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Discussion Starter #1
Guys-

I know there are a lot of mechanically savvy engineering types here, so I'm wondering if it's possible to get the "Reader's Digest" skivvy on how bore, stroke, firing interval, and cylinder configuration generally translate into performance.

I know that bigger bore, shorter stroke "oversquare" bikes are generally faster revving with power coming at higher rpms, and that smaller bore, longer stroke bikes rely more on low-end torque for their giddyup.

So since our SC's are definitely oversquare, you'd think that while they produce pretty good HP compared to other retro-style bikes in their class, they might be a little weak in the torque department. But the grin we get from the pull that occurs when we crack the throttle down low proves this is far from the case.

So how does firing interval (like 270 degrees v. 360), and cylinder configuration (vertical twin, V-twin, L-twin) factor into the equation to produce an engine with certain performance characteristics?
 

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Chilehead
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Theoretically, torque depends only on displacement and compression ratio, and is independent of bore, stroke, number of cylinders, etc. This of course assumes equal cylinder filling in all cases.

Practically, cylinder filling depends on many things. Larger bore means (generally) more valve are which means better cylinder filling in theory, though reality is smaller valve area will fill better at low revs, larger area better at high revs, thus more high-rev torque with larger valve area (which is why 4V heads will give more top end then 2V heads on an engine of the same bore/stoke).

In addition, torque is not linear, but only is produced on the power stroke. My cylinders will produce a more even torque, but the average torque will be the same. The flywheel (and other rotating mass serve to give a smoother average as well, but do not affect the actual average torque).

Also, there is a maximum linear piston speed that needs to be observed so as not to require exotic materials, so shorter stroke means higher reliable revs.

Tom
 

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Theoretically, torque depends only on displacement and compression ratio, and is independent of bore, stroke, number of cylinders, etc. This of course assumes equal cylinder filling in all cases.
You would think that the longer stroke motors would have more torque for a given displacement. This is because the rod journal is farther away from the axis of the crank, necessitated by the longer conrod, and would exert more lever action on the power stroke.
 

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Con rod length can vary regardless of stroke. Longer con rods are something BMW performance enthusiasts have long played with to change power characteristics (moving the pin higher in the piston). It provides the piston greater leverage during the power stroke and changes the acceleration/deceleration curves of the piston speed.

I don't know if Duc tuners have found any benefit in such changes.

Engine layout is a much bigger and more confusing topic. Engines like the "Big Bang" motor of the current R1 involve trying to change the way power is delivered to the rear wheel with the intent of reducing wheelspin. On pavement, that kind of thing only becomes an issue as HP gets much higher than ours. For us, I think engine layout has a bigger impact on vibration, engine width, and the practicality of air-cooling than anything else. Obviously L-twins and Boxer twins have a layout that is well suited to air-cooling, while L-layout V-fours and (especially) Boxer-fours are more problematic. Thus those engines are normally water-cooled in current designs-. VW and Porsche managed to air-cool Boxer fours and sixes, but only with giant fans and big shrouds that are not fun on a motorcycle.
 

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Chilehead
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You would think that the longer stroke motors would have more torque for a given displacement. This is because the rod journal is farther away from the axis of the crank, necessitated by the longer conrod, and would exert more lever action on the power stroke.
Yes, but if you have a longer stroke you have a smaller piston area.

Assuming equal charge and CR, you will end up with the same PSI pushing on the piston.

Twice the stroke means half the piston area (for the same displacement). PSI x area = force. Force x lever arm (stroke/2) = torque. Do the math, you get the same result in both cases.

Regarding conrod length, you can use the same conrod length with both long or short stroke. However, in thory a longer conrod is more efficient than a short one, but only if they weigh the same. As I said, the reality is more complex than the theory.

Tom
 

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Guys-
I know that bigger bore, shorter stroke "oversquare" bikes are generally faster revving with power coming at higher rpms, and that smaller bore, longer stroke bikes rely more on low-end torque for their giddyup.
The bore/stroke ratio does not by itself determine powerband or quantity of power. Various configurations just LEND themselves to certain dynamics that work well to achieve the desired (high/low rev, trq vs. HP) result. An induction system that fills the cylinder equally at high or low rev would yield a torque-y powerplant regardless of HP. Typically at issue is intake flow at high rpm vs. low rpm. There are compromises that are addressed in the intended usage: Long stroke engines with smaller valves will flow well and fill the cylinder at lower revs, they can't do so at higher revs so they crap out. (Sounds like a Harley...) Big valve/high lift engines won't fill the cylinder so well at lower speeds and the so you'll likely lose some power down low, but that's expected if you're aiming for maximum HP at higher revs. (Sounds like a 600cc from anyone...)

Getting a good compromise is the tuner's art. Our Ducatis are typically tuned for a nice balance tipped slightly in favor of torque.
 

