using model aircraft digital optical tach on full sized aircraft

[orvie6]

Thanks guys for the responses..
To help clarify the issue.. I have an experimental aircraft that uses a full VW engine in direct drive.. I have a shielded ignition system using a Magneto..so the 'wire to distributer' wont work, nor will my Tiny Tach that has the wire that wraps around a spark plug wire..Since it is direct drive, I thought that using the 'model airplane tac' with extended photocell would be the best way to monitor rpm..and as usual...that would only be a reference..Im sure that I could tell if there was a problem with the engine by listening to the noise..and since I dont plan on flying this is any kind of bad weather, nor is there room for two people..it is all relative!!
Thanks again for the replys