mikem wrote:

One other BIG difference between EI and JPI:
JPI probes are grounded through their sheath, while EI probes are
electrically isolated from their sheath. There are large currents
that flow in the engine and airframe due to where the alternator and
battery are located, and because the airframe is used as a grounding
path. The resulting ground voltage drops create ground loops between
the various JPI probes, while EI avoids this problem by isolating
their probes. JPI installations can be very troublesome get working
properly. As a retired instrumentation engineer, dealing with low-
level analog sensing methods most of my working life, you couldn't
give me JPI system...


IF you follow the JPI installation instructions, including wiring the
ground to the engine block, not to the panel ground, it works fine right
out of the box. If you don't ground the instrument to the engine block
per the instructions, you'll have the problems you described.

For a 6 cylinder system, the JPI offers more functionality, as the EI
system is limited to 16 measurements, 12 of which are used for the EGTs
and CHTs on a six cylinder engine. Add oil temp, carb temp, voltage
monitor and OAT and you've used up all the spare capacity. I'm a happy
JPI customer and would buy one of their units again.