Here is a bit of knowledge that the readers of this thread may be interested in. The primary turbocharger in BD's street twins is basically a Silver Bullet with a slip-cover compressor and 1.10AR T4 turbine housing.

 

Ok Don--

You're welcome to continue denouncing "talkers" if you like, but if you have a beef specifically with ME, I will await your PM and we can iron this out.

The difference in compression ratio between two engines is well known, and changing piston bowl volume is easiest way to do it. What examples of "outlandish reasons" have you heard? It's one thing to comment on the MEANS that a change was effected-- it's another to comment on the ENDS of that change. To which are you referring?

Maybe you'd care to explain WHY the factory has changed compression ratio from 16.4 on the SO trucks to 17:1 on the HOs to 17.2:1 on the later HPCR trucks.

With that, I'll conclude my involvement in this thread.

 

I'm sure I have never heard there being a 2mm difference in piston bowl size between the SO and HO, some form of different piston bowl design has been stated, along with headgasket differences, and head design differences, but never substantiated.

Emissions, lower static timing with a higher compression ratio.