BMW S1000RR valves

By Kevin Ash

Pictures: BMW Press

BMW’s new S 1000 RR is certainly a vindication of the path superbike development has taken over the last two decades as there’s surprisingly little real innovation compared with the Japanese norm, despite the Germans’ strong desire to be different. It’s more in detail that the new bike can be singled out, such as with its butterfly-controlled exhaust system where crossover pipes are opened depending on rpm (note this is not an EXUP-type pressure wave disrupter, it’s more akin to Honda’s H-VIX system) and the variable fuel pressure injection system.

(click on image for full size)The bike’s F1-type valve train and cylinder head design are also unique on bikes, and this is one of the key areas BMW is likely to have as an advantage in getting maximum power. Whereas the S 1000 RR’s rivals have their camshafts running right across the tops of the valves, so the cam lobes can push down on them directly, the BMW uses offset camshafts with small finger followers each operating its own valve. At first this seems like it might add reciprocating weight to the valve train, but BMW insists the set-up is some 50 per cent lighter than a conventional system using inverted steel buckets and shims. This could well be so, as the design eliminates the sideways swiping force on the top of the valve experienced on other engines which in turn means the bucket has to be strong and therefore heavy. Indeed, on some earlier designs with the shims on top of the buckets, it wasn’t unknown for the cam lobe to sweep the shim right out of the bucket.

(click on image for full size)The reduced weight of the S 1000 RR system is one factor that allows very high revs, but moving the camshafts to one side has also allowed the designers to take greater advantage of the narrow valve angle: the inlet valve is set at 11.2 degrees from vertical and the exhaust is 13.3 degrees, about as narrow as you get in this class. In itself this is useful as it allows for a flatter combustion chamber, very important with the S 1000 RR’s widest-in-class 80mm bore, but equally important, with the inlet camshaft shifted the inlet ducts are straighter and more vertical, considerably enhancing intake efficiency. In other words, they don’t have to curve to avoid the camshaft as they do on conventional designs.

(click on image for full size)On the exhaust side the camshaft has been shifted in the same direction (ie the finger followers are also pointing towards the rear of the engine) but this doesn’t get in the way as the exhaust tracts curve to exit horizontally anyway.

The whole cylinder head is kept exceptionally compact by the use of an intermediate gear between the crankshaft and the camshafts. This halves the speed of the camchain drive sprocket meaning the sprockets in the cylinder head on the camshafts can be much smaller, half the size of conventional ones, and shortening the camchain length also improves the valve timing accuracy.

BMW S 1000 RR review