Ya Don you are pretty darn close I bought that little OS Pet .099 with a two position throttle (one big hole and one small one in the barrel) in 1964 at Ed's Covina Hobby east of Los Angeles. Hmm still flying it.

Sorry guys just could not resist throwing that old harmonic tack into the fry. Ok there is one more type that has not been mentioned yet probably because it not used to often.

So far we have the old harmonic vibrating pin, The optical sensor which senses rapid changes in light density, The electronic type that counts firing pulses from an electronic spark ignition and one more thats avalible for glow engines and that is the the magnetic hall sensor. This last one is rare but is electronic but counts a rotating magnet attached to the prop driver bushing.

I know this one is out there because it is included with complete telemetry accssory package for the Aurora 9 radio system. Amazingly that system include your choice of two types both a sensor for an optical tack with a little bulb pointed at the prop and if you choose a magnetic hall sensor type these are for inflight real time readings.

Now I think the original question was which of these types is more accurate and I think the only realistic answer is it does not matter not one wit. Think about it what we use tacks for is referance only and as long as the repeatability is good then so what if it a little off we are only using it to referance changes we may have made.

The vast majority of my engines are two stroke glow and for those I choose to tune high speed to a pinch and I find this is actually better for me and my ear rather than using a tach. What I do use a tack for is for comparisons between props, fuels, plugs and so on and on. This is why for two stroke glows I always have the carb line accessable for a pinch cowling or not.

John