BMW R100 1.6 high ratio rocker

High Ratio Rocker Design

Here’s a sneak preview of the high ratio rockers I’ve been working on.

With the stock rocker shaft position and my tapered pushrods, 1.45 is the highest obtainable ratio before the pushrods clash with their tubes. So the shaft will need to be moved which means custom shaft and posts. Shooting for a ratio of around 1.6 which will give 14.2mm max valve lift with the Schleicher 340 cam.

At a ratio of 1.6, the valves clash so I might have to sink the exhaust valve a bit or go for 37mm exhaust valves.
Port flow would need to be optimised to suit otherwise there’ll be bugger-all gains.

Material will 4340 high tensile steel. Seriously tuf stuff so can get the weight down.

Why not a lumpier cam instead?

Yes, a cam change would be easier. But other high lift issues start to rear their heads.

Lift velocity is limited by the 22mm tappet diameter so to make things work, need to go for smaller base circle diameter, which means smaller shaft diameter and more deflection. Also, you can’t fit the higher lift cams with decent base circle and shaft diameter through the hole in the case.

A friend loaned me a 2 piece high lift roller cam by one of the well known airhead aftermarket suppliers from back in the day. The shaft diameter is way small so can fit in the hole. But flexed so much, it morphed into 2 pieces :uhoh

Soooo, to overcome the 22mm limitation and keep up area under the lift curve, people have used mushroom lifters and enlarged the case hole to get in a fat cam then fit a sleeve for the stock flange bearing.

In some cases, going for rocker ratio instead of cam lift has advantages for valve train dynamics which I don’t yet fully understand. One thing is that because velocity and acceleration are multiplied by the rocker ratio, ramp action is quicker and you get some more duration. However, this may or may not benefit your cause depending on the setup. It’s complex!

 

 

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