Quote:
Originally Posted by mkoesel
Right. So, if an over square engine is good for high revving operation then why did BMW force an under square design onto the ~8000 RPM S54, only to turn around and reverse that for the ~7500 RPM S55. You see? Your rationale does not cover it. I am not saying you are wrong, but there is very clearly more to the story.
More torque? Ok, let's prove it.
How much more torque, then, from the engine with the longer stroke? Can you calculate it? Before you run those numbers, let's keep in mind that any increase you make in stroke while keeping displacement and cylinder count the same means a commensurate decrease in bore.
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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).