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      09-27-2013, 12:51 PM   #80
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Drives: 2015 M3 6MT
Join Date: May 2013
Location: Seattle

iTrader: (0)

From experience with bikes where the difference between bore & stroke is a lot bigger than among cars a larger stroke/bore ratio is used for and results in torquier, low revving engines.

Here's a vivid example of my current V-twin bike vs. a similar sized V-twin of very different character.

Ducati 748 Bore x Stroke: 90x58.8 mm ( My bike )
Harley Davidson 883 Bore x Stroke: 3.0x3.8 inch

In my practical experience from bikes of all types the bore x stroke makes a huge difference in the engine character where long stroke yields an engine with low end grunt and lower peak rpms while a short stroke yields a much racier character with a weak low end but high peak rpms and hp.

The S55 vs N55 difference is however relative minor but still the lower piston speed at a given rpm and larger fuel intakes (valves ) should make room for higher rpms. It makes me wonder if there is a sigfiicant room left that will be utilized to push peak power and redline significantly higher up in the rpm range with a comp. pack. Are the small Mitsu blowers up to feeding the required pressure?

The reason BMW has not implementd a shorter stroke before is likely to find the best balance of both HP and TQ for a road car. They've done a good job at that. The shorter stroke is likely also part of why the M4's TQ is rated lower than that of an Alpina B3.