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Maximum Fuel Flow 390 with FM150

I'm seeing about 16.5 on takeoff. IO-390 Tbolt, FM-150, dual PMAGS, Field elevation 726'. I had to modify my snorkel a little to fit around my B&C starter, so it's an even less optimal route for intake air.

-Rick
 

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I believe I see about 16.5G, though I am not at sea level, field elevation is 2600 and I usually have my mixture about .75" out during take off.
 
Got me curious, so I checked it this morning. At rotation, 2650 rpm and 18.8 gph. Fm 150, Thunderbolt IO-390, dual p-mags, Hartzell composite prop, field elevation 520. Have you calibrated your red cube? Perhaps, that is giving an erroneous fuel flow.
 
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I was told a representative engine for the IO390EXP (since I can’t find one for the EXP engine) would be an IO360A1A times about 7% for the greater horsepower. I will have to look for a manual for that.

Nevertheless, I just learned that fuel flow is not a good measure of engine performance, as it is influenced by air flow, exhaust flow, and the actual horsepower the engine produces. Perhaps, my engine is producing more horsepower than yours at rotation? A better check of whether the engine is lean or rich, I was told, is to fly at 3500 feet and 24 squared. Lean to peak EGT on one cylinder and record that temperature. Then, return to full rich for 45 sec to a minute and record the EGT for the same cylinder. The difference should be 185 - 230 degrees. Higher difference (say, 300) would indicate the mixture is too rich. Lots to learn.
 
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I’ll probably go through the AFP steps to correct stopping short of swapping the inlet orifice and call it good. The engine runs well and the plane does a little better than Vans published numbers.

Call Don and get the procedure for checking full rich mixture by reference to EGT. The approach largely eliminates altitude and density factors.
 
My procedure for setting up the mixture at idle was (going largely based on memory)

At idle, pull the mixture about half out and then start leaning. if the Manifold raises more than an inch, then you need to lean.
 
Mark,
Dan, I’ll give Don a call as you suggest. He knows this stuff so well it reinforces how little I do.

I called Don. He is the one who told me the procedure in my previous message. Great resource and easy to talk to. :rolleyes:
 
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FYI - I contacted Jeff Schans, Lycoming Thunderbolt manager and a great guy, and he said the engine we have is a YIO-390-A3B6. Maybe everyone else already knew this.;)
 
Generally a good rule of thumb is 1 gal of fuel for each 10 hp for takeoff. For example 160 hp should be using around 16 gal per hour on takeoff. This allows a rich enough mixture to give you good cooling on hot days. These numbers are at sea level.
 
Generally a good rule of thumb is 1 gal of fuel for each 10 hp for takeoff. For example 160 hp should be using around 16 gal per hour on takeoff. This allows a rich enough mixture to give you good cooling on hot days. These numbers are at sea level.

I think .9 GPH is actually a little closer than 1.
 
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I went thru my G3X log data. On average taking off near sea level in summer:
RPM 2720
FF 17.1 GPH

Stock FM-150 on a 390 Thunderbolt all as shipped no mods.
Didn't open the bore on the snorkel at the lower end.
 
The fuel flow recommendation that I put forth is not mine, it’s Mike Busch’s numbers....:)

Doesn't matter. Facts are facts. 16 GPH for 160 hp is a BSFC of 0.60 assuming 6 lbs/gal. Too fat to develop full power.

Example below, a graph from the FAA dyno. It's a mixture sweep done to find detonation onset. Rich is on the left, moving progressively leaner to the right. I've stripped out a bunch of data not relevant to this discussion, including the detonation onset on the lean end.

With this 540K, a BSFC of 0.060 meant a power loss of about 4% and a lot of wasted fuel. Sure, CHT was a little lower, but it's better to cool with air for a whole bunch of reasons.
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Two more graphs, real data.

As others have noted here, the 390 manual says 105 lbs of fuel per hour is the minimum for rated power, or 17.5 GPH. 29.92"hg and standard density is assumed in order to achieve rated power, so if it's hot outside, or the aircraft has climbed a bit, fuel flow should be less than 17.5. And Lycoming didn't pick the figure without consideration. It assumes the worst case pro-detonation operating condition, specifically 475F CHT, 245F oil, and 100F inlet air.

The chart below is a mixture sweep to determine detonation onset, again using an IO540K. Chart in the previous post was full throttle. This one can be seen as a condition just a bit into initial climb, as MP drops to 28", with everything really hot departing a quick turn fuel stop in the desert.

As before, 0.50 BSFC is just on the rich side of maximum power. More important (at least to Lycoming's lawyers) is the margin between 0.50 and detonation onset at about 0.44 BSFC. Point is, there is no reason to think you need more fuel to avoid detonation. Lycoming's recommendation of 17.5 GPH already added the margin...and remember, it assumes worst case temperatures. Want absolute max power? It's found a little leaner than 0.50, and given a good cooling system, it's safe to go there. Keep temperatures down, and the detonation line will move to the right, or disappear entirely.
 

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Climb a bit more, still at 2700, but MP now down to 26". BSFC of 0.50 is still hanging in there like a fly on Pence, just short of max power.

I'm posting this one because we can now see peak EGT. Most of the reading material says to stay away from the 50 rich of peak area, as it is the most pro-detonation mixture range. Absolutely true, and you can see it here, with 50 ROP right at detonation onset....although again, remember, that's with temperatures at maximum.

So where is 0.50 BSFC? 160 ROP, which is why the "lean test" for fuel flow (comparing full rich EGT with peak EGT) is generally considered too rich beyond a 200F spread.

Forgive me please, but there are three ways to cool, (1) throw fuel at it, (2) do it with air, or (3) reduce throttle. One of them is costs money and spews extra lead on the landscape. We already have too many brain dead citizens; more lead won't help ;)
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BTW, this stuff is near universal. Here's a chart for a Continental IO550 plotting BSFC against power, CHT, and EGT.

0.60 BSFC is literally off the chart.
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io360

If I read this chart right, then we have about 0.4777 for an io360.

(86 lbs/h / 180 hp) = 0.47

(L)IO-360-M1A Oper & Install Manual 60297-36.jpg
 
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