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Fuel Burn

vomatic

Active Member
What are you experiencing for fuel burn w/ 160, 180 and 200 hp engines? I am trying to decide what to put in my -8. My thought is the 200 hp, but looking for any input. Thanks.
 
Hi

I have a 8 with a 0-360 (180) C/S prop and at 170kts TAS I burn 9gph at 165kts TAS I am down to 8.2gph. Or at 188kts 13.5gph:D
 
Opinion follows: Forget the 160 HP. I read once that pulling the power back on an O-360 to match the speed of an O-320 results in nearly the same fuel flow. If that applies to a 200 HP compared to 180 HP, then you have a strong case for the 200 HP. Since few if any people clamor for LESS power, the 200 HP may have a stronger resale value.
 
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Forget the 160 HP. I read once that pulling the power back on an O-360 to match the speed of an O-320 results in nearly the same fuel flow. If that applies to a 200 HP compared to 180 HP, then you have a strong case for the 200 HP. Since few if any people clamor for LESS power, the 200 HP may have a stronger resale value.
You will hear this a lot. Do a search on this subject and "Dan". Dan has the big engine in his -7 and has been very vocal on the subject of his low fuel burn at low power settings.

What you may be more interested in is the flight characteristics of an O-320 powered -8 vs. that powered by an O-360 or IO-360 (200HP) engine. The big engine will always climb better than the small but the question is, how does it "feel".

Just be careful if you take advice from a guy flying behind an O-290 like me.
 
My airplane typically burns 8 gph from chock to chock on X/C trips and on local stuff too. I usually fly at ~65% power. I have a 160 hp 0-320
 
fuel burn

Our engines burn about .45 lb. of fuel per horsepower per hour, perhaps a tad less when aggressively leaned. A 160hp at 75% would use 160 X .75 X .45= 56 lb./hr which divided by 6 Lb per US gal. = 9 gallons per hour
 
Our engines burn about .45 lb. of fuel per horsepower per hour, perhaps a tad less when aggressively leaned. A 160hp at 75% would use 160 X .75 X .45= 56 lb./hr which divided by 6 Lb per US gal. = 9 gallons per hour

Cross,
I am a novice so please correct me if I am wrong. There can be considerable variation in fuel consumption for a given hp depending on leaning and also rpm. Lycoming's Curve # 12883 (Operator's Manual for the I0-320-D) for 75% power shows fuel consumption at about 8.3 g/h leaned to "Best Economy" @ 2,200 rpm rising to about 10.4 g/h leaned to "Best Power" @ 2,700 rpm. Another example; at 65%, fuel consumption ranges from about 7.1 g/h at 2,000 rpm and "Best Economy" to 8 g/h at 2,700 rpm also at "Best Economy" (a good argument for a C/S prop????)

Vomatic,
My 9A with an 0-320 (carb) and a C/S prop at 8,000 ft density altitude gives a TAS of about 173 mph and uses 6.8 g/h at 2,200 rpm, manually leaned to "Best Economy". From Lycoming Curve # 13380 for the 0-320-D (Operator's Manual) this fuel flow gives about 57% power. I am not sure how applicable this % power calculation is to my engine as I have 9:1 compression (standard is 8.5:1) and one LS electronic ignition.:confused:

Fin
9A Flying
 
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Our 360...

......generally figures to 9.6 GPH, David....but then again, since we built this airplane for speed, we use it kinda hard, generally running 65-70% power, near 200 MPH cruise speeds.

Welcome,
 
Forget O-320?

Yes, Dan has been very vocal about the amazing fuel burns he gets on his big engine, while running aggressively lean of peak on very long cross-countries. If that sounds like your kind of flying, then go for it. But just imagine how much lower the fuel burn would have been using the same procedures on a smaller engine. The laws of physics still apply -- 200 hp takes more gas than 160 hp.

On the other hand, if most of your flying is touch-and-go's or local fly-in lunches, where you're either full-throttle a lot or it's just not worth the hassle of LOP, then the O-320 still makes a lot of sense. Especially if you're considering a carb. Especially with fuel over $5/gallon.

My O-320-powered RV-6A with C/S prop used to get around 7 GPH doing touch-and-goes in the pattern, which, honestly, was probably where most of my flying was done. The same thing in the Piper Cherokee 140 I had before (with the same engine) used to cost at least 10 GPH. The difference is that at 2300 fpm, it takes less than 30 seconds of full-power climb to get to pattern altitude, and then with an RV you're nearly idling the rest of the time, to avoid running over the spam-cans. A C/S prop makes a much greater impact on climb performance than horsepower does. The difference between a C/S 160 hp and a C/S 200 hp is probably about 700 fpm, but full-power climb at 200 hp is probably over 20 GPH.

