(01-15-2018 11:49 AM)W/R Wrote: So how does BMW coax 125hp on an air cooled motor but the CB1100 is sub 90hp?
Is it because of the design - 2 cylinders both hanging out in the breeze as opposed to a tightly packed I4?
First, design. Tolerances between components must be very exact to ensure proper sealing, and proper heat transfer, for the pistons, cylinders, and valve train.
Second, materials. The materials must be able to withstand higher heat and pressure for longer periods of time.
Last, intent. If Honda or any other marque wanted to get 125 HP out of an air-cooled I4, they'd be doing it. The CB1100 is ~1140cc, the old BMW R1150 was 1130 cc. They both make about 85 HP (crankshaft, of course).
When BMW went to the R1200GS, the power increased to 109 HP at the crank, with 1170cc's, hardly any more displacement than the CB1100.
The latest R1200R Roadster makes 125 HP at the crank from the same 1170 cc's. And that's a twin. I4's can usually make more HP because they can spin much faster. Not so the CB1100, its redline is very low, really about the same as the BMW's.
Cooling isn't an issue, there is plenty of airflow over the front of an inline four.
So it really comes down to intent, since Honda certainly have the technical resources to build an 1100 cc I4 producing 120 HP.
The final question is why don't they? Probably because air-cooled bikes making that kind of heat and power still present durability and reliability issues that are very easily resolved by introducing liquid-cooling into the design.
I was going to sit back and eat popcorn but the intelligent and thoughtful questions are just too hard to resist. And I was planning on cutting back on my screen time.