On Jun 27, 2013, at 7:19 = AM, Colyn Case wrote:

Walter,

Can = you please elaborate on:

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.

I'm specifically wondering = what is changing at critical altitude. The wastegate is closed, = cooling is minimal, but does TIT not correlate in the same way to = turbine speed?

Colyn

On Jun 27, 2013, = at 9:04 AM, Walter Atkinson wrote:

**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=92s 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=85on 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=92re reducing the amount of fuel available for cooling and the CHT=92s= go up.**

No. = See above.

**With most Continental engines, this doesn=92t = 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=92re = 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 =93GAMI lean test.=94 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.
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=85but 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=92s = ARE CRITICAL, and you=92ll want to watch these while you=92re fiddling = with the EGTs. Continental says the max TIT is 1750 and the max = continuous is 1650. I am more conservative, and have a =93yellow = warning=94 set at 1550 and a =93red alarm=94 set at 1650. As you lean = the engine, TIT=92s 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=92ll cook your = cylinders if you operate them anywhere close to that in normal cruise = flight. I have a =93yellow warning=94 set at 400 and a =93red alarm=94 = 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=92s 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=92d pull the power = back, but have never had to do this=85yet.**

**As to CHTs = during cruise flight. Get the fuel flow to the cylinders balanced = first=85effectively 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=92s the only variable IF you=92ve balanced the fuel = flow/cylinder power output. Working CHT=92s first just won=92t yield = good results=85you=92ll wind up doing it over and = over=85**

That's excellent = advice.

Back to precautions you asked = about:
1) If you=92re running LOP, be sure you = have the engine producing less than 75% of its rated power; below 65% if = you=92re conservative.

= WHY???

2) Be sure that every cylinder is LOP = and that the hottest EGT is at least 30 degrees below the cylinder=92s = 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=92s 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.

Best = regards,

Walter Atkinson

Advanced = Pilot = Seminars