With the local fly-in scenario, you're probably flying with your buddies, so naturally you're balls-out all the way. And you're not going to mess around with LOP when you're yacking on the radio and flying a loose formation for a 20 minute hop. So with 200 hp, you're probably burning 10 GPH impressing your buddies while they're burning just as much (at 90% power) to keep up. The guy in your formation with the O-320 will probably say screw it because he can't keep up with your 200 horses anyway and he'll pull the power back to 50% to save the gas. And he'll still land within 5 minutes of the rest of the group. But he'll have spent half the money to get there.

Don't get me wrong -- I'm just as impressed as the next guy with raw horsepower. But don't buy a big engine because someone told you it won't cost any more to feed. If you're worrying about fuel burn you're probably a guy like me who has to consider how much fun you can afford.

The worst of all worlds is to build a plane that you can't afford to fly.
 
Don't get me wrong -- I'm just as impressed as the next guy with raw horsepower. But don't buy a big engine because someone told you it won't cost any more to feed. If you're worrying about fuel burn you're probably a guy like me who has to consider how much fun you can afford.

To me, it's all about density altitude, rather than top speed. The larger displacement engine is more capable of solving potential problems, or skipping the (climbing) problem altogether; and a C/S prop is still a farther advantage.

If higher altitude mountain area flying isn't in the plans; and the fact that smaller engines still pull RV's around much better than typical Cessna/Pipers.........then a smaller engine is okay.....I guess. :D

L.Adamson
 
I live in Colorado and fly in the mountains a lot. Talk to most RV pilots here and none (Ok maybe one or two) will recommend a 160 HP engine. Maybe it is a guy issue. Others will state that they do fine with 160 HP up here. A C/S prop is also highly desirable.

Fact is (right or wrong) that when someone here looks for an RV to buy (or are wondering about an engine on a construction project) my suggestion is 180 HP or better. So at least here resale value is essentially zero for a 160 HP RV.

So minimum 180 HP engine and if you can...a C/S prop and you will have a great plane. Whether the 200 HP is worth it...I would not let fuel flow be the deciding factor. Use the power when you need it and throttle back other times (plus leaning...plus one electronic ignition).
 
No you're not wrong, leaning has much to do with spicific fuel consumption. The .45Lb/hp I mentioned is approximate and assumes leaned to best power. In my opinion, many carbourated engins can't be leaned LOP, at least at higher power settings. With a higher the compression ratio the fuel burn per hp should be lower. You may have noticed that using my, somewhat crude formula, gives 9 gal/hr which is pretty much in the middle of the range from lycoming's charts. I've found that in real world flying this formula works reasonably well.
Chuck Ross RV4 flying in Vernon BC
 
For my 7A I debated between the 200 & 180hp engines and ended up with a Mattituck TMX IO360, 180hp with dual Lightspeed II ignitions and MT CS prop. My fuel burn is ranging between 7.8-8.2 gph at about 70% running 50deg LOP. I am very happy with that, especially hearing that others are around 9gph with 180hp. This is getting me 168kts IAS consistently at around 6000' and my intersection fairings are still not on.
 
O-320 mountain flying

I live in Colorado and fly in the mountains a lot. Talk to most RV pilots here and none (Ok maybe one or two) will recommend a 160 HP engine. Maybe it is a guy issue. Others will state that they do fine with 160 HP up here. A C/S prop is also highly desirable.

Fact is (right or wrong) that when someone here looks for an RV to buy (or are wondering about an engine on a construction project) my suggestion is 180 HP or better. So at least here resale value is essentially zero for a 160 HP RV.

So minimum 180 HP engine and if you can...a C/S prop and you will have a great plane. Whether the 200 HP is worth it...I would not let fuel flow be the deciding factor. Use the power when you need it and throttle back other times (plus leaning...plus one electronic ignition).

You're saying a plane that climbs at over 1000 fpm at full gross at 5000' is a problem in the mountains? Then how do all those guys with 172's make it up to Leadville to get their picture taken?

The guy I sold my 6A to immediately took off on a trip through the Southwest to the Grand Canyon. When he got back I asked him how it did at all those 7000' airports out there. He response was "Gee, I guess I never really thought about it. I was flying with a guy in an Archer and compared to him it felt like a rocket".

And as far as resale value goes, I should have asked a whole lot more for mine. It sold twice in less than a month for near what I had in it (the first guy had to back out because he wasn't a citizen and couldn't get it registered).

I suspect that when gas starts pushing $7, you're going to see people's attitudes about O-320-powered RV's change in a big way.
 
