What you need is a frequency switch (just like a shifter light). There's a chip that'll do this. LM2917. I spent ages reading through the spec sheets for this chip and it's quite easy to work with. Basically pick up the ignition pulse, which is then converted to a voltage. Run this voltage through a comparator which has a on/off limit of whatever voltage coresponds to your desired rpm. this will turn on a transistor, that switches a relay (for safety) and changes your wire splice as we discussed.
Done it yet?
The above method will work 'cos that's how I built my shift light. Just need to spend many hours reading up about electronics. If you (unlike me) studied electronics then this should be a breeze.
This method also works to control the RC valve, but there's a lot more electronics involved. I've had one working on the breadboard that would start moving the RC valve at whatever rpm I set, to fully open at whatever rpm I set, but it wasn't at all reliable
If I'd have known then what I know now I wouldn't have dropped electronics in favour of mechanical engineering at college. I'd have done both
