There is no specification of what sensor is used to get the ingition signal to the UC.
I have made three types of RPM interfaces used to test cars.
The first was inductive: Made with a currentclamp. These are used for measuring large currents through a conductor. The construction is similar to a transformer: A couple of windings on a splittable iron-core which goes around the sparkplugleads give an isolated and reducted output. The output was filtered and put into a comparator to obtain a nice square signal.
Also a possibility is an optical way of detection: put a piece of reflective material on a moving part of the engine (dynamo?) and detect this with a photo-reflective cell. This looks easier and electronically it is, but the BIG drawback is: you have to know the relationship to the real RPM of the motor.
Third for dieselmotors there are piezo-electric sensors which are clamped tightly around the diesel fluid lead. each time the valve opens the pressure inside the lead (usually around 80 bar I believe) drops which gives an output on the piezo device.
And of-course the idea of shielding all computer-parts from the spark-induction is a very good idea.