After installing an EDM-900 in my p.ponk 0-470-50 equipped Cessna 180 and flying it for over 50 hours I have developed a theory to explain what I can only describe as “unusual instrument indications”. Prior to the install of the EDM-900 I had a six cylinder egt with 25 degree incrimination. This was difficult to read and virtually impossible to determine which cylinder hit peak egt first. I also had no indication as to what the current fuel flow was. I have checked the intake manifold and it has NO leaks.
My first surprise from the EDM-900 was my fuel flow at cruise power (2400 RPM/23 inches m.p.). At 1000 feet and full rich the fuel flow is about 18 gallons an hour. The hottest cylinder is almost always number 1 at about 1450 degrees egt. The problem I had was that cylinder would peak at 1490. This was something I just could not get my head around. Full rich operations were only about 40 degrees rich of peak egt. I was unable to get a 75 degree rich of peak egt (recommended best power) at any power setting. Climbing up to 3000 feet helped to get a little more fuel to the fuel/air mixture. The number one cylinder was able to maintain almost 75 degrees rich of peak but required a full rich mixture setting with a fuel flow of about 18 gph. EGT temperature differential was about 250 degrees. The EDM manual indicates a carbureted engine should have a temp differential of about 200 and a fuel injected engine about 100.
I also noted some “odd” reading when applying carb heat. There is a definite increase in fuel flow when carb heat was applied. The increase was small but notable. The more puzzling indication was the EGT temps. With no other movement of any engine control, with partial carb heat at 40 degrees the EGT of the number one cylinder LOWERED from 1450 to less than 1325. This now allowed leaning the engine to get a 75 degree rich of peak egt setting with a fuel flow of LESS than 16 gph. Another “odd” reading was the cylinder egt temperature differential went from 250 degrees to less than 70.
My only explanation for this is the carb heat is allowing the fuel to better vaporize and burn more efficiently. Basically, with the use of partial carb heat, my engine is running about 150 degrees cooler, using 2 gallons LESS per hour and has the efficiency (egt temperature differential) better than one would expect from fuel injection.
Normal carb temperatures with no heat are about 15 degrees. So, only about a 25 degree rise in carb temps changes the efficiency of this engine from day to night. I would have never known this without an advanced engine monitor. Live and learn.