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Originally Posted by Jeroen Here's another interesting article about the myth of long stroke/high torque
The author correctly states: Quote:
So if torque is what accelerates a race car, why don’t we use engines with 2-inch diameter cylinder bores and 6-inch long crankshaft strokes? Obviously there are other factors involved.
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First of all, Stroke does not necessarily decide the amount of torque. It determines the rpm range at which torque is delivered.
Long stroke engines have the ability to provide torque at low rpms, But dont rev high. Ex: Royal Enfield Bikes. They can lug quite a lot of load at low rpms but dont rev so high.
Another consequence of this: Power is product of torque and rpm (Energy*Rate). So long stroke engine do not provide that high power outputs. (high torque*Low rpm)
Short stroke engine can rev relatively high and provide torque (torque figures need not be that high) at those rpms. Ex: The duke 200. It provides almost the same power as the royal enfield, but much lesser torque at the same rpm as the enfield. It provides its torque higher up the rpm band.
Hence the higher power output, (Moderate torque*high rpm).
The reason why race cars dont use a large stroke small bore engine is that race engine need very high power output. That is, they need torque at higher rpms. Except during starting, the race cars travel at very high speeds and do not need low end torque. They need torque at maximum possible rpms. This allows them to harness good amount of energy (not necessarily very high) at Incredibly high rate( very high rpms).
Simplest possible example i can give is the one ive posted else where on this forum but am unable to find.
A cycle with a large peddle can be ridden by a person with a very heavy load at slow speed. Even without the load he will not be able to achieve much faster speeds because of the limit of his leg movement speed (Engine parts inertia).
A cycle with a small peddle can be ridden by the same person at very high speeds, but cannot carry high loads at low rpms (short peddle so very low torque). He will be able to reach very high speeds because his legs dont have to move that big a distance (as compared to big peddles) to turn them at a higher rate.
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The intake manifold, cylinder head runner volume, and camshaft timing all have a much more significant impact on the torque curve than the stroke – and are much easier and less expensive to change.
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This is a very general statement. All of them put together may have (at the very most) an equal effect on torque as stroke. But Stroke determines the very basic and fundamental behavior of the engines.
That is why almost all passenger cars are undersquare (relatively larger load and lower rpm range). Commuter bikes are undersquare for the same reason as well. They do not need high power output, They need good low end torque to minimize usage of high rpms and achieve good mileage.
Extreme examples of this is marine engine. They have VERY high torque outputs at Very low RPMs (Large engines (parts) have high inertia).
Super bikes are oversquare to achieve high power output. Super cars have oversquare engines for the same reason.
Extreme examples of this are Race engines.