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Rising fuel prices: 160 hp or 180 hp?

n567vb

Active Member
I'll be picking an engine soon. I thought it would be interesting to hear people's comments:

If you were building again, considering the rising price of fuel, would you consider going with 160 hp instead of the 180 hp engine, or would the performance downgrade be greater than the fuel savings?

Thanks
Vince
RV-7 QB
 
I don't have any personal experience (yet), but I've heard rumors that for a given power output, a 180 uses less fuel. Of course you need to compare apples to apples, ie. FI vs. carb, lean-of-peak vs. rich-of-peak, same weight, etc.

So, say you run a 160 @ 75% (120hp), that would be 66% for a 180. Maybe someone with personal experience can give you hard numbers.

Clear skies,
 
Power is nice at high density altitudes! You can always throttle back but you can't randomly add 20 hp to your engine for those high density altitude trips.
 
180

if I were doing it over, I wouldn't build. really. not with 200$+ fill-ups. and uncertainty about petrol prices in general, avgas in particular. having said that, I would go with 180hp. at least. . . the initial cost is about the same, and you can simulate 160 hp by reducing the power setting. you can't simulate 180hp with a 160hp engine. . .

flying soon. . . I hope!!
 
False economy

It tkaes the same amount of fuel to make a certain horsepower no matter what engine your using..There are pumping losses if you partially close the throttle on the bigger motor but those are small when comparing ROP vs LOP operations.

FI'D motors are usually more efficient (1 to 1.5 GPH less burn) than carb'd engines, because they can run LOP.

As others have said..You can't get another 20HP when you want it out of the 160HP motor.

Add to that the resale value (assuming they'll be worth anything with rising gas prices), the 320 engined airplanes are worth less.

Well the choice is clear...Use the 360!..If you want fuel economy go fuel injected and an Electronic ignition that will advance the timing at low MP..either a Pmag or a Lightspeed.

The AFP FI is compatible with ethanol blends should we be forced to go that way in the future.

Frank
 
rising fuel

This discussion has me rethinking the RV-9 with an O-235. Incredible economy. I also pulled my Longeze plans out and looked at them again. Interesting times.
Oh, by the way, go with the O-360 but set it up for auto gas. ;-)
DM
 
The debate about whether an 0-360 can be as (or more) efficient than an 0-320 has been going on for years as you will find by searching the threads.

If fuel costs are your big concerns, you might consider going with a low compression version of either the 0-320 or 0-360 so that you would have greater flexibility in your uses of autofuel. For example, you could get a 150hp 0-320 or ~170hp 0-360 that will run on any grade of auto gas. I kind of wish I had gone with a 150hp 0-320, rather than the 160hp 0-320 on my -9, for this reason, but supposedly I can at least run high octane autofuel if I want anyway.
 
Fuel cost (burn rate) is directly related to horsepower demand, not engine size. Engine size simply determines how much horsepower supply you have to meet that demand.

If you're just poking around at sightseeing VFR altitudes, 90-100 horsepower will be available from nearly any engine and your burn will be low. If you need to get somewhere quick or need power to climb to mid-teens and keep moving fast, you're going to need the displacement to do it, and there's very little penalty in having the extra displacement all the time.

Last time I checked, the weight difference between a 320 and a 360 is something like 17 pounds, and the cost difference is less than $1k, but the extra horsepower could save your bacon one day when you really need it. If you are going to run a C/S prop, just leave the throttle wide open and pull the RPM back to the 150-160 hp point and you'll have the same fuel burn as a wide-open 320, within measurement error.
 
Last time I checked, the weight difference between a 320 and a 360 is something like 17 pounds, and the cost difference is less than $1k...

I am in the midst of a conversation with Sue at Aerosport about these issues, and she just told me that the weight difference is about 10lbs for their engines, and the 360 is only $200 more than the 320...these two facts alone are swaying me back to a 360.:cool:
 
I wouldn't base an engine choice on the price of fuel. If you're that on the brink perhaps the better question is can you afford aircraft ownership at all. Fuel prices fluctuate and a drop is not unlikely. Then, won't you feel silly puttering around behind a wimpy 320 when you coulda had a 360 you could always throttle back? The difference in pumping loss is buried in the noise of pilot leaning technique.

