Engine monitors, ANR headsets, and all sorts of other dodads you can spend money on - are they worth it? Warning, long technical post.
I was going to post this in answer to another thread but I think it is a worthwhile one on its own. Not trying to claim I am right, just offering up a point of view that will hopefully start an interesting and thought provoking discussion. I should also point out that this post only applies to you if you operate a normally aspirated carburated engine. And it also only applies if you have a budget. If you have unlimited cash and panel space, fill your boots.
Carburated engines have horrible mixture distributions - fit one with an engine monitor and you will find 100 deg.+ variances in EGT's between cylinders, and the consequent variances in CHT's , made even worse by baffling variances. Leaning until the engine runs rough (leanest cylinder starting to not run) and then pushing the mixture control in 1/4 of the distance from the rough running point, will always result in a close to optimum mixture setting. Confirmation can be seen in the exhaust after landing. White deposits is too lean, gray brown is perfect, black is too rich. I compared leaning techniques in 4 airplanes fitted with engine monitors. And after leaning to 50 degrees ROP with an engine monitor, the mixture control was in exactly the same place as when I did it the old fashioned way. The thousands of dollars worth of engine monitor wasn't telling me anything I couldn't figure out on my own. Now an engine monitor will also tell you if you have a bad plug, a bad mag, an induction leak, or an engine starting to tear itself apart. But you can hear or feel all of those things as well if you are familiar with the engine and know what to listen for. That is unless your $1000 ANR headset has masked these symptoms - and they will! So what would I spend the $3000 the headset and engine monitor cost on instead to minimise engine wear and maximise safety?
Carb air temp gauge. The way we manage carb heat in most airplanes is stupid. In icing conditions, we periodically apply full carb heat just in case, which then screws up the mixture to the cylinders. And then run it cold, waiting for it to ice up, and hoping that we notice it in time. Then apply full heat, hope it isn't too late, scare our passengers who have to listen to the engine choke on the ice, then go back to full cold induction air. Repeat. Pratt and Whitney figured out long ago that the optimum mixture temp (measured right in the cab throat) is 7 deg. C. This temperature prevents icing, maximises the charge density, and atomizes the fuel efficiently. So after take off you just run whatever carb heat is required to produce a 7 deg. C mixture temperature, then lean, and fly along safely and efficiently with nothing else to worry about.
Anti-corrosion spraying of the airframe interior. As I work on airplanes that are parked on the coast, the amount of surface corrosion that occurs is amazing. It is a non-stop battle that we occasionally lose. Have the entire airframe interior sprayed with ACF-50 or Boeshield. Wings, tail, fuselage, all the nooks and crannies. You will save major structural repairs down the road. Additionally, because the airplane has to be seriously stripped down to do this, more so than is usually done at most annuals, you get to have a really good look at the places in the airplane that don't normally get looked at.
Gas money to fly more often. The single most thing you can do to improve the life of an aircraft piston engine is fly it more. Preferably at least once a week, but at least every couple or 3 weeks. Any longer and corrosion is starting to form on internal parts (especially around here) And that will eventually cause premature wear of parts as well as cylinder problems.
Magnetic drain plugs. Oil filters are very good at trapping almost all particles, but the mirco impurities of steel still make it thru. We install magnetic drain plugs in all the engines and always get a finger smear of metal sludge off them that the filters missed.
Oil additives. CamGuard is an amazing corrosion inhibitor with anti-scuff additives. It is excellent for use in infrequently flown engines, or the particular models of Lycoming engines that like to eat their camshafts. AvBlend is an excellent additive for removing and preventing lead deposits in valves and guides. If either of these issues are a concern to you, these products are worth considering. Both are a bit pricey and I suspect that they are not approved to be used together.
There is my take on where to spend your flying $$$ to make our engines more reliable and to save future maintenance costs. Free advice but you get what you pay for. Really looking forward to other opinions, both pro and con.