watkins,
I disagree on the JPI. I think a lot of folks have misconceptions about the utility of those devices. As Jeremy says, they can be very useful in many ways.
They provide fuel flow information. If all you do is take off here, fly from here to there at one power setting, and never experiment with different power settings, you are right--a fuel flow computer is a waste.
On the other hand, with my airplane, I can vary fuel flow in cruise from 6.7 gph up to about 17, depending on what I do with the prop and throttle. Winds aloft different today? Use a different power setting, and this works even for a fixed pitch prop airplane. Remember, he's going to upgrade engines as well.
Do you have the time to do flights and experiment with several different power settings to see what works best for a given situation? I don't, or maybe I'm just too lazy. With the fuel flow, you can adjust the power setting, lean and look at the fuel flow. Look at the GPS and you've got some powerful information. Now, change the power setting, and compare.
Secondly, I agree that these things aren't all that helpful for leaning. That said, what makes you think your single probe EGT is mounted in the "right" cylinder? My engine alternates between two different cylinders as to which peaks first. Best way to lean these little Lycs is to pull it till it gets a little rough, then back off.
Don't fool yourself that you have enough information to lean by EGT if you only have a probe on one cylinder. That is straight from Lycoming, by the way.
But, one of the best uses of these devices is seeing where any problems are before they destroy something, and as a diagnostic tool for trouble shooting.
At one point, we were replacing an airplane with a new one. Aircraft Division asked me what I wanted in the new airplane. I told them the standard avionics, PLUS an EDM 700. Anything else, I could care less about. I used that device a LOT for a lot of things.
If you are comfortable running only one power setting, you are correct, you can learn to predict your fuel requirements pretty precisely.
But, I can sure make one less gph work for me these days.
MTV