BTW, counter to an above post, higher compression yields improved efficiency. Ask the custom engine builders (not a mere engine assembler). 91 mogas works just fine in a 8.5 or 9:1 Lyclone. For similar rationale as displacement, I wouldn't spec a 7:1 when the price delta of 87 vs. 91 is 5%. We fly for fun, not pucker-lipped penny-pinching. Choose performance, you won't regret it.

John Siebold
Boise, ID
 
Cost of fuel....

Ok lets assume that the engines weight the same and that they will burn the same fuel per hour per HP used.:rolleyes:

The 360 will still burn more fuel.:D

Think about it. When you take off are you going to only push the throttle in part way.:eek:
There would be a number time that you would feed more fuel to the 360 then you would to the 320.

If you want cheap operation, go with the smaller engine.

I am not saying that anyone should not go with bigger if they want it, but you will need to pay for it.

Kent
 
Yes but

The 360 will be climbing at max power for less time...Thus it will burn the same fuel as the O320 assuming you don't have a lead foot..I mean hand..:)

Personally if you want real fuel saving go with a 540...you think I jest but I believe the Io540 can be leaned to better fuel specific consumption numbers than the 360...and you won't be climbing for very long at all!

OK the 540 is a bit heavy..but Seriously there is no advantage to a 320 over a 360

Frank
 
320 vs. 360 fuel use

A few considerations:

fuel use has more to do with indicated airspeed and drag than the motor. An o-320 making 120 hp (75%) burns about the same as an O-360 making 120 hp(67%). The plane will be making about the same speed for a given fuel burn, regardless of the motor

All things being equal, the engine with the higher compression ratio will be more efficient (keep mogas/100LL in mind though)

Balanced fuel injection and good instrumentation, properly understood and used, will make a far bigger difference than the 320/360 choice.

An electronic ignition will allow much more aggressive leaning in low-power cruise, and can pay for itself in short order with $5 avgas.

My RV-8 has a 200hp IO-360 with stock 8.7 to 1 pistons, well-balanced fuel injection, and electronic ignition. I flew to OSH a couple of years ago with a buddy who has a very clean, O-360 carb RV-8A. We topped off together at each end, and I burned about 15% less fuel, even though I was flying wing much of the trip.

There has been a great deal of discussion about this in the Cirrus community, where we have SR20s (210 hp TCM IO-360) and SR22s (310 hp TCM IO-550). On trips of up to about 300nm, the Sr22 will typically burn less total fuel, because it climbs much faster and cruises faster. SR22s pulled back to SR20 speeds in cruise have similar fuel burns.

I would prioritize like this:

1 good multicylinder engine monitor
2 fuel injection
3 electronic ignition
4 clean, straight airplane
5 efficient prop
6 most displacement you can afford, after paying for 1-5
 
What if we factor in the HP gains with EI and a tuned exhaust to the 320? Most EI manufacturers claim about a 6% HP gain, and a Vetterman tuned exhaust is supposed to gain another 3-4HP, right?

So, a 160HP 320 now becomes more like 172HP with EI and a tuned exhaust. Will this make a difference? Would probably climb better...then fuel burns would be less on the 320?

I'm not telling, I'm asking...:confused:
 
Sorry Frank....

The 360 will be climbing at max power for less time...Thus it will burn the same fuel as the O320 assuming you don't have a lead foot..I mean hand..:)

Personally if you want real fuel saving go with a 540...you think I jest but I believe the Io540 can be leaned to better fuel specific consumption numbers than the 360...and you won't be climbing for very long at all!

OK the 540 is a bit heavy..but Seriously there is no advantage to a 320 over a 360

Frank

To move X mass (the plane) from MSL 0 to MSL 10000 takes a give amount of energy (HP * time). If you take less time and use more HP, it is the same as using less HP and taking more time. I know that it won't work out if you use 0 time or infinite HP, but within the norms of airplane engines it works out.

In fact climbing faster would work against you because of the increased drag from going faster.

Sorry no free lunch.;)

Kent
 
In fact climbing faster would work against you because of the increased drag from going faster.

Not if both aircraft are climbing at the same indicated air speed, with different vertical speeds.
 
exactly

And beside you can dial in more pitch and climb with less horse power if you want to...then the fuel burn in the climb would be the same anyway.

Cubic displacement is almost irrelevant...Except if you can have more then why not...:)
 
True...

Not if both aircraft are climbing at the same indicated air speed, with different vertical speeds.

