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I'm wondering what the limiting factor(s) for bore/stroke ratios are, and how over-square an engine can go in the search for higher rpm and hp (motorcycles specifically)...

I know that rpm is limited by an average piston speed of roughly 25 m/s, and reducing stroke allows for higher rpm, since it lowers piston speeds. Many sport bikes have b/s ratios of only about 1.6:1 to 1.8:1, with piston speeds just under 25 m/s. It seems like the valve springs are probably the limiting factor for rpm, and they just set the ratio as high as needed to stay under 25 m/s, meaning the over-square limit would not be reached.

Assuming the valve system could handle it (such as desmodromic), what would limit the bore/stroke ratio and how high could it get? The highest production ratio i could find is Ducati's Desmosecidi RR at 2:1 (86 x 43 mm). I'm also wondering why it kept the same 14,000ish rpm as other litrebikes despite having lower stroke (around 17,500 rpm at 25 m/s), desmo valves, geardriven cams, and a perfectly balanced 90 degree V4.

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(From an engine kinematics perspective)

Increasing bore-to-stroke ratio (B:S) has two potential effects


  1. It reduces piston-to-head clearance

    In order to maintain the same piston displacement and compression ratio (CR), the gap between the piston at top-dead-centre (TDC) and head has to become smaller. This is because a larger bore implies a smaller stroke for the same displacement.

    I crunched some numbers for a simple flat piston with dimensions similar to the Desmosedici engine (0.25 l). At 13.5:1 CR the clearance between the cylinder head and piston is 3.19 mm, so the Ducatisti engineers don't really have a lot of room to play with.

    I crunched some more numbers for different bore-to-stroke ratios.

    • At B:S 1.6 the clearance climbs to 3.70 mm
    • At B:S 2.5 the clearance falls to 2.75 mm

    It might not sound like much difference but it is what it is.

    I'm not qualified to comment on how much this kind of difference in clearance can make to tooling and manufacturing costs.

  2. To maintain piston-to-head clearance, you have to reduce CR

    Note that CR affects thermal efficiency, and subsequently torque and power output (I'll keep constraints like knocking/auto-ignition out of this discussion).

    Crunched some numbers for B:S 2.5:

    To maintain 3.19 mm clearance at TDC the CR needs to drop from 13.5 to 11.65.

    That's roughly 4-5 % loss of efficiency. All else equal, if the engine originally makes 170 hp, you'd have to make do with about 8 fewer horses with the increased bore.


Now one could combat the expected loss in torque with higher revs, which leads in to your second question.

There could be many reasons why the engineers chose to limit the rev counter to a certain value, including material limits, reliability requirements and (possibly) rotordynamic concerns. It's not kinematics holding the rev limit back, it's something else that is known only to Ducati.

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Another factor is a "strength of materials" issue, combined with the increase in weight of a larger bore piston.

The reciprocating forces at TDC are tremendous, and they are the type of force (tension) that contributes to fatigue. Compression forces at BDC are far less an issue in terms of connecting rod/pin/piston stresses.

Weight is a huge factor, because if I recall, that vector is multiplied against the RPM vector that is squared, when calculating tension forces in the piston pin area during TDC reciprocation. In any case, piston weight is critical, but so is piston strength.

That said, I swear I remember an oval-piston Honda bike engine that revved 20K+ RPM, and that was almost 3 decades ago. But I don't recall the BxS being grossly oversquare.

  • That Honda was the NR500, which used 2 conrods per piston and 8 valves per cylinder. – Hobbes Aug 30 '16 at 9:27
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Another factor is the combustion chamber geometry. @Zaid already mentioned the piston-to-head clearance. But as your bore grows, so does the surface area of your combustion chamber, so you will lose more heat to the walls, reducing efficiency.

Reducing the stroke also reduces the torque produced by the engine (the force of the ignition is applied to a shorter arm), making the engine less drivable at lower rpm.

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At higher RPM's (right around 12,000 rpm for most applications), the time it takes for valves to open and close is way too long using only spring tension, engines need an engineering solution for this, therefore it adds to the complexity and cost of the engine. It's doable and it's been done, but a high-revving engine will always be pricier.

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    While your answer is valid for conventional valvetrains the Desmosedici engine mentioned in the question doesn't suffer from valve float. I believe the OP is looking for other reasons besides valve float for why the redline is where it is – Zaid Jul 25 '16 at 22:08

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