No you're not wrong, leaning has much to do with specific fuel consumption. The .45Lb/hp I mentioned is approximate and assumes leaned to best power. In my opinion, many carburated engines can't be leaned LOP, at least at higher power settings. With a higher the compression ratio the fuel burn per hp should be lower. You may have noticed that using my, somewhat crude formula, gives 9 gal/hr which is pretty much in the middle of the range from lycoming's charts. I've found that in real world flying this formula works reasonably well.
Chuck Ross RV4 flying in Vernon BC

Did you perhaps mean "best economy"? I believe best power is around .55 lbs/hp/hr, while best economy is around .43 to .45. The term best power is a little misleading - it means the most power for a given amount of air going into the engine (rpm and MP). It does not mean most power for a given amount of fuel - that is best economy (LOP). There is a large difference in fuel flows between best power and 50 LOP, about 30% more. You will fly faster, yes, but not proportionally.
 
150/180/200

I fly a 200 hp CS IO360 RV8, a friend flies a 180 hp fixed pitch IO360 RV4 and another with a 150hp fixed pitch O320 RV6a.

We flew to OSH together and the IO360's were getting 3.5-4 gal/hour less fuel burns at the same speeds. The 180 and 200 horse IO360's got almost the same with the edge going to the 200, probably because of the CS prop.

These fuel burns were confirmed by the flow gauges and fill ups at fuel stops.

The IO360's had capability of 'smoking' the 320 by over 25 knots...adding the benefit of pure entertainment value.
 
If fuel gets to $7 a gallon

GA will be so close to death that it won't matter if you have an O-320 or O-360.

Besides, I can throttle back and get essentially the same fuel burn as an O-320.
 
I fly a 200 hp CS IO360 RV8, a friend flies a 180 hp fixed pitch IO360 RV4 and another with a 150hp fixed pitch O320 RV6a.

We flew to OSH together and the IO360's were getting 3.5-4 gal/hour less fuel burns at the same speeds. The 180 and 200 horse IO360's got almost the same with the edge going to the 200, probably because of the CS prop.

These fuel burns were confirmed by the flow gauges and fill ups at fuel stops.

The IO360's had capability of 'smoking' the 320 by over 25 knots...adding the benefit of pure entertainment value.

Steve,
I am not doubting you but more information is needed such as; What were the mixture settings and rpm for the three aircraft? Did they all have the same type of ignition system. Even with the same engine/prop I would expect the 6A being a side by side and nose wheel to burn more fuel than an 8 or 4??

Fin 9A
 
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Thinking about this topic a bit more. Two identical aircraft except that one has a larger engine, flying side by side would require about the same HP (assumes equal prop efficiency). If both these aircraft have a FP prop then I would expect that the aircraft with the smaller engine will have to turn higher rpm to produce about the same HP as the larger engine. Lycoming's curves show that for a given HP, higher rpm will result in higher fuel flows. So can we say that when comparing aircraft with FP props, the larger engine should be more fuel efficient for the same HP??? However if the two aircraft have C/S props and fly side by side at the same rpm then I would expect the fuel flows to be about the same?? The larger engine would have a fuel consumption advantage at higher HP as it would be able to lean to "Best Economy" up to 75% power while the smaller engine, in an attempt to keep up may exceed 75% power and would have to run a richer mixture.
What does all this mean to me with an 0-320? In the real world, for my cross country flights I have decided I want to cruise at 150 kts TAS at around 8,000 ft as this gives good speed and OK fuel consumption (6.8 g/h). I set my C/S prop at 2,000 to 2,2000 to take advantage of the lower fuel flows at lower rpm and as I am well under 75% power I can lean to "Best Economy". For this type of flying the larger engine would be of little benefit to me as 150 kts TAS is the speed I want to fly at. Even if I had a bigger engine I would generally limit myself to 150 kt cruise so as not to incur lower mpg. So I am happy with the 0-320. However if Vans had allowed the bigger engine I would have seriously considered it it in my 9A.
Comments welcome.

Fin
9A
 
Steve,
I am not doubting you but more information is needed such as; What were the mixture settings and rpm for the three aircraft? Did they all have the same type of ignition system. Even with the same engine/prop I would expect the 6A being a side by side and nose wheel to burn more fuel than an 8 or 4??

Fin 9A

I believe engine configuration and management, including mixture uniformity and ignition and prop make a much bigger difference than which RV it is. I fly next to an RV4, same engine and prop, and I consistently can burn 1 gallon per hour less. Both have dual Lightspeeds, but I'm FI and he carb. I have flown side by side on trips of 1200 n.m. with RV8's, and burned within 1 gallon +/- of the 8's (60 gallons each way at our cruise speeds).

Lots of stuff in the archives on this topic, plus, the "reference standard" articles on Avweb by John Deakin - must reading for understanding engine management.
 
Fin
As I recall I was running around 2300@18 inches 160TAS at 12,000 MSL, not sure about the other o360 but the o320 was at 2500 and close to full throttle.
All mag ignitions and I was leaned to 75ROP and was seeing 6.4gal/hr on the flow meter.

