The New And Improved Unofficial R65 Forum V2

Technical Discussion => BMW Technical Q&A, Primarily R65 => Topic started by: Steve on March 17, 2024, 08:40:31 AM

Title: The BMW R65 engine is very over-square! What does this mean?
Post by: Steve on March 17, 2024, 08:40:31 AM

If bore equals stroke we call that engine square. If bore is greater than stroke we call it over-square. With a bore of 82mm and a stroke of 61.5mm The R65 is VERY over-square.

Compared to a square engine of the same displacement, an over-square engine has less torque because there is less distance between the center of the crankshaft to the center of the rod bearing, which is to say that it has less static leverage. Also, at a given RPM the piston travels more slowly because it has less distance to travel, therefore it pumps air more slowly which results in poorer air/fuel mixing. Therefore it must rev higher to sufficiently boost the air velocity and get good power.

Fortunately, the over-square engine has bigger valves, therefore higher potential air flow if high RPM is possible, and it IS possible because the shorter stroke places less stress on the engine. It’s safe to give it a high red-line. Race cars tend to have over-square engines while stodgy Harley-Davidsons have under-squares.

Thus the R65 has less power at low RPM than the comparison engine but more at high RPM. This is obvious to all R65 riders. Personally I don’t like to run at sustained high RPM, so if I need to quickly accelerate I drop down a gear, throttle open, and wait a half second or so for the power to build. At 4500 RPM it starts to really take off. The delay is exacerbated by my having the original heavy 1979 flywheel and a rather clunky transmission. On the other hand, once I am zooming I can lean her over further without scraping a head due to the shorter stroke.

Title: Re: The BMW R65 engine is very over-square! What does this mean?
Post by: Bob_Roller on March 17, 2024, 10:33:45 AM

I got my ‘81 R65 in early ‘81, the national speed limit on rural interstate highways, was 55  mph about 90 kph .
When the national speed limit was increased, at least here in Arizona to 75 mph, 120 kph .
Engine doesn’t sound ‘happy’ at that speed, so I usually stay at 65-70 mph, 105-110 kph .
Fuel consumption also increases, with the lower speed limit, I could get a consistent 45-50 mpg .
Now with increased speed limit and 10% ethanol in the fuel, at best I get 38-39 mpg .