David Crouch:
I am thinking of putting in a JPI FS-450 fuel computer in my B24R Sierra. Any experience with these or similar units?
The JPI products are good units; I have installed several. I consider the JPI 700 EGT/CHT to be far more valuable on our planes, versus the 450 Fuel Flow unit. You have to check with JPI on their STC status, but you could probably get 337 field approval for a 450 if you had to take that route (simple fuel system, and thousands operating in the field). My thoughts follow:
All-cylinder CHT is the single most valuable instrument to prevent engine damage and extend cylinder life. It will indirectly show EGT, with a time delay. It will give warning of risky CHT levels, no matter what the cause (too steep a climb on a hot day, for example; or leaky baffles).
All-cylinder EGT is a convenient tool for leaning and fuel economy, for detection of failed spark plugs, to give a quick indication of an under-performing cylinder, and to help manage the heat being transferred to the cylinder heads. It can help you avoid continuous operation in the 40-50 degrees rich-of-peak range that the GAMI folks have proven to be the hardest on the engine (in terms of the sharpest pressure spike and the most heat transferred to the cylinder heads).
A supplementary oil temp indication, available on the JPI, provides an added reading that is often more accurate than the OEM gauge (which is usually in need of an overhaul). This oil temp indication gives an actual temperature value, and stops flashing when the oil is warm enough for takeoff. This is also helpful to ensure proper constant-speed prop operation, and to preserve adequate oil pressure despite CSP cycling.
The JPI will show buss voltage in a form that is more recognizable and very useful, as compared to the OEM ammeter.
The JPI gives a flashing alert of an out-of range event (oil temp, voltage, EGT, CHT, etc.), making it far more likely to be noticed in time to deal with it effectively.
As far as tracking fuel consumption, on our planes this is a non-issue (or should be). Our tank setup is stone simple (wet tanks, one tank per wing, no bladder, no crossfeed, no fuel transfer (except on the relatively few Continental powered planes), predictable and repeatable unusable fuel in level cruise, and a highly predictable average rate of fuel consumption.
The 450 is of the most value on planes with complex, multi-tank setups with fuel transfer requirements, and bladder tanks; especially on twin-engine planes. Even then, the 450 has its limitations. It is no more accurate than the reliability of the person who is inputting the loaded fuel volume. You still have to know what your usable fuel really is. You still are no more protected against an unexpected loss, like a leaking vent or drain valve. Your only protection from that is keeping your fuel gauges accurate.
I’m not suggesting that you go out and do it if you aren’t yet experienced and comfortable with it, but on our planes it is quite easy to predict total fuel usage to within fifteen minutes of a dead engine (two to three gallons usable remaining). Part of the “experience” aspect involves a thorough understanding of how this limits your maneuvering options. It is effortless and totally safe to predict fuel usage to within an hour’s remaining fuel. An hour basically means fuel is still visible at the lower rib when you look in the filler neck.
To my mind, it is money better spent to have your fuel senders and gauges overhauled, and to install the bonding straps described in the Service Bulletins and on BAC, to gain accurate fuel gauges. Combined with visual fuel level confirmation (15 gallons/tabs, 20 gallons/top of slots, and full), and known fuel consumption tracked over a couple of hundred hours, accurate gauges let you cross-confirm all the available info. As there are several points of cross-reference in this process, this combination leaves relatively little room for pilot error when compared to inputting fuel data on the JPI, a task which virtually every 450 owner has admitted doing incorrectly more than once. Accurate fuel gauges are also the only thing standing between you and nothing but wind noise, if something drains your tanks in flight (and several things can in fact do that). Unexplained fuel loss may also let you detect an engine compartment fuel leak (if you don’t smell it first) in time to land instead of burn.
My recommendation is to make the all-cylinder EGT/CHT a priority, in conjunction with getting your fuel gauges accurized. There is more elaboration on this on BAC. And having expressed my philosophy on this, I’m sure that our many happy 450 owners will weigh in on why it was worth the money to them!