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The bore/stroke ratio does not by itself determine powerband or quantity of power. Various configurations just LEND themselves to certain dynamics that work well to achieve the desired (high/low rev, trq vs. HP) result.
Yes, a very clear way of stating it. More valve area makes for better breathing makes for more rpm, but a large-bore/short-stroke motor doesn't HAVE to have bigger valves, it just allows bigger valves.

Also, longer stroke motors used to run into piston speed issues limiting high rpms, but metallurgy and design have make a lot of steps in that direction that have allowed piston speeds to climb. Our piston speeds would have led to possible seizure in the 60's and perhaps even 70's. Piston speed was the limiting factor on rpm some time ago, but advances (probably mostly due to the Space Age) shifted the limiting factor to valve-float. Ever since, we've been making incremental steps to reduce valve-float.
 

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Discussion Starter #8
Thanks for the interesting info guys. Sounds like there are a lot of variables to tinker with besides those I mentioned.

Can anyone tell me where crank interval comes in?
 

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Regarding the question on firing interval I think this is much more influential on HOW the power (torque actually) is produced and managed by the rider than in the quantity of the power or torque. An engine that has more evenly spaced firing intervals will produce torque that is easier to manage at the rear wheel. As Major mentioned at the power our bike produces this is not much of an issue...but at 150 RWHP it is.

As others have mentioned the torque is dependent upon how much energy can be extracted from the fuel which in turn depends upon issues such as the ability to fill the cylinder, the ability to get the spent gasses out before the intake stroke fills it again, etc.... These are all dependent upon many factors such as valve size, valve timing, ignition timing, CR, etc...with engine geometry playing an issue as well.

Many of the latest advancements in engine design (particularly in the car world) have been instituted to allow for a engine to produce both decent low end torque (which also means decent low RPM power) while still allowing for the engine to be efficient at higher revs and therefore produce decent peak power.

As an example my 1991 BMW M5 has a 24 valve, 3.6 liter inline 6 good for about 310 HP. Some of the newest 6 cylinder engines of equal displacement from Nissan, etc...produce about the same peak power but make better low RPM torque. Much of this can be atributed to variable valve timing and other advancements in engine control.

Our 2V Ducs do well at the low-medium RPM range but fall flat against a 4V design I think mainly due to the ability of the 4V engine to breathe. There is no reason to rev the engine high if it cannot breathe. It makes nice noise but little else.
 

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Yeah, having variable control over those factors really has made a difference in the past few years. Variable Valve timing, Variable intake length, Variable exhaust chamber volume, they've all made engines better at producing great power over a wide range of rpm.

By the way, VVT isnt' just on cars. Honda put it on a couple bikes 2 or 3 years ago (the interceptor was one, I think - too lazy to Google it).
 

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valve float

Computer modeling has allowed realistic modeling of the valve train, and eliminated valve float from conventional vale springs. Desmodromic valve system in Ducati was previously used to overcome the float.

If you measured each cylinder in real time individually while running, you would find that there is cycle to cycle variability between each cylinder. The lower the number of cylinders, the more a variance between the cylinders will effect the smoothness of the engine.

The more sophisticated the fuel injection system is, the more the variances in the production of power in each cylinder can be adjusted for power vs. fuel consumption etc..

Without measurement you may not know that you have caused the cylinder head temperature to rise to a level that makes the engine unreliable depending on how the bike is tuned.

Usually when we refer to the bikes as running lean, in fact we really mean less rich. Since a completely lean mixture, would have no fuel, only air, and the spark wouldn't ignite the air by itself, and the engine would produce no power or heat.

For the most part it's when the spark event occurs within a range that starts at a certain point before top dead center and a range after top dead center.

A lot of this engine tuning information can be obtained from air-cooled piston engined aircraft engineering diagrams and technical manuals from the early days's before magazines started pushing partial charts with peak horsepower, and peak torque numbers, but didn't really explain the whole story.
 

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Ducati had a good idea with the three valve motor in th ST I thought. I had an old Mercury that had a 351 Cleveland in it, which had 2V and 4V heads, which referred to two barrel and 4 barrel carb and not number of valves. The 4V heads had larger valves, which were to handle the large CFM air/fuel mix. In reality, the 2V heads were "better" street motors with a 4 barrel carb. There's a hell of a lot to engine design, and to be honest I think they learn more everyday.
 

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Chilehead
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Ducati had a good idea with the three valve motor in th ST I thought. I had an old Mercury that had a 351 Cleveland in it, which had 2V and 4V heads, which referred to two barrel and 4 barrel carb and not number of valves. The 4V heads had larger valves, which were to handle the large CFM air/fuel mix. In reality, the 2V heads were "better" street motors with a 4 barrel carb. There's a hell of a lot to engine design, and to be honest I think they learn more everyday.
The V in the case of multi-barrel carbs stands for 'Venturi', i.e. the technically correct word for 'barrel'.

