John Barrett wrote:

I probably know the answer to my question before I as= k but I am running my TSIO 550 lean of peak and I want to be sure I am using= adequate precautions and am doing the best for the engine.

To that end, if my engine baffling is not as good as i= t could be and I intend to continue to improve it, will a GAMI test now resu= lting in, say, a 1 gph difference in FF for peak EGT among the 6 cylin= ders be useful for me to send to Tornado Alley for GAMIjectors? In other words, if I improve my baffling after= I get George Braly=E2=80=99s fix, will that investment still be relevant?

John,
I=E2=80=99ll take a shot at this and defer to/welcome other comments from th= e experts.

First of all, baffling changes will not significantly affect EGTs=E2= =80=A6and vice versa. EGTs are a closely-related measure of the power being produced by the cylinder=E2=80=A6 and a host of o= ther factors related to combustion. CHT=E2=80=99s are a measure of how much h= eat is being transferred to the cylinder=E2=80=A6and how much is being remov= ed by cooling air. Again, lots of other factors affect cylinder temperatures, and while logic would tell you that high EGTs will y= ield high CHTs, the relationship contributes little as compared to other fac= tors. So with this in mind, consider these points.

To run a TSIO 550 lean of peak, it must be operated below 75% powe= r. My personal rule is to run it at or below 65% of rated power. Engine component heat, and particularly cylinder heat i= s the limiting factor, and the TSIO-550 can produce almost 100% of rated pow= er up into the teens if you leave the controls full forward. You=E2=80=99ll f= ind power setting info in the engine manual, and if you have a modern engine monitoring system you can probably p= rogram this into the monitor to give a direct readout of horsepower or a per= centage of max rated=E2=80=A6 With or without this, you need to derive this i= nformation from total fuel flow (LOP only) or a combination of MP and RPM, but it=E2=80=99s important that you k= now--and operate the engine at lower power settings--if you want to run LOP.= (NOTE: Some experts such as John Deakin =E2=80=9CThe Pelican=E2=80=9D say that y= ou can run these engines LOP at 100% of rated power AS LONG AS YOU CAN KEEP T= HEM COOL, but then acknowledge that as a practical matter this can=E2=80=99t= be done with current production aircraft engines=E2=80=A6they produce too much heat for the available cooling systems at high power setti= ngs.

When you run rich of peak, some of the fuel is used directly for c= ooling and passes through the engine without burning. As a matter of course, it cools by evaporation, and leaves nasty l= ead and other deposits behind=E2=80=A6on plugs, valves, etc. So when you run= LOP and eliminate this source of cooling, you must offset it by either redu= cing power (total heat produced) or increasing cooling (better baffling). So as you lean the mixture toward max cylinder E= GT, you=E2=80=99re reducing the amount of fuel available for cooling and the= CHT=E2=80=99s go up. With most Continental engines, this doesn=E2=80=99t ha= ppen 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= =E2=80=99re running about 65% power) as you lean, while the other cylinders a= re still rich of peak EGT. You need to note the fuel flow when the first cyl= inder reaches peak EGT. As you continue to lean, the EGT (and CHT to a lesser extent) of the first to peak starts g= oing down, while the remaining cylinder EGTs (and CHTs) continue to go up. E= ventually, all cylinders reach peak EGT and start decreasing. Note the fuel f= low when the last cylinder reaches peak EGT and just starts to decrease. The difference in fuel flow between f= irst and last cylinder EGT to peak is the =E2=80=9CGAMI lean test.=E2=80=9D I= deally, the fuel flow between first and last should be less than 1 GPH; use o= f GAMIs can tune this to less than half a gallon per hour, BUT NOTE THAT THIS IS OPTIMIZED FOR ONLY ONE POWER/RPM setting, s= o you should test/set up the engine at the normally-expected cruise power se= tting.

