purely playing with engine ideas - short stroke big bore

[Raux]

So,
as I sit and ebay i realize the idea of putting together a big-case, short stroke motor sounds interesting.

things like finding the shortest stroke big crank.
749 with a 58.8mm stroke (shortest crank for the large motors, a 696 crank is 57.2 but not sure if will fit in large case)

and biggest bore pistons (that I know of)
1198 with a 106mm bore

gives a 1.80 bore/stroke ratio (for example 1199 is 1.84)
and a displacement of 1040cc

I wonder what kind of power this would produce. and how difficult would it be to buildup.

AND if the 696 crank would work

ratio would be 1.85
and displacement 1010cc

maybe making a sub 1000 cc would be good for some racing series so a
bore/stroke of 105/57.2 would be 990 cc and a ratio of 1.84

the revs with the right rods could be easily 12,000-13,000
with the 1198 valves and large airbox it would be a monster motor.

of course this is a 4v

the biggest 2v bore I've found is 102mm
so with the 58.8 and 102 it would be a 1.73 and 960cc
and with the 57.2 and 102 it would be a 1.78 and 935cc

[wannabfast]

theoretically, it would need to be an extremely high revving engine
if you can pull off 17k+

however with a short stroke crank in a big case with a large bore, you would probably need custom con-rods to get it to move to the top of the cylinder

would be fun to see i think

[Raux]

yeah that's what I was thinking too. I know the small case cranks are smaller diameter so tons of mods to get them to fit in the large case...
so the 749 crank would be the one.

pretty sure I've seen this done on a 4v but not on a 2v motor

but not sure a 2v could breath enough at the higher revs

dammit. why did i talked myself out of this

[Link]

yeah that's what I was thinking too. I know the small case cranks are smaller diameter so tons of mods to get them to fit in the large case...
so the 749 crank would be the one.

pretty sure I've seen this done on a 4v but not on a 2v motor

but not sure a 2v could breath enough at the higher revs

dammit. why did i talked myself out of this

You just saved yourself about $15K

[xsephirot]

Liquid cooled 2v? Sounds like a very interesting project. Or maybe it'll be easier just to drop in a 1199 engine if that is possible haha.

[Raux]

I think a short stroke 1198 motor with a 749 crank is the best way to make it happen.

obviously 67.9-58.8 = 9.1/2 = 4.55 difference in deck height, so either shaving the cylinder down or gettin longer rods will have to be worked.

yeah yeah, why do down in displacement,but just sounds like a cool project to try.
now just have to find an 1198 salvage to start.

what would be even more fun is stick it in an updated 888 (which I've been told by a racer is the best handling sbk of the 4vducs)

yeah well, just dreaming here.

[wannabfast]

Liquid cooled 2v? Sounds like a very interesting project. Or maybe it'll be easier just to drop in a 1199 engine if that is possible haha.

the st2's were liquid cooled

[Speeddog]

You're going to have a bit of a struggle to get a good compression ratio with a short stroke crank.

at one point, I was gathering parts to slip a 696 crank into a DS1k motor, making a 794cc engine.
But even with some Pistal 11.8:1 DS1k pistons, that would yield 9.6:1 with the short crank.
Not very racy.

[brad black]

/\ what he said on the comp issue.  that's the first problem.  nothing custom rods and pistons can't help alleviate.  you can also machine a head to fit on different stud spacing, as the spacing is (mostly) not that different and they're comparatively all in the same locations (just further out) and all the heads start as a similar blank for a specific engine type usually.  but that may lead to you being stuck with smaller valves.

the 696 crank should fit fine in the bigger cases, as i think all the blanks are the same.  i believe it still has the 40mm big end pin of all the old little motors.  big motors are 42mm.  the manual gives a rod big end size of 43.67mm, so you'd have to assume it's 40mm with bearings.  so you're custom rods would need different big end diameter well as length, but all that stuff is easy to sort once you've got the engine requirements set and you're ordering rods.

i personally don't believe it'll make 3/8s of make the beast with two backs all difference over an otherwise identical engine with less oversquare settings.  if you want to rev it you need to have it able to make power at the desired rpm.  749R is a good example.  it's made to make high rpm power, and everything about it is done to do so (once the race kit is in).  bore and stroke is just a part of it.  put std 749 heads on one and you'd get std 749 power.  put 749R heads on a 749 and i would expect it'd be much the same as the 749R power wise, just less reliable in the longer term if the power is used (like racing it).

horsepower is in the heads and the supply and evacuation thereof.  once you have a power level set you can then decide on the torque level which is often cc based or defined by a cc limit and that then specifies the required rpm.  a 140hp 1198 will probably be a much easier to ride bike than a 140hp 749 and in a race sense cheaper to maintain because you won't be revving the snot out of it.  but it won't feel as flash as the flat torque curve will make it feel a bit boring.

so it depends why you want to do it.  generally larger motors make better road bikes, as they're more forgiving and require less dedication to maintining corner speed.  dedication to maintining corner speed is probably what causes most single vehicle motorcycle crashes on the road, and smaller motors that produce less midrange power due to the basic max torque output per cc constant give you less abilty to recover from lower corner speeds.  i see it as something of a safety thing, others would call it lacking in balls.  but how fast you are prepared to ride on the road really tends to be a measure of how much risk you're prepared to accept or how stupid you are.  i don't appreciate people going up the inside of me on the wrong side of the road kneee down into a blind corner just so they can show me how superior they are, as the car that avoids them and their stupidity will probably kill me.  but now i'm digressing and ranting.  what was the question again?

if you want to make it feel revvy (like all the 748/853 owner go on about) and racy just put a closer ratio gearbox in it.  much cheaper, even a custom one for a 696.  actually, the smalll 6 speed is pretty close ratio apart from a low first.  has a big jump tp second.  and lighten the crank assembly.

i don't believe feel is that related to bore and stroke.  and i don't think most people have ridden enough of similar bikes to understand the changes you can make to things as simple as cam timing, exhaust configuration and the like and the difference they make to engine feel.  take the midrange out of something and one person will say the top end feels great.  the next will complain about no midrange.  like guys who traded their 1198 on panigales for instance.

but, if you want to do it, do it.  it's only money, and not mine.

and the truth is very, very few people have ever made two identical versions of the exact same motor apart from bore and stroke to show the differences.  and if they have they tend to be coy about it.

