It is not particular to any type of engine. At least its not supposed to
be. I tried it on a Briggs and Stratton mower and if you hold it just right
it reads right around 3,000 rpm. On the Explorer, it seems to work best on the
front cylinders. But I think the electronic ignition on it fires only on power
stroke. It may be picking up other cylinder's spark when probing the rears.
I am going to try to filter the input and maybe a different type of antenna
today.

 "The government cannot give anything to anyone that they have not first
 taken away from someone else."