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That's interesting to hear from Dinan. Can't guarantee it's correct but this is what I picked up back in 2012 when the F30 was newer and it seemed there weren't many N26 on the block.
N20 is the ULEV (Ultra Low Emissions Vehicle) motor while the N26 is the SULEV (Super Ultra Low Emissions Vehicle) motor. This equates to the N26 having additional emissions equipment:
-V-band downpipe-to-catback exhaust flange (N20 is slip on)
-Electronic waste gate (N20 is pneumatic)
-Larger cats (detailed by Dinan)
-Crank case oil vent recirculation (I believe that's what it is at least - N26 has a tube from valve cover leading back to the intake post-MAF while the N20 does not)
I'm guessing their tune difference is due to the differing wastegate types (electronic Vs pneumatic). I know when Burger Tuning was initially releasing their F30 tunes the Stage 2 tune produced slightly less horsepower on the N26 than the N20 due to lack of ability to control the wastegate effectively. I'd imagine stock, they should dyno the same given OE boost levels (10% HP difference between N20 and N26 at peak RPM is a pretty substantial difference).
What baffles me is why BMW produces both N20 and N26 in states with high emissions standards. It was my impression the N26 would be in the tighter emission states (CA, NY, and a few others) while the N20 would be the standard version yet here in CA I see tons of N20 in addition to some N26.
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