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      09-26-2013, 09:21 AM   #36
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Drives: The best kind of F30 :)
Join Date: Aug 2012
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Originally Posted by Boss330 View Post
First point:

S54 was NA so it needed all the help it could to create some decent torque. To increase torque you can go FI or longer stroke... Since the S54 is not FI they had to go longer stroke. Makes perfect sense on a NA engine that also has to combine HP with a minimum of torque in a "heavy" car.

The S55 uses FI to overcome the torque deficit a short stroke has and can reap the rewards a short stroke and large bore gives you without suffering less torque.

A oversquare design has so many benefits for high performance (see my previous post) that it is commonly accepted as the best design for high rpm and high HP.

The N55 has allmost the same CC but has a 89,6 stroke and 84mm bore.

So, the S55 has 9,6mm less stroke and "only" 5,4mm more bore than the N55. The N55 has a 12% larger/longer stroke than the S55, but "only" 6% less bore. I'm pretty sure that difference would be noticeable and measurable in a comparison of the two engines (in a similar state of tune, like in NA conditions).
I don't agree with your last paragraph. Bore is used as a common easy measuring yardstick, but it is actually piston area that counts. As bore is squared in the formula Pi D^2/4

This gives us piston area of 63.053 for the S55 and 55.417 for the N55. Which makes the N55 12.1% smaller in 'effective bore' vs 10.7% larger in stroke.

The weird thing is.....long strokes are FI friendly, as there is more density of Fuel/Air in the cylinder, it takes longer to burn. Also, smaller bores make detonation easier to control.