[Raux]

Compression I was thinking could be dealt with

there's 4.55mm needed to be added to the rod, or removed from the barrel, or added to the piston.

A custom piston with a taller deck, not just HC pistons.
A shaving of the barrel
A custom longer rod.

Now, doing a custom HC, taller-deck piston would be just crazy and probably the best.

As far as the crank.

I recently picked up a lightened starter gear for an 1100HM S... didn't fit on the 696, so that's why I was thinking there would be more issues.

and obviously cleaning up the heads for more air flow, larger airbox, etc etc would be done to open up the intake system

The thinking behind all this is the 1199, Ducati had reached the limitations on the 1198, so they went more oversquare.
So why not emulate that to some extent.

But yeah, I suppose it would end up being a more revvy, racer type motor. But hey, I told the wife I want a track bike

If I could just get that 999 frame I have here, this motor... and then, and then and then....

[Duck-Stew]

A +1 to Brad's comments.

Here are my additions:

Don't shave 4.55mm off anything.  Timing belt, exhaust system & throttle body fitment.

Don't get a piston w/a 4.55mm taller deck-height.  If you're trying to rev it, a heavier piston isn't the way to go.

Longer (& stronger) custom rods are the answer there.  But, you'll also be needing a custom higher compression piston to go with it.

Last point: (only mentioned in case you actually build it) NEVER cut a cylinder head to achieve proper squish band, cut the barrel instead.  Cutting the head changes chamber cc's, and (in the case of a Duc) reduces the beveled surface @ the perimeter which defines the squish band.

[Raux]

i thought the length of the rods affect rev limits?

[wannabfast]

i thought the length of the rods affect rev limits?

i think it has more to do with the stroke and cam timing but this is my theory though..

[TAftonomos]

Long rods/short stroke = awesome.

You drop the piston speed down, which means the damn ports on the head don't have to be HUGE to let the motor breath up in the stratosphere...which means it will have better response on the mid/low and take off.  You can rev it to the moon (assuming you have the cam to do it) and the stress on the motor is lower (as the piston speed is down).  Deshroud the valve areas in the head and let the thing breathe....

Since you are going to make pistons...make the squish area huge, and you can get the CR way the F up there 2 as there is no where/place for the charge to detonate...

I had a hand in building a 2.0l Honda motor about 10 years ago.  Static CR was 14:1, ran on pump gas, and made way over 100hp/liter.  IIRC it dyno'd around 250WHP and made a torque as well.
This was a long rod/short stroke motor.....

[Moronic]

I think a short stroke 1198 motor with a 749 crank is the best way to make it happen.

obviously 67.9-58.8 = 9.1/2 = 4.55 difference in deck height, so either shaving the cylinder down or gettin longer rods will have to be worked.

yeah yeah, why do down in displacement,but just sounds like a cool project to try.
now just have to find an 1198 salvage to start.

Fun to speculate about, but it seems to me the main thing you would achieve is a reduction in torque.

Think about it. All else equal, torque is roughly proportional to displacement. So, you stroke down an 1198, you get a smaller engine, and proportionally less torque.

Which also means, at similar revs, less power.

So, with the shorter stroke, you cut the piston speed and so can rev it harder - assuming the 1198 valve train can handle more revs.

If you add revs to the point where the head flows as much mixture as it did for the 1198, and uses it just as efficiently (quite a trick), then you will have 1198 power but at higher revs (and still less torque).

Think of it another way (and all this is just different words for the points Brad Black made above):

The key advantages of a shorter stroke are 1) you can increase the bore for the same engine size, which allows bigger valves; and 2) more revs for a given piston speed.

If you simply shorten the stroke of a given engine (i.e. do not widen the bore as well), then you miss out on advantage 1. And without 1, there is not a lot in advantage 2.

what would be even more fun is stick it in an updated 888 (which I've been told by a racer is the best handling sbk of the 4vducs)

yeah well, just dreaming here.

If my Web source is right, then the 999R shares the 749's stroke of 58.8mm, and its bore is 104mm - only two mm down on the 1198. So a 999R motor would just about give you what you're looking for, and probably much more cheaply. (Less fun, granted, depending on what you find fun.)

So, squeeze a 999R motor into an 888 chassis and there is your dreambike.

Alternatively ...

The S*R* Monster chassis are derived from the 888 design, I believe. And the last of them took a 999-based motor.

So, quick and dirty option: squeeze a 999R engine into an S4Rs.

Even easier ...

Get an S4Rs. 100 x 63.5mm is still pretty good, and I think that is the shortest stroke of all the non-R big-bike motors pre-Panigale - indeed, shorter even than the 888's.

Often when I ride mine, it occurs to me that the short stroke is a big part of why I like the engine so much. Which suggests your theorising might be close to the mark after all.