Thanks Bob, that passion ford picture confirms what I thought :) I'm gonna have a go at making the loom bit with diodes as they only cost £2 off fleabay for enough to make 10 looms so if anyone else wants one let me know ;)

looking at that simple little circuit using two diodes and a zenner its converting the two signels from each coil (4 cyl wasted spark is two coils) to a single coil and dizzy type signel to run a std tacho

Thats the general idea!

Finally got round to creating the bit of loom for this using the diodes, bit of a mission to get it working but I got there in the end with some help from my brother who's good at soldering :) Engine idles around 500 rpm and when reved up, where I thought it was close to the redline from the sound, it's only doing 3krmp :)

You could always check calibration of the rev counter with a decent timing light. Some of those rev counters have a trim screw in them.

Bob