The huge difference in perfomance is climb to altitude, the more powerful engines simply run away from the smaller engines in a dramatic fashion. That performance advantage is well worth a couple gallons of LL to me.
Also even at higher fuel flows with the same percent of power settings I am covering more real estate adding to some efficiency.

I owned a 150 horse RV6 for 600 hours with the 200 horses, more is better.
 
Fin
As I recall I was running around 2300@18 inches 160TAS at 12,000 MSL, not sure about the other o360 but the o320 was at 2500 and close to full throttle.
All mag ignitions and I was leaned to 75ROP and was seeing 6.4gal/hr on the flow meter.

Steve,
Thanks for the extra information. I am not trying to be critical, just trying to understand better. I appreciate this is the result you saw but something does not seem right. You were burning 6.4 g/h and the 0-320 6A was using 3.5 to 4 g/h more (post #18). So the 0-320 was burning about 10 to 10.4 g/h? At 12,000 ft MSL would likely give a density altitude of at least 13,000 to 14,000ft?? The 0-320 at this DA and 2,500 rpm would pushing to make 60% power. The "Best Power" fuel flow for the 0-320 (160 hp) at 60% and 2,500 rpm is about 8.8/9.0 g/h (I think). The mixture must have been very rich for the 0-320 to be using 10 to 10.4 g/h. At 60% power and 2,500 rpm the 0-320 could have been leaned to "Best Economy" and used about 7.5/7.6 g/h. If the real % hp was less than 60% then the 0-320 should have had a fuel flow closer to yours. :confused: :confused:

Fin
9A
 
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Fin, you are right. I remember fueling up after a 2.5 hour leg and the IO360's took within a tenth or two of 20 gallons and the O320 took about 26 gallons.
This was almost a year ago and at this stage of my life, yesterday is a little fuzzy.
According to my calculator the burn difference was likely slightly less than 3 for the entire trip
The fuel burn at 12,000 was more like 1.5-2 gal/hr in favor of the IO360's but down low where we cruised going up to OSH the fuel burn difference was more pronounced due to the larger engines loafing compared to the 320.

I do recall Bryce doing some high altitude mental math figuring how many gallons fuel savings it would take to trade up to a larger engine....but then we be using up that advantage with faster speeds and more fuel flow.

I occasionally fly xcountry with a number of RV's. Next time I will pay closer attention to the precise numbers and report back. There are some Rockets in our group also that were reporting similar fuel burns to the Io360's at cruise.
There is no doubt a efficiency advantage to the larger engines at the same airspeeds.

Fuel efficiency aside, watching my VSI stay pegged at over 2000'/min through 6000MSL is almost priceless.
 
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It really should be easy to resolve this bigger engine gives better fuel consumption debate by comparing book figures. From the Lycoming curves for the IO-320D, fuel consumption at 65% (104 hp), 2000 rpm and leaned to Best Economy is 7.1 g/h.
I would be interested to know what fuel flows the Lycoming curves show for the 200 and 180 hp engines at the same 104 hp, 2000 rpm and leaned to Best Economy. The curve I am looking at for the IO-320 is called, "Part Throttle Fuel Consumption" and shows Fuel Consumption and HP at various rpms and for both Best Power and Best Economy. My understanding is that the 200 hp engine has a higher compression ratio so I would expect it to have better official fuel consumption figures. I am guessing the 180 hp engine with 8.5:1 would be about the same as the 8.5:1 0-320??
How about it? Anyone with the bigger engines willing to take the challenge and gives the official figures? :rolleyes:

Fin
9A
 
I have an RV-7A with a Superior XP-360 with FADEC electronic ignition. I typically fly at economy cruise, 65% power at 154 kts TAS. My fuel burn is about 6.2 gph. Since I don't have a fuel totalizer, I just look at Hobbs start/stop. Extremely efficient.

The key is that the FADEC system constantly leans the engine and optimizes each cyclinder for fuel flow, much like a car.

Paul
 
From 2001

You can tell it was years ago from the fuel cost. The trip had four different RVs from O-320 wood prop through O-360 constant speed. This is from my website.



The two tankfuls where everyone flew together gave a little opportunity to compare the fuel efficiency of the four RVs. The fuel burn order was consistent, but three of the airplanes were very close on fuel usage. The order was, from least burned to most:

180 HP O-360, fixed pitch Sensenich

160 HP O-320, constant speed

160 HP O-320, wood (lightest airplane)

180 HP O-360, constant speed (heaviest airplane by quite a bit)

Hmmm! That inconsistent data probably says that pilot leaning techniques overpower powerplant/propeller choices for efficiency.

For the whole trip, including my solo, fuel hogging legs, I burned 7.4 GPH using actual flight time. The group legs where we flew around 150 to 155 knots, were lower. Total fuel cost was $221.05 for 12.9 hours.




My airplane burned the least fuel and I think that was entirely due to leaning. I have a carb and mags.
 
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