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O-320 Fuel Burn at Full Throtle

MauiLvrs

MauiLvrs

Well Known Member
We were wondering what others are seeing for fuel burn at full power with O-320s.

At 100% power we are only able to burn a little over 12 gal/hr.
The engine manual say that the fuel burn should be about 14 gal/hr.

The CHTs are higher than we would like and 2 gal/hr would sure provide some extra cooling. The fuel burn at 75% power is about 10 gal/hr at sea level which is about what the manual says it should be.
 
Just to confirm that you're at 100%, what are the RPM and MP readings when you see these higher than desired CHT's? Also, what CHT's are you seeing and at what airspeed does this take place?
 
29"+ at 2700 RPM.
29" unless power reduction is required to keep CHTs down.
Just verified again that both the throttle and mixture go to the stops.
CHTs run 430 and sometimes up to 450 at 110-120 knots.
 
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Some hypothetical numbers

Best power is usually at around 0.5 pounds per HP per hour. If you could get 160 HP from your 320 then you'd need to burn 80 pounds per hour and that would be, at approx. 6#/gallon, 13.33 gph. If you run richer, you will see higher gph, of course.

That said, isn't the full rated power at 29" or more? They rate these engines at sea level. For 27" ambient, you are at about 2300-2400 density altitude. 27" is 93% of 29". 13.33 x .93 = 12.4. Does that come pretty close to what you are seeing?
 
hevansrv7a said:
They rate these engines at sea level. For 27" ambient, you are at about 2300-2400 density altitude. 27" is 93% of 29". 13.33 x .93 = 12.4. Does that come pretty close to what you are seeing?
Click to expand...
Those numbers seem to be about right for 3000 feet.
But even at sea level, we never seem to be able to get close to 14 gal/hr.
with 29" @ 2700 rpm.

Using the number of .7 pounds per HP that we found gives about 14 gal/hr which is what Lycoming says it should burn at 100% power.

Wondering if the economizer circuit is functioning correctly.
Interested in what other are getting before we call Lycoming.
 
Dave & Trina, after a lot of reading and research I enlarged the main jet. At least Aerosport Power did. They said typically an O-320 in an RV typically run lean. I now have a jet with a #39 hole in it.

At 29.4" and 2680 rpm it takes about 14.5USG.

You can see much more info here.

The jet size in the cruise is irrelevant because fuel flow is controlled by the red knob.
 
MauiLvrs said:
What was your fuel burn before and how were your CHTs affected?
Click to expand...

Sorry I wasted my time writing to you. If you don't read the reference I gave you, you will not know the answers to your questions. They are all there.

WRT Kelly, I think Aerosport Power know their business.
 
Steve Sampson said:
Sorry I wasted my time writing to you.
WRT Kelly, I think Aerosport Power know their business.
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You weren't wasting your time. The reference was very useful. Let me rephrase the question as it wasn't clear. What I didn't find in the reference was the fuel burn and CHT @ 75% power at sea level.

The concern in the Kelly article in large part was that drilling out the jet would damage the tip.

If we can't resolve the fuel burn rate our may be the same way that you did and replace the jet with one of a larger size (as opposed to drilling it out).

Looked through our test data and we were getting a temp increase of about 100 deg in #1 & #4 and 150 deg. increase on #2 & #3.
 
Dave,

I would second what Steve is saying - I was seeing 430+ after my O-320 was overhaulled and drilled the main jet to reduce the CHTs. Now they are generally under 400. I have no idea how high they would go before as I would throttle back and increase speed when the hottest cylinder got to 430. I don't have a fuel flow so can't tell you what it is flowing. Enlarging the jet by one drill size will have a noticeable effect.

I would suggest drilling the jet and seeing what happens (I am told the maximum enlargement should be 0.007"). If it reduces the CHTs you have found the problem and can consider fitting a new jet if you prefer. If the CHTs are still high you have lost nothing as you would have bought a new jet anyway.

Pete
 
Dave & Trina, your 75% question I really can not answer because that is not a power setting or height I use.
I do run around a lot at 58-60% power and there is a good graph here where you can see all the parameters. The engine is running at 21lph ( 5.5usg) at about 2500'. Its running on P-mags so it runs on fumes.
(PS double click the graph to get it bigger.)

You can see the temps but do check the scaling factors in the key on the right.

Even the 58 to 60% power is probably overstated since the mixture is very lean. just by richening a little I think the speed would increase but the MPG would drop.

With respect to drilling the jet, that is what Aerosport Power do, and in fact the jet I am now using and relating to theis flight I drilled.(Or at least a friend with a pillar drill drilled while I watched.) The beehive jets they use now I think the fuel comes out of the holes in the sides not the end anyway. You are drilling the jet inlet not outlet. The Kelley stuff might have been right once but I suspect it is garbage now.
 
Steve Sampson said:
Even the 58 to 60% power is probably overstated since the mixture is very lean. just by richening a little I think the speed would increase but the MPG would drop.
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Steve,

Off the original topic, but I calculate you are at about 50% rather than 58% to 60% power. Lycomings Part Throttle Fuel Consumption graph on page 3-22 of my manual shows at 5.5 g/h and 2,000 rpm (from your graph) and a very lean mixture (best economy/peak egt or leaner) would give 80 hp (I assume it is 160 hp engine?).
The real figure could be marginally different as our RV installations with straight through exhaust pipes and electronic ignitions are likely to extract slightly more hp for a given FF than a standard installation. Counteracting this is the fact that Lycomings graph is for an injected I0-320 which I think produce slightly more power at the same FF than our carb models.

Fin
9A. 0-320 D1A
 
I'm guessing that Lycoming will not be as responsive in finding a solution as Aerosport Power :(

Some carb models have a way to fine tune the economizer circuit. I don't see a way to do that on the MA4, other than increase the jet size.
 
Finley Atherton said:
Steve,

Lycomings Part Throttle Fuel Consumption graph on page 3-22 of my manual shows at 5.5 g/h and 2,000 rpm (from your graph) and a very lean mixture (best economy/peak egt or leaner) would give 80 hp (I assume it is 160 hp engine?).
Click to expand...

Sig, yes I would rather a little over rich than lean on full power, though as you see I use it minimally. I had it richer but decided I wanted it a little closer to the Lyco spec., but it was a feeling rather than any good justification. Wanting it richer than Lyco indicate makes sense because at full power the distribution is very poor, though as you see when leaned right out everything closes up nicely.

Fin, yes it is nominally 160hp though Bart said a little more. But I expect you are about right.

A very interesting thing shows up though in that last graph. I will repeat it here. I try to run at,
2000rpm
21lph
22", not because it is easy to remember but because I have noticed it to be very economic. If you look at the graph, up until just after 9:28 I am actually running with about 22lph. (Its very hard to adjust) Then I get it right.
Several things happen:
- The grey line at the top is a computed MPG and you see it jumps up from around 7 to perhaps 7.3. Clearly the efficiency has increased with less fuel.
- the speed is unchanged. (I can get leaner still, 19lph, but then the speed drops away fast.)
- The EGT tighten right up.
- The CHT actually widen out slightly with #1 dropping faster than the others.

On the return flight I set the fuel flow up but in fact it is running more like 20.5lph. The MPG clearly increases but in fact the airspeed has already dropped a little. Perhaps 4 to 5mph on average though the air was quite turbulent. (Which will cause me to raise another post.)
 
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