Here=E2=80=99s the rub: You need to be sure all cylinders are at l= east 30 degrees below their peak EGT when operating the engine in cruise power, and this is critically important when LOP. Idea= lly, all cylinders are all operating at about the same amount below their in= dividual peak EGTs, but the actual temperatures are not significant=E2=80=A6= it=E2=80=99s the peak value and delta below--for each cylinder--that=E2=80=99s significant. If the fuel flow is not well bal= anced among the cylinders, the first to peak EGT will get so lean by the tim= e that you get the last to 30 degrees below peak EGT that the first to peak s= tarts mis-firing. My cylinders start mis-firing at about 120 degrees LOP as a reference point. Min= e are well enough balanced (no GAMIs) that I can operate the richest cylinde= r at 60 degrees LOP and the leanest cylinder (first to peak when leaning) will remain below 100 degrees LOP. Th= at equates to about a 0.5 GPH fuel flow difference FOR MY ENGINE=E2=80=A6but= this varies a lot from engine to engine, and with the power/RPM setting.

BTW, the TIT=E2=80=99s ARE CRITICAL, and you=E2=80=99ll want to wa= tch these while you=E2=80=99re fiddling with the EGTs. Continental says the max TIT is 1750 and the max continuous is 1650. I am more conserva= tive, and have a =E2=80=9Cyellow warning=E2=80=9D set at 1550 and a =E2=80=9C= red alarm=E2=80=9D set at 1650. As you lean the engine, TIT=E2=80=99s will i= ncrease 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 a= bout 1500 on my TITs in cruise flight; I lean the mixture more if they go ab= ove 1550 in cruise flight.

Continental says the max CHT is 460 degrees. I am pretty convinced= you=E2=80=99ll cook your cylinders if you operate them anywhere close to that in normal cruise flight. I have a =E2=80=9Cyell= ow warning=E2=80=9D set at 400 and a =E2=80=9Cred alarm=E2=80=9D at 420. I o= nly 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 c= ase, when my CHT=E2=80=99s get to 420, I increase the fuel flow (low boost i= f not already on) , richen the mixture control (climbing ROP anyway) and/or i= ncrease airspeed. As a last resort, I=E2=80=99d pull the power back, but have never had to do this=E2=80=A6yet.=

As to CHTs during cruise flight. Get the fuel flow to the cylinder= s balanced first=E2=80=A6effectively you are making each cylinder produce approximately the same amount of power (yields a smoo= ther running engine)--and heat. Remember that this will be optimized only fo= r 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 m= any things to change the cooling air flow across the cylinders--and that=E2=80= =99s the only variable IF you=E2=80=99ve balanced the fuel flow/cylinder pow= er output. Working CHT=E2=80=99s first just won=E2=80=99t yield good results=E2=80=A6you=E2=80=99ll wind up doing it over and over=E2= =80=A6

Back to precautions you asked about:

<= span style=3D"mso-list:Ignore">1) If you=E2=80=99re running LOP, be sure you have the en= gine producing less than 75% of its rated power; below 65% if you=E2=80=99re conservative.

<= span style=3D"mso-list:Ignore">2) Be sure that every cylinder is LOP and that the hottes= t EGT is at least 30 degrees below the cylinder=E2=80=99s max EGT (without consideration of the actual value, nor the values of any o= ther cylinder)

<= span style=3D"mso-list:Ignore">3) Keep the TITs below 1650 degrees; ideally below 1550.<= o:p>

<= span style=3D"mso-list:Ignore">4) Keep the CHT=E2=80=99s below 420; ideally below 400 (I= f you do the above things, they will likely be in the 300-340 degree range)

As to GAMI=E2=80=99s: if you can get the engine to run smoothly, w= ithout mis-firing, with the richest cylinder (last to= max EGT as you lean) at least 30 degrees LOP you probably don=E2=80=99t nee= d GAMIs. Older Continentals won=E2=80=99t do this; the newer ones have a Con= tinental version of GAMIs (flow-matched injectors) and are better, but not as good/close as the fine-tuned GAMIs. A second mea= sure of evaluation is fuel flow. If the fuel flow between the first and last= cylinders to peak is greater than 0.5 GPH, GAMIs will improve this, and lik= ely result in a smoother-running engine.

LOTS of stuff here; hope it helps both setup and understanding=E2=80= =A6

Bob