Based on hard data, there are some significant misstatements in your post. I will comment in line.
**To run a TSIO 550 lean of peak, it must be operated below 75% power.**
Many of these engines are operated above 75% power as a matter of routine while controlling CHTs. This usually requires operating it LOP. As Confucius say, "Do not tell man something impossible when he already do it!" <g>
**but it’s important that you know--and operate the engine at lower power settings--if you want to run LOP. **
There is absolutely NO reason not to operate the engine LOP at high power settings. Literally, thousands of pilots are doing so quite successfully.
**When you run rich of peak, some of the fuel is used directly for cooling and passes through the engine without burning. As a matter of course, it cools by evaporation, and leaves nasty lead and other deposits behind…on plugs, valves, etc. **
The heat of vaporization of any extra fuel in a ROP mixture is not even a blip on the radar screen in BTUs available for cooling. The reason a richer mixture runs cooler than, say 50dF ROP, is that the richer mixture burns slower placing the thetaPP further from TDC. This results in lower internal cylinder pressures and fewer BTUs transferred across the thermal boundary layer. Extra fuel does NOT cool a cylinder. A full rich mixture relying on heat of vaporization for cooling would not be 1 degree cooler!
**So when you run LOP and eliminate this source of cooling, you must offset it by either reducing power (total heat produced) or increasing cooling (better baffling). **
When LOP, the leaner mixture slows the burn even mores than a richer mixture and cooling is even better. It is possible, therefore, to operate at higher power settings LOP with the same CHTs as ROP, or at the same power setting with cooler CHTs.
**So as you lean the mixture toward max cylinder EGT, you’re reducing the amount of fuel available for cooling and the CHT’s go up.**
**With most Continental engines, this doesn’t happen at the same fuel flow for all cylinders, that is, one cylinder reaches peak EGT at a given fuel flow (around 16-17 GPH if you’re running about 65% power) as you lean, while the other cylinders are still rich of peak EGT. You need to note the fuel flow when the first cylinder reaches peak EGT. As you continue to lean, the EGT (and CHT to a lesser extent) of the first to peak starts going down, while the remaining cylinder EGTs (and CHTs) continue to go up. Eventually, all cylinders reach peak EGT and start decreasing. Note the fuel flow when the last cylinder reaches peak EGT and just starts to decrease. The difference in fuel flow between first and last cylinder EGT to peak is the “GAMI lean test.” Ideally, the fuel flow between first and last should be less than 1 GPH; use of GAMIs can tune this to less than half a gallon per hour, BUT NOTE THAT THIS IS OPTIMIZED FOR ONLY ONE POWER/RPM setting, so you should test/set up the engine at the normally-expected cruise power setting.**
Well, sort of. You have correctly explained the GAMI Lean Test. The spread between the first and last to peak in GPH is called the GAMI Spread. 1gph is pretty poor. GAMI shoots for a Spread of 0.3 and most engines will run "smooth enough" with a Gami Spread of 0.5 or below. While the GAMIjectors were originally "optimized at 2300rpm (not any particular power setting) testing has shown them to remain quite well balanced along a wide range of rpms and power settings. The GAMI Spread will change very little UNLESS you have induction leaks.
**Here’s the rub: You need to be sure all cylinders are at least 30 degrees below their peak EGT when operating the engine in cruise power, and this is critically important when LOP. Ideally, all cylinders are all operating at about the same amount below their individual peak EGTs, but the actual temperatures are not significant…it’s the peak value and delta below--for each cylinder--that’s significant. If the fuel flow is not well balanced among the cylinders, the first to peak EGT will get so lean by the time that you get the last to 30 degrees below peak EGT that the first to peak starts mis-firing. My cylinders start mis-firing at about 120 degrees LOP as a reference point. Mine are well enough balanced (no GAMIs) that I can operate the richest cylinder at 60 degrees LOP and the leanest cylinder (first to peak when leaning) will remain below 100 degrees LOP. That equates to about a 0.5 GPH fuel flow difference FOR MY ENGINE…but this varies a lot from engine to engine, and with the power/RPM setting. **
How far LOP one should set the mixture is dependent upon the power being produced, not some magic number that always works. For example, at 65% power, 20dF LOP is adequate. I operate routinely at 87% power and set the mixture at 70-80dF LOP.
**BTW, the TIT’s ARE CRITICAL, and you’ll want to watch these while you’re fiddling with the EGTs. Continental says the max TIT is 1750 and the max continuous is 1650. I am more conservative, and have a “yellow warning” set at 1550 and a “red alarm” set at 1650. As you lean the engine, TIT’s will increase with EGTs, and will continue to increase for a short time after the first EGT peaks are reached, and then start decreasing. I normally see about 1500 on my TITs in cruise flight; I lean the mixture more if they go above 1550 in cruise flight.**
I know of no scientific data to support your recommendations of keeping TITs under 1550. Doing so may keep you 100dF ROP or more, but it is doing nothing for the life of the turbo. The 1650 TIT limit is for max rotor speed at max temperature. The turbo may be safely operate at 1649 all day long without concern. A s a matter of fact, that number is conservative unless one is at critical altitude.
**Continental says the max CHT is 460 degrees. I am pretty convinced you’ll cook your cylinders if you operate them anywhere close to that in normal cruise flight. I have a “yellow warning” set at 400 and a “red alarm” at 420. I only see these temps during high power climbs between about 12,000 and 16,000, when the engine is still producing lots of power but the air is starting to thin/provide fewer molecules to carry away the heat. In any case, when my CHT’s get to 420, I increase the fuel flow (low boost if not already on) , richen the mixture control (climbing ROP anyway) and/or increase airspeed. As a last resort, I’d pull the power back, but have never had to do this…yet.**
**As to CHTs during cruise flight. Get the fuel flow to the cylinders balanced first…effectively you are making each cylinder produce approximately the same amount of power (yields a smoother running engine)--and heat. Remember that this will be optimized only for a small range of power/RPM settings. Then go to work on balancing the CHTs. This is (at least theoretically) much easier than balancing the fuel flows and EGT peaks because you can do many things to change the cooling air flow across the cylinders--and that’s the only variable IF you’ve balanced the fuel flow/cylinder power output. Working CHT’s first just won’t yield good results…you’ll wind up doing it over and over…**
Back to precautions you asked about:
1) If you’re running LOP, be sure you have the engine producing less than 75% of its rated power; below 65% if you’re conservative.
2) Be sure that every cylinder is LOP and that the hottest EGT is at least 30 degrees below the cylinder’s max EGT (without consideration of the actual value, nor the values of any other cylinder)
That depends on the power being produced.
3) Keep the TITs below 1650 degrees; ideally below 1550.
No scientific reason for this recommendation concerning 1550.
4) Keep the CHT’s below 420; ideally below 400 (If you do the above things, they will likely be in the 300-340 degree range)
Excellent advice.
I hope my response has improved your appreciation of the issues. If I can expound on any of these, feel free to ask. You would benefit significantly by taking the APS class as all of these things will be shown with data to support what I have said above.