I’ve been chatting with some Bultaco experts about an expansion chamber designed to my Alpina cylinder. The short answer is that I can’t find evidence of one. Even when going with aftermarket exhausts, people stick with the trials style design (basically a straight-walled pipe all the way back), or start messing with the cylinder. One guy said he did put a standard Pursang pipe on an Alpina 350, and it made no power at all, worse than stock. If people want more power, they go with a different cylinder or start opening up the exhaust port and raising the port heights.
I used the Jennings formulae and this pipe is what I came up with for the stock cylinder.
Bultaco factory literature listed the Model 213 Alpina 350 as making a peak 21.6 HP @ 6500 RPM. I’d like to keep the power curve tractable and gradual, like an Alpina’s, while bumping up the peak power just a tad for the street. I used 7000 RPM as my peak HP target, while still trying to keep some power on the low end and keeping it from getting peaky as much as possible — probably why it looks a bit wonky. I went for the longest headpipe and the shallowest (most gradual) cone tapers practical within Jennings’s guidelines. The cone angles shown are 6° divergent & 10° convergent.
I have no idea if all those variables would interact as I’d hope, or just be a lousy exhaust. I’m guessing it wouldn’t be any WORSE than an Alpina’s straight pipe, but for all I know the bottom end could be weaker without any greater speed on top. It’s all a big crap shoot, but I’ve got to start somewhere. Furthermore, I’m actually mating the Alpina barrel to a 360 Frontera lower end, which has much less flywheel. That might turn out to be an impractical combination, but having something freer revving would be better for the street, even with (or maybe especially with) the conservative porting.
For comparison, here are the measurements for Tommy Robb’s 250cc roadracing pipe, and Bultaco’s Kit America pipe for the 250 Metralla. These really aren’t applicable to my bike at all, but are included here should anybody else be interested.