1. Does the valve snap open when the pressure overcomes the spring rate? Or is it a gradual opening over a few 100/1000 rpms?
A ‘snap’ probably isn’t a proper term or analogy here. The best example or advice I can give you is to simply run the motor with the PV cover off. Press the valve in (duplicating spring tension) and then ‘release’ it indicating an overcoming of said spring tension. This will give you an idea of how quickly it is overcome. In my opinion, when my engine is running right, it’s almost instantaneous and ‘feels’ good as well as ‘sounds’ different.
2. Does the Rotax ignition programing actually change the timing at <7500 rpm to allow the firing of the cylinder to occur as the piston draws even with the closed exhuast valve (on the down stroke)?
During the Florida Winter Tour, I’ve been able to see and use the Coil Tester and I can say that at 7500 RPM’s your coil begins an immediate and progressive advancing of the timing. Even when bench-testing, you can notice an ‘audible’ note change of the ignition timing.