Does someone have the specs for the various BMW cams?
Lifts, durations, crank angles, LCA's. Can anyone explain these durations to me, i.e. 308, 320, 324 & 336. At what lift is this, or is it just off the base circle?


Would that it were so easy. Unfortunately, some of those designations are like the various caliber designations of firearms, in that there are a bunch of different ways of measuring the same thing. From the four particular ones you cite, I'm guessing that you're looking at a German website that refers to those numbers.

In real general terms, European convention is to measure cam duration at 1 mm of valve opening. American cam makers seem to agree on .050". So immediately you can see that the US and Euro duration figures are not directly comparable. Add the fact that some call it Valve lift, but they really measure lifter rise, which is different, because a BMW rocker ratio is 1.39:1 Also, roller cams behave differently (in terms of timing) than flat-tappet lifters due to the geometry changes as the cam attacks the lifter. It won't take long for your head to explode.

Nobody measures duration off the base circle, because that would reveal the different strategies that various makers use for engineering the take-up or quieting ramps, the gentle rise of the cam that takes up the clearance in the valve train, and begins the job of opening the valve and letting it gently back down onto the seat. This critical bit of design can affect the flow characterisrtics of a cam design, as well as the longevity of the components in service. Cam manufacturers are understandably reluctant to reveal this proprietary information, and they appreciate the market need for some comparability. Thus the above mentioned standards, which are pretty well followed.

What is a realistic compression ratio for pump premium gasoline given that my bike is dualplugged?

If you have a nice healthy ignition suystem and good valves, seating correctly, you could probably run pump gas with 10:1 CR, and a stock 308 cam. Longer duration cams effectuively reduce actual compression levels, so you could even go a little higher with a 320 or 336 cam. I've had no experience with the rather new 324 degree profiles, so I can't say on them.

Any reliability mods for sustained high rpm use?

There are tons. I'd suggest searching the archives and doing some reading. That question goes way beyond what can be answered in one post.

-Tom Cutter

(Originally posted to the Airheads list)