I bought my 231 about three years ago. It came with an LB engine, original engine instruments, a Merlyn wastegate, a prepurchase/annual (at my expense of course), and a warning from the previous owner to run it extremely rich, like 13 GPH, if I didn't want to overhaul at least a few cylinders every thousand hours -- a warning which I immediately ignored. Instead, I adopted a strict policy of running either full rich (takeoff, climb, as the POH and Continental engine manual instruct) or cruising at engine settings which were guaranteed to produce 65% horsepower or less. This typically meant running at about an indicated 27" MP and 10 GPH (after the big pull.) I say indicated because, without a decent engine monitor, I decided to track fuel and oil consumption (among other things) in a spreadsheet so that any significant deviation from baseline fuel/oil consumption, which might indicate a problem, would be obvious, and after a couple fill-ups my spreadsheet indicated that my overall fuel consumption not including taxi but including climb was only about 8.5 GPH. And also, my MP gauge disagreed with the Kollsman Window at my sea level airport by about an inch. The idea was to try to run LOP and low power to keep everything as cool as possible, avoid there being a red box (26" and 10GPH isn't, 27" and 8.5 GPH could be, both should be <65% power) and try to stay outside of it if there were a couple cylinders pulling more than their weight until I could do some upgrades. Since I only had one CHT, I tried to go with a very conservative 360dF max CHT in cruise in case some undetected issue with baffling or mixture resulted in a cylinder other than the instrumented one actually being the hottest running cylinder. In climb, though, CHT could easily hit 380 or, on hot days (like the third day I owned it, climbing out of The Dalles with OAT 110dF) creep up towards 400 and necessitate stair-stepping or circling to cool things down even when attempting to cruise-climb at higher airspeed.
The obvious first upgrade for most people would have been an engine monitor. In fact, I had hoped to have one installed shortly after purchasing the aircraft but a number of failed pieces of avionics impacted my training for my instrument rating, so my first batch upgrade money ended up being spent on a GTN650 (it had a IIMorrow and an inop KN 53 and KA 87) and a new audio panel (the PM 1000 squealed whenever I turned the landing light on, so I had to choose between being visible and talking to CTAF/tower when landing.) Given that I knew my CHT could get hot even full rich (and with an indicated 25GPH flowing, it shouldn't have been a fuel issue) and I had to exercise some degree of heat management in climbs I also felt that the intercooler needed to happen sooner or later. I reasoned that since I could only do one or the other at annual, I already knew the engine was running pretty hot, and heat kills these engines, it made more sense to make the engine cooler than to be able to see six too-hot CHT readings instead of just one. I opted to buy the Turboplus intercooler first. I believe it was the first one supplied to a customer after Turboplus restarted production.
I had the intercooler installed at annual. This was supposed to necessitate modification of the Merlyn wastegate. I'm not sure what the history of the Merlyn installed on my plane was. Merlyn themselves didn't have record of it being installed on that plane, and it appeared to have already been modified to be fitted with the Turboplus intercooler even though my plane obviously wasn't intercooled. At any rate, it went back to them for modification and I got what looked like a completely new unit back (mine was over 20 years old; my shop was impressed at its longevity) and some updated paperwork for the unit and a fresh 337 written up for my aircraft for their modest refit fee. Solid after-sale support in my view, particularly with it being for the wrong owner of the wrong plane. (While we're on the subject, Turboplus after-sale support has been excellent. Jeff's checked up on me a couple times to make sure that a couple details with installation were done right, made sure I was pleased with performance, and he managed to get me digital copies of the installation and fuel adjustment instructions on extremely short notice while my plane was in service away from my home base and I couldn't get my hands on the originals. Getting a bit ahead of myself, John-Paul at GAMI and the Electronics International folks have been excellent as well.)
The intercooler had the desired effect, allowing me to climb at Vy to the high teens without CHT exceeding 380dF indicated on my sole CHT gauge. Without an engine monitor it is somewhat difficult to provide more commentary on the efficacy of the intercooler beyond "It made my engine run a lot cooler, enough that it'll probably pay for itself in enhanced cylinder life." As part of the installation, fuel flow was recalibrated. By this point I knew my mixture balance was probably somewhere between mediocre and poor, but I appeared to be able to run very slightly LOP. Not much, though, before it got rough. According to my spreadsheet my fuel consumption increased very slightly for my second year of ownership, but I only reduced my cruise MP by about 1", down to about 26", which suggests I was generally cruising at a higher power setting. Supporting that, I got probably three to five knots more in cruise as well. Folks with a 231 with a vintage panel and no intercooler will have to make their own decision on which mod to do when, and you want them all, but I believe I made the right choice to do the intercooler first and enjoy an extra year of operation with the significantly reduced CHTs, even if I couldn't actually observe it for most cylinders.
Second annual saw installation of the engine monitor (CGR-30P&C, I chose them because I liked the flexibility with respect to mounting the pair of round gauges over on the left rather than needing to mount a large format on the right or consume part of the radio stack or massively rework my panel when I already considered it possible that I would massively rework it for Dynon Certified or G500, a good call since G3X seems like it is within my future budget.) My CHTs had an extremely wide spread, my mixture was all over the place, and my GAMI spread was about 1.2 GPH. I'm not sure two of my cylinders ever really got LOP, and another two were pretty close to them. I felt pretty good about forcing myself to bumble along at 8.5-8.7 GPH in cruise for the last 130 hours, because running it a whole lot harder would have been pretty bad for the richest two cylinders. I flew it twice after getting the monitor before ordering GAMIs. GAMI got it right on the first go, my spread went down to 0.3 GPH and I finally got to fly my plane for what felt like the first time.
Short flight, low altitude (only went up to 9500) but I can finally, finally run the plane at a real 65% power. CHTs stay under 360 at 31"/10GPH with the cowl flaps closed, I should have no problem running similar power levels up to the high teens/low flight levels, and the plane managed 150+ knots TAS at 9500' in this power configuration. The 231 as it came from the factory was not Mooney and Continental's best effort, but with some time, attention, a couple truckloads of money (though still tens of thousands less than the price of a comparable 252) for some aftermarket parts, it can be made into pretty close to the plane it ought to have been. I'm really looking forward to more flying this summer -- up in the high teens where it's cool, quiet, there's no traffic to deal with, and the plane performs at its optimum.