[olddog]

Just a passing though. I suspect the size of the vacuum can could be optimized to get the smoothest signal. If you could figure a way to plumb this all up on top of the engine, temporarily, such that it could all be removed with no ugly after affects, then use an air cylinder instead of a can. You need a way to lock the cylinder shaft, although allowing it to flutter might be interesting. Now you can vary the volume of the vacuum can, while the engine runs.