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Yet another takeoff fuel flow question.

sailvi767

Well Known Member
I am trying to dial in a new motor. IO540 with 9.5 to 1 pistons, RSA10 servo, Raven CAI. I have problems with rising EGT’s at lower power settings. I have exhausted options looking for induction leaks. I am starting to think I am flowing to much fuel. FF on takeoff is about 29 GPH however with the current weather and temps in the SE density altitude is above 3000 at my home airport. The interesting thing is that when I get the rising EGT’s pulling power turning base leaning the mixture from full rich corrects the EGT’s and they come right down. Is it possible I am so rich that fuel is burning in the exhaust creating the high EGT’s?
Is 29 GPH to high for a 3000 foot plus DA on my engine configuration for takeoff? I have searched the forums and can’t find a takeoff FF chart for a 3000 foot DA. Any advice appreciated!
George
 
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Need more data. Once past peak egt, they keep going down as you get richer. Pulling power back from WOT changes things, so not surprised to see egt rise in that case, as the servos at full rich tend to be more conservative (i.e. richer than necessary) at the high MAP extremes. Afterfires are not likely at that fuel flow rate.

29 gph WOT and 3000’ is much richer than necessary. Wasting fuel and likely reducing power output. Not sure that density altitude directly relates to air fuel mixture. Either way, wot sea level should flow in the neighborhood of 25 gph.

Larry
 
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A very rough fuel flow gph number for takeoff is 10% of HP. In other words, a 300 hp engine needs 30gph to be reasonably rich at takeoff power. This is NOT anywhere near correct for cruise power, but for takeoff it’s usually close, and will normally be 200-300 rich of peak EGT.

So 25 gph is in the ballpark for an IO-540 at sea level. Slightly less at 3000’.
 
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Did some testing today and I think excessive FF is the problem. Dialed the mixture back a bit to get 27.5 GPH and it was like a different engine. No issues with EGT’s pulling the power back and the aircraft was making a lot more power off the touch and go’s. The kind of power I hoped to find with the engine swap!
G
 
Still seems rich to me.

The formula I’ve seen suggests 27.5 gph should generate 330 hp. Are you supposed to get that much out of your 540? A 260 hp 540 should be drinking about 22gph according to the formula I know, which is half a pound per hour per horsepower. 260 hp, 130 pounds, 21.7 gph.
 
I think you're fat on your fuel flow - my IO360 runs about 15 at takeoff power at 2700' MSL, you are roughly 1.5 times the engine and should be burning roughly 1.5 times the fuel - not 2.0 times.
 
I agree, with the higher compression pistons I do want to be careful dialing the fuel flow back but 29 GPH is high and the engine seemed very happy when I dialed it back to 27.5. Not sure at this point if I want to make actual changes or handle it with the red knob. In colder weather where the DA is not above 4000 on downwind at a airport 800 feet AGL 29 GPH might look better! I also got about a 110 RPM rise on a idle cutoff check. Might need to lean the idle a bit.
G
 
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