Rpm....
The rpm gage gets it's reading from the MPEM (multipurpose electronic module). The revolutions of the engine can be picked up at the ignition coil if you have to substitute another gage.
In the forum, you'll see the majority of us refer to our boats/pwc's by engine cc's, not horsepower. So, your Challenger would be the 1998 model with twin 717's (85 horsepower)...........:cheers: