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      09-26-2013, 09:37 AM   #39

Drives: No BMW for now
Join Date: Jun 2006
Location: Canton, MI

iTrader: (1)

We are going in circles.

I still don't believe the S54's design was chosen for performance reasons (engine torque), but instead for cost reasons (using the existing architecture to get to 333hp). Hence, I am skeptical that they've gone the route of developing a new architecture for what is by all appearances a less compelling case for it with the S55. Alpina shows us the feasibility of a production 410hp twin turbo BMW I6 with an 84mm stroke and 89.6mm bore. Surely 430hp or more is also possible. S55 revs higher, yes, sure. But not 8000 RPM like the 91mm stroke S54 does.

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).