I should have said IAS instead of speed.

But still if both plane/engines (320/360) climb at same VSI and the 360 is using more HP it will be climbing at a faster IAS and therefore more drag (less efficient).

If you choose to use the same HP from each engine, they will climb the same.

If you choose to fly at same IAS but with higher VSI on the 360 you will get to alt faster but use more HP and fuel.

frankh said:
And beside you can dial in more pitch and climb with less horse power if you want to...then the fuel burn in the climb would be the same anyway.

If you want, but who with a 360 is going to want to.:p

Kent
 
When you study the LYCOMING power charts, you will find that the 360 uses less fuel per hour per hp produced than the 320. Yes you will burn more fuel when you make more power.

I have been flying a 320 CS for over 10-years and 2,100 hours. I fly with a lot of airplanes that have 360 and they typically will burn less fuel than I do at the same speed. At and below 150 KTAS (2,300 RPM for me) we typically burn the same fuel but at any power setting higher, I burn more fuel.

I have a 360 CORE sitting in my hangar to build up as a replacement. I will do that after I do the 320 to keep flying.

If you look at the TCDS for the 320 and the 360, you will find that there is only 10 pounds difference. They have the same bore but the 360 has a longer stroke. IMHO, the stroked engine makes it more fuel efficent.
 
Our flying club operates an O-320 Skyhawk, an O-360 Archer, and an IO-360 (200 hp) Arrow. A few years ago I added up the fuel burned over a three year period for this fleet (about 2600 hours total), and found that the Archer burned 19% more gas per hour than the Skyhawk on average. Probably part of this is due to pilots running the Skyhawk at lower power on local flights.

The interesting part was that the Arrow burned 5% less than the Archer. This was undoubtedly due in part to a more efficient engine. It may also have been partly because the Arrow has a fuel flow gauge, making it easier to lean precisely.

Regardless of engine choice, I'd recommend investing in a fuel flow transducer. Lets you see right away how much that last 10 knots costs...:)
 
RV9A, Penn Yan Superior 10-360 F.I., M.T. 3 blade constant speed prop, 2 Slick magnetos.
Cost of engine was same as 10-320, a few pounds heavier, and 1 inch wider.
Our airport is at sea level, and full power, 2700 rpm, climb is around 2100 fpm @ 115 mph indicated with about 18.5 gph fuel burn.
As soon as possible I reduce rpm to 2500 and fuel burn drops to around 15.5 gph. I also try to stay at 25" m.p. on roll out, but it's difficult because I think there's some ram air effect as speed increases so m.p. usually increases. Takeoff is so fast, that I'm too busy to play with the manifold pressure setting. I don't reduce manifold pressure until I level off. Haven't really gone that high yet to have to add m.p.
I normally cruise at 23 square and showing a little over 9 gph. Not sure how accurate that is with the Dynon D10.
I'm glad I got the 10-360. Just makes more sense.
Jack
N99552
 
All my friends with 0-360s in their RVs use less gas than I do with my 160 hp 0-320. I think the constant speed propellers they have are the biggest factor. My RV-6 has a fixed pitch propeller and turns around 2800 RPM at 8000' and WOT. Piston strokes per hour are a big factor in fuel consumption.

If I were building, I'd go for an 0-360 and a constant speed propeller. People can banter theories and numbers all they want, but the facts speak for themselves.
 
RV9A, Penn Yan Superior 10-360 F.I., M.T. 3 blade constant speed prop, 2 Slick magnetos.
Cost of engine was same as 10-320, a few pounds heavier, and 1 inch wider.
Our airport is at sea level, and full power, 2700 rpm, climb is around 2100 fpm @ 115 mph indicated with about 18.5 gph fuel burn.
As soon as possible I reduce rpm to 2500 and fuel burn drops to around 15.5 gph. I also try to stay at 25" m.p. on roll out, but it's difficult because I think there's some ram air effect as speed increases so m.p. usually increases. Takeoff is so fast, that I'm too busy to play with the manifold pressure setting. I don't reduce manifold pressure until I level off. Haven't really gone that high yet to have to add m.p.
I normally cruise at 23 square and showing a little over 9 gph. Not sure how accurate that is with the Dynon D10.
I'm glad I got the 10-360. Just makes more sense.
Jack
N99552

9GPH at 23 squared..>That must be ROP right?...What does it do LOP...Should be around 7 to 7.5GPH at 24 squared...Should improve your MPG somewhat..

