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      09-27-2013, 08:16 AM   #76
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Originally Posted by mkoesel View Post
And speaking of people commenting through other channels, a knowledgeable forum member sent me this regarding the earlier discussion about the relationship between stroke length and torque:
Agree that it's not as simple as oversquare means low torque and undersquare means loads of torque.

However undersquare (with a large bore and short stroke) means the possibility to fit larger valves increasing flow.

Some info I found doing some searching:


A longer stroke will have greater port velocity at a given RPM, more torque due to more leverage on the crank, will achieve it's greatest efficiency at a lower RPM, and have less peak potential than a shorter stroke motor. Smaller combustion chambers are also more efficient, with the flame front having a shorter distance to travel- this leads to being more detonation resistant, and having an advantage for emissions.

Bigger bores with shorter strokes have the potential to turn higher RPM's, and larger/more valves will fit into bigger combustion chambers. Since the HP race involves turning ever higher RPM's to make more power, the oversquare motors have increased in popularity particularly when it comes to motorcycles.

Due to the emissions requirements, the lack of widespread consumer interest in 10,000 RPM cars, and the better reliability from lower RPM's, most car/truck engines are nowhere near as oversquare as motorcycle engines.

While it would be interesting to check how one bore/stroke combo would compare to another, it's important to remember that any results you find would be different with other engine combo's. Port size/shape, camshaft profile, exhaust backpressure, intake shape/length, piston style, compression ratio, and a thousand other items would affect the results. Each change would favor one bore/stroke ratio over the other. It's the overall package that counts, not one specific item.

Huge ports, aggressive camshaft profiles, short intake (high RPM type motor) would likely show a preference for an oversquare motor.


Agree that on a turbo motor many of the points above aren't that critical and a undersquare engine would have sufficed.

But, if bore/stroke info is correct, i give credit to BMW M for choosing a engine concept that is generally better for a high performance, high rpm engine. Especially given the work involved in this...