I highly recommend the combined digital-analog JPI EDM-700 system. The 2.25 inch device fits neatly into space below the clock in the upper left corner of the panel. The analog bars provide a quick means of comparing cylinders, and the "normalize" function (after engine is stabilized in cruise, hit "normalize" and all four bars are set to equal height halfway up -- any change in one cylinder is immediately apparent as that bar goes out of line with the other three) is great for detecting problems when you're not monitoring the gauge closely. The digital readout below the bars can be left in scan mode (shows EGT and CHT side by side for one cylinder at a time in rotation -- about 3 seconds and then it steps to the next) or manually cycled to one cylinder (or to the other available digital data -- battery voltage, oil temp, CHT cooling rate in degF/min, and difference between hottest and coldest cylinder). The system now includes an available fuel flow readout if you want to put a flowmeter in the fuel line (wasn't available when we bought ours -- wish it had been).
We had the EDM-700 engine analyzer on our Grumman AA-5A Cheetah for five years (700 hours) before we sold the plane. The installation we had was a full four-cylinder EGT and CHT plus oil temperature. It cost about $1900 for the parts – I got it through a Gangster, but I forgot who; post a note and you’ll hear from him. The system was installed by my own mechanic during engine reinstallation following a major overhaul, so it’s not possible to give an accurate installation cost. In my opinion, it’s an outstanding piece of equipment for managing the mixture and throttle, but there are some caveats.
Based on our Cheetah experience, we put the EDM-760 with fuel flow in our Cougar, and again it's great. The fuel flow information is tremendous in terms of knowing how much is aboard (precision fueling is important in the Cougar in order to manage available payload) and in monitoring operation in flight. Twice I've been able to isolate the dead plug on run-up, thus making it a quick plug swap instead of a dead plug hunt. Now that we have the Tiger, there’s another JPI EDM-series engine analyzer in our future.
It’s important to view the information as relative rather than absolute. Installation and other variables can make the numbers you see be different from actuality. For example, our Cheetah had a Tanis preheater that uses the CHT probe wells on the cylinders, so we use the spark plug gasket thermocouples. Further, because of the way the engine compartment and baffling are laid out, we had to mount these on the bottom plugs. As a result, the temperatures read 40 to 80F higher than the cylinders’ actual temperature, and 40F higher on the rear cylinders than the front. Also, due to placement of the EGT probes, the back cylinders read 50-100F higher than the front cylinders. Thus, the information is useful from a relative standpoint (i.e., one cylinder is reading 100 higher than usual) rather than an absolute value (i.e., cylinder #3 CHT is 420F). As a result, the analyzer is great for spotting problems (via unusual readings), leaning (when peak EGT occurs rather than what the actual temperature is at peak EGT), and checking operation (identifying a bad plug during mag checks).
Tanis ran some tests of the “actual” temperatures (i.e., those measured at the tip of the thermocouple in the CHT well – the “official” values used in certification) versus those measured other ways. They found that the spark plug gaskets read up to 80F high, with the largest difference at the top of the climb. Spark plug gaskets on the lower plugs read hotter than those on the top plugs. Tanis' new "washer" type thermocouples (Tanis P/N TAS 2144K, $60 each) that fit onto the Tanis preheater probes in the CHT wells were accurate at climb, idle, and descent, and read 30F colder than the tips at "high speed cruise." A guy I know with a PA-28R-200 got those probes and is very happy with them. We have since replaced our gasket thermocouples with this type. However, if were installing both systems at once, I'd get the Tanis dual probes (Tanis P/N TAS 100-K, $165 each) that replace both the preheater probes and the other thermocouples. They read the temperature at the tip in the well, and so are totally accurate.
That said, I find the system most useful on a regular basis for leaning in cruise and, at higher density altitudes, in climbs, for checking spark plugs during run-up/mag check (EGT rise on each cylinder when single mag selected), and for monitoring CHT during descent to prevent shock-cooling. It’s also an excellent backup for the installed oil temp gauge. We installed the probe that feeds the analyzer at the inlet port to the oil cooler, thus reading the oil temp at its hottest. The installed gauge reads from a different part of the engine where the oil is a little cooler. As a result, the analyzer oil temp usually reads about 10F hotter than the installed gauge, and provides assurance that I know the max oil temp when the engine is running in the summer heat. On the other hand, in the winter, the systems provides a cross-check of the vernitherm. When the oil is too cool, flow through the cooler stops, and the oil temperature on the analyzer drops to engine compartment ambient. (Boy, was I surprised the first time I noticed an oil temp of 60F in flight! - at least until I figured out why). When the vernitherm opens again, the temperature rises to match the installed gauge. Another interesting piece of information, and an assurance that an otherwise hard-to-check but critical piece of equipment is functioning properly.
In addition, it has helped sort out an engine problem. Without any prior symptoms, our Cheetah's O-320-E2G engine with the high compression STC began running a little rough today. The #3 cylinder EGT showed about 60F higher than normal, and the #3 CHT about 30F lower than normal. The other three cylinders were all at normal indications. There was noticeable but minor roughness. At times, the RPM would go up about 50 and the #3 EGT would drop back to normal, then the RPM would drop back about 50 and the EGT would rise back up to the 60F-higher value. On ground run-up, all was normal at 1800 RPM, and I got the usual 30F rise in EGT on #3 when going to either L or R mag, with no noticeable roughness. Takeoff, climb, and cruise performance seemed pretty normal, although the roughness was apparent at full power.
On the advice of several mechanics, I did a mag check while airborne, and sure enough the engine ran pretty smooth on the left mag, but rough as a cob on the right, isolating the problem down to one plug -- the #3 right (lower) or its associated leads/section of the mag. My mechanic said, "Let's look at the easiest thing first -- pull the plug." We did, and found a blob of lead you wouldn't believe -- looked like a giant booger, complete with trailing snot deep into the plug. We then pulled the other three lower plugs and found measurable but not particularly significant leading on them. The top plugs were pretty clean. After cleaning and regapping the plugs, the engine runs fine. The point is that with the analyzer, I was able to isolate the problem straight down to the #3 cylinder right (lower) spark plug/wiring. That alone might have saved hours of trouble-shooting.
Based on our experience with the Cheetah, we put a JPI EDM-760 in our Cougar, and now have a JPI EDM-930 in our Tiger. After 15 years of flying with JPI EDM's, I wouldn't have it any other way. The only question in my mind would be which of the ever-expanding array of JPI EDM's to install.