Frank
 
Somebody elses $.02

I was looking at the ER tanks made by Hotel Whiskey Aviation and sold on the Safeair1 web site. Their claim is that an RV-6 with 150 hp will be able to travel farther than an RV-6 at 180 hp. They don't have them for a -9 yet!

Their quote:

?Customer demand led to the additional capacity provided with the new RV-6 ER Tank design which incorporates a "tee-head" at the outboard end of the fuel tube. Overall capacity of the ER Tank system is approximately 8.8 gallons which adds 170 nautical miles of range to the typical 150 h.p. RV-6. With the ER Tanks, you can effectively improve the overall performance of your plane by eliminating fuel stops during long cross country operations.?

I?m only adding THEIR $.02, I have no experience in this matter but I thought this information pertained to the discussion.
 
9GPH at 23 squared..>That must be ROP right?...What does it do LOP...Should be around 7 to 7.5GPH at 24 squared...Should improve your MPG somewhat..

Frank
Frank
Not really sure if the fuel flow is accurate. Have to do some flow checks and calibrate. Probably ROP and staying on the safe side. Now that phase 1 is done, and after paint is on, will start doing some long flights and will get a better idea.
On my One Design with basically the same engine/prop, and at altitude, I cruise at 21"/2450 rpm, and can see 7.8 gph.
Stay posted
Jack
 
Which to Use 320 or 360?

After reading this column the following becomes apperant. The 360 adds about 15 pounds to the plane. At the SAME power the 360 will burn LESS fuel. Having more power in an emergency can be a plane and LIFE saver. With a CS prop you can set the RPM at a fixed 2700 RPM and use manifold pressure as % power.
So if you can use that gray material between your ears you will cruise at a lower power setting. The engine will last longer at the lower power. A $1000 does not make much of a differance.
 
Our flying club operates an O-320 Skyhawk, an O-360 Archer, and an IO-360 (200 hp) Arrow. A few years ago I added up the fuel burned over a three year period for this fleet (about 2600 hours total), and found that the Archer burned 19% more gas per hour than the Skyhawk on average. Probably part of this is due to pilots running the Skyhawk at lower power on local flights.

The interesting part was that the Arrow burned 5% less than the Archer. This was undoubtedly due in part to a more efficient engine. It may also have been partly because the Arrow has a fuel flow gauge, making it easier to lean precisely.

Regardless of engine choice, I'd recommend investing in a fuel flow transducer. Lets you see right away how much that last 10 knots costs...:)

Alan,
Your calculations do not take into account the differing cruise speeds of these 3 aircraft. Think MPG, not gallons per hour.
Charlie Kuss
 
Alan,
Your calculations do not take into account the differing cruise speeds of these 3 aircraft. Think MPG, not gallons per hour.
Charlie Kuss

No argument. However, the MPG numbers would be proportional if all three engines were installed in the same kind of airplane.

I've got an O-360/CS and am happy with this choice.
 
As someone who put in a small engine (135 HP O-290-D2) I have chuckle whenever I ready these threads.

First off, there is simply no replacement for displacement. Bigger is better, yatta yatta yatta.

Here is something no one has mentioned yet, weight. From what I've found on the web, the O-235 is 240 lbs, O-290 is 263 lbs, O-320 is 272 lbs, the O-360 is 282 lbs, IO-360 is 320 lbs.

Depending on the prop, you can add an bunch of weight, or not. I have heard a constant speed prop can come in at 80 lbs by the time you finish with the controller, plumbing, cable, etc. On the other side, my Catto prop is all of 9 lbs.

Why am I saying all this? Because weight is the enemy! Build a light plane and it will climb close to the same rate as an overweight plane. (1,800 FPM for my -9 w/ full fuel and two aboard.) That weight also decreases your fuel efficiency.

The extra weight also cuts into your useful load. My -9 came in at 990 lbs empty with a useful load of 760 lbs. I just can't put that much fuel, people, and stuff in the thing.

Regarding climb performance, how much climb performance do you "need"? Sure going up at 2100 FPM is great but even an O-235 power RV-9 goes up faster than a 172 and cruises faster too.

The truth is, not many people pull back the throttle to save gas when taking a trip. Sure my plane is slower but fuel stop to fