches
with the same theoretical max for the rod the max rpm for the 803  is approx. 11591 rpm

quite a change for 8.8mm in stroke length!

[Speeddog]

Ducati wants more power from their engines.

In some arenas, they are constrained by a maximum displacement rule.
So, the straightforward path to more HP is higher RPM.
The straightforward path to more RPM is shorter stroke, larger bore, especially when you are constrained by a maximum displacement limit.

Shorter stroke results in lower average piston speed for the same RPM.
AFAIK, average piston speed isn't really the issue, it's just an easy-to-calculate number that makes it easy to compare with.
Piston acceleration is the issue.

There are difficulties that must be overcome to make use of this RPM:
A) Piston is larger diameter, so it either gets heavier (which is a problem for the piston pin/rod/crank), or gets thinner everywhere (piston durability problem).
B) Need to be able to flow more air, but not a huge problem, as the bigger bore makes it easy to fit larger valves.
B1) But with the larger valves comes a larger area combustion chamber.
B2) Larger combustion chamber area means the chamber gets thinner to avoid losing compression.
B3) This negatively affects valve timing (avoiding valves hitting pistons) and reduces combustion efficiency (flame front moving a longer distance in a thinner chamber).


Ducati appears to be 'prioritizing' bore over stroke in their effort to get more power out of the same displacement engine.

[stopintime]

Next question:
why would a four cylinder bike rev higher - smaller mass to move/accelerate?

[Raux]

Next question:
why would a four cylinder bike rev higher - smaller mass to move/accelerate?

I think you still have the stroke formula to deal with.
piston speed/acceleration are the same forces no matter the number of pistons.

[Speeddog]

Next question:
why would a four cylinder bike rev higher - smaller mass to move/accelerate?

For the same displacement, 4 cyl will rev higher than a 2 cyl, as both bore and stroke are smaller, so parts are lighter and don't have to move as far.

[Raux]

For the same displacement, 4 cyl will rev higher than a 2 cyl, as both bore and stroke are smaller, so parts are lighter and don't have to move as far.

well the honda cbr600 has a longer stroke than the 696, but i expect their rods can handle more feet per minute due to like you said, lighter/less weight to move.

so using the same calculations i had before instead of say 5000fpm as a max they can have 7000fpm as max (arbitrary numbers of course)

idiot search on my part. but was trying to make a point... with the correct stroke of 42.5mm using the same fpm max (which from what I heard has a lot to due with construction and material) theoretically your max is @ 18,000 rpm

but as Brad says in later posts, there are other factors.

[Speeddog]

well the honda cbr600 has a longer stroke than the 696,
~~~SNIP~~~

What?

[brad black]

current cbr is 42.5mm.  rods break on tdc overlap when there's no gas force to help slow the piston, so all the decel and accel of the piston is handled by the rod.

[Raux]

eh sorry. did a search and hit the bore not the stroke... my bad... but the point is the same.. i corrected my previous post

[NorDog]

Another thing to consider when thinking about high reving multis vs Ducati twins is reciprocating weight.  The smaller the valves, the less reciprocating weight a valve spring has to over come.

I think it right to say that Ducatis get more RPMs for a given reciprocating valve weight because they have no valve springs to worry about, thus no valve float problems.

Oversquare bore/stroke + desmo = higher reving big twins.

I don't know of any other big twins that rev as high, though I guess there are some.

Anyone care to clue me in on that?

[Moronic]

I don't know of any other big twins that rev as high, though I guess there are some.

Anyone care to clue me in on that?

Aprilia? KTM? I've not checked but I think they are pretty close.

AFAIK, piston acceleration is still the rev limiter, bringing us back to the original question.

I believe the big plus for the desmo system at present is it allows a steeper cam profile, given the max rpm.