Chicago Region BMW Motorcycle Owners Assoccrbmw

Piston Engine Displacement refers to the volume displaced by the pistons in an engine as they move up and down in their cylinders. To determine engine displacement one needs to know the engine cylinder diameter (the “bore”), the distance the piston(s) move up and down (the “stroke”), and the number of cylinders the engine has. Engine displacement is NOT affected by whether the pistons have domed or flat tops, combustion chamber size or shape, cam and valve gear, whether a turbo or supercharger is involved, or what fuel is used.

Let’s see this work. Does your R1200GS really displace 1200cc’s? With a little research I found the bore and stroke dimensions. This info is usually in the owners handbook and can certainly be found in repair manuals. The cylinder bore is 101 millimeters and the stroke is 73 millimeters.

Multiply bore times bore
Multiply times .7854 (converts a “square” to the area of a circle)
Multiply by the stroke
Multiply by the number of cylinders

101 x 101 x .7854 x 73 x 2 = 1169732.3484

Remember, I gave bore and stroke in millimeters and I want a result in cubic centimeters, so divide by 1000. 1170cc’s.

Actual engine displacement is almost always a little less than “advertised”. Since licensing, insuring, taxing, and usage of vehicles depend on these numbers, I believe this is done for legal reasons.

BMW model designations often refer to engine displacement, but also often not. On the car and motorcycle sides, BMW model designations prior to WW2 almost never had to do with engine size. The first BMW motorcycle was the R32. It was not a 320 cc bike, it was a 500. The bike on the cover of the February 2009 edition of “ON” (BMWMOA magazine) is an R12. It has a 750cc engine. Post war there were also models which did not follow the “rules”. The R68, the R27, and the R69 are examples. On the car side, the 528e and 325e of the mid ‘80s both used the same engine, a 2700cc design. The 318i from 1992 on had a 1900cc engine. And today, a 550i and a 750i are actually powered by a 4800cc engine. And the current 335i and 535i have 3000cc engines.

So now we have a new F650GS which is actually an 800cc bike. Its cylinder bore is 82mm and the stroke is 75mm. There are two cylinders. (You can do the arithmetic- follow the example above). Why did BMW choose not to name it F800 with another two- letter suffix? (F800GS is already in use). Hell, I don’t know. To “fool” insurance companies and/or states issuing licenses? Of course not. These people look at actual displacement, not a model name. Was BMW trying to appeal to the owners of the “old” F650GS single? I would think 800 would be more appealing than 650. I have to assume this was the decision of some marketing “genius”.

Hey Jon what up!
I believe Europe like in France has some funny laws about motorycles and their related horse power/engine figures with regards to age-licensing- and insurances...who knows.

Anyway in '81 my new Guzzi was called a G5 1000 but actually it was 949cc's just under 950cc's which would have put me in another class "as in more cash" for the insurance premium.

John:

Nice write-up!

I've never seen the bore and stroke numbers listed without the cc displacement also listed, but it's interesting to see how it's figured.

I have an Aprilia Mille R. It's an "liter bike" (1000cc) but actual displacement is 997 (from memory).

In France, the max HP is 100. If it's got two wheels, that's all the power you're allowed. It's probably enough but requires some makers to detune their bikes for use in France. BMW does this for their hex-head bikes. Both my bikes have more than 100, but I could live with this rule if I needed to.

Tax based on displacement is common in Europe and in many states. Insurance rates are based on the model of bike. BMW 1200s touring bikes are lower cost than Honda 600 sport bikes. The reason is the demographic of the typical owner. Lots of accidents and limited resources and regard for safety on the sport bikes.

My Aprilia costs more to insure than the BMW for this reason, I'm sure.


-TB