Quote:
Originally Posted by CanAutM3
For a given displacement, a longer stroke does not mean more torque. While it is true that a longer arm means more torque for a given force, this is offset by a smaller piston area on which the cylinder pressure is applied therefore yielding less force.
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OK, my explanation above was very simple and imcomplete, just to show typical different between NA and TC engines.
In theory you are right, given displacement and the differences will weigh up for each other, but in real life that will not be the case. I was a bit categorical when I stated that longer stroke equals higher torque.
Will try to explain a bit more:
Given two NA engines with different stroke but same volume, rpm etc. Also we say they have the same exh/inl-valves, timing, inlet and exh systems, (this will not be natural since they will breathe differently, but just now that will be the case just to simplyfie things).
The one with a slight longer stroke will achive better "filling grade" given the same conditions, (by filling grade I mean the engines ability to fill the cyl with fresh air, M engines vary between 100 to 110% filling, this means theoretically a cyl with displacement of 1 litre will fill up with 1,1litre air during inlet-stroke=110%)
The reason for this is that is that the "long-stroke engine" have better ability/longer period of time to create "speed" on the inlet air through the ducts, (due to the slight longer stroke) and thus continue to fill with air after BDC in the inlet stroke and again create a slight above atm pressure in the cyl. This again will create better combustion->higher pressure->increased force->higher torque.
Please bare in mind that this is just one of and a quite cost effective ways to create better breathing or filling grade of a NA engine, there are many ways to the goal as usual, many of them much more expensive. F1 engines for example are not very long stroked, but still create a filling grade up to 130% or more, (but those engines are far from cheap