Tom
 

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the 3v motor seems like a good example of an underutilised design filling a gap that had to be created for it by dropping the 2v motor from that particular model. in isolation it's a nice motor, compared to the 4v motor of identical capacity it seems pointless.

firing interval is a bit wacky. a 90 degree v has perfect balance compared to a parallel multi, which is why lots of parallel twins run 270/450 to mimic the 90 degree twin. but a 90 degree v is a big motor and hard to package.

if you're designing a motor for a bike the bike itself is already set, and competition models will set the required power and possibly capacity. from that the head configuration is pretty much set. some companies are now going to twins whereas previously they would have gone 4's (ER6 for example), possibly more for a point of differentiation and perhaps more likely for the public perception that twins are more rideable or user friendly.

the twin cylinder myth has more to do with the reality of an engine that is restricted by piston speed and ultimate head flow and the tuned rpm range that defines than anything to do with number of cylinders imo. you can tune a 750 4 cylinder bike to have lots of midrange and peak at 100hp around 10,000 rpm just like many v twins do, or tune it to rev to 13,000 and give lots of top end.
 

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Guys-

So how does firing interval (like 270 degrees v. 360), and cylinder configuration (vertical twin, V-twin, L-twin) factor into the equation to produce an engine with certain performance characteristics?
The cylinder configuration and firing order have more to do with the smoothness of power delivery than anything else. I believe the flat twin provides the smoothest power delivery, like in the classic BMW motorcycle engines. The L twin is next, followed by the V twin and then the vertical twin being the roughest. The upshot is that the smoother the power delivery from a given configuration, the lighter the practical flywheel you can use for a given displacement, among other advantages. For a motorcycle, the L twin of a Ducati is probably the most narrow configuration in the direction of vehicle travel, thus reducing wind resistance and allowing more design freedom in placing the engine more under the rider, for example. One cool trick that has been done is to provide secondary balance by weighting the drive gears in the motorcycle transmission so that they double as balance shafts.

Also, the larger the number of cylinders for a given displacement, the smoother the power delivery, but at the expense of more weight and complexity.

See here for a good explanation of engine balance:
http://en.wikipedia.org/wiki/Engine_balance

and this is from the above Wiki page:
http://www.timberwoof.com/motorcycle/V-TWIN-7.html
"One benefit of the way the pistons move is that this engine does not need a large flywheel. The reason is that the two pistons are continuously exchanging kinetic energy. Whenever one piston is stopped, the other one is moving at maximum speed. The rate of one piston's acceleration always precisely matches the rate of the other one's deceleration. The system stores kinetic energy (though because of friction in the bearings, not as efficiently as a simple flywheel would)."

So, that's why we can run such light flywheels in our 2 cylinder engines!
 

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coolest thread ever. This is like drinking coffee with Kevin Cameron...
 

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Discussion Starter #18
dirkwright - thanks for the links and explanations. This is fascinating stuff, although being a mechanical engineering ignoramus my eyes glassed over at the equations. The visuals are cool though - they're worth a thousand equations as far as I'm concerned:cool:

So is it safe to say that engines designed and manufactured to require less balancing are inherently more powerful, at least power to weight-wise, because power that would've been required because of the extra weight of balancing and any related mechanical inefficiencies balancing requires can instead be transmitted to the rear wheel? Or are there sometimes mechanical forces created by balancing that can actually be transmited into usable power, whether HP or torque?
 

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dirkwright - thanks for the links and explanations. This is fascinating stuff, although being a mechanical engineering ignoramus my eyes glassed over at the equations. The visuals are cool though - they're worth a thousand equations as far as I'm concerned:cool:

So is it safe to say that engines designed and manufactured to require less balancing are inherently more powerful, at least power to weight-wise, because power that would've been required because of the extra weight of balancing and any related mechanical inefficiencies balancing requires can instead be transmitted to the rear wheel? Or are there sometimes mechanical forces created by balancing that can actually be transmited into usable power, whether HP or torque?
Thanks.

Vibration is wasted power, basically, so yeah, an engine configuration that is inherently more balanced can produce more power for a given displacement and other considerations like CR. An engine that vibrates more needs stronger fasteners, a heavier flywheel, stronger casings, and other components. So, the engine with the less vibration can be lighter and more powerful. I believe that's the reason boxer engines are typically used in piston engine aircraft, for example. The BMW boxer twin is great too, but has the problem of heads getting scraped on the pavement when leaned over. Obviously, a short stroke and short connecting rods would help in that motor when used in a motorcycle.
 

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Discussion Starter #20
...The BMW boxer twin is great too, but has the problem of heads getting scraped on the pavement when leaned over. Obviously, a short stroke and short connecting rods would help in that motor when used in a motorcycle.
Yeah, I've seen the ugly results of what happens to bike and rider when an old beemer boxer gets dipped too low in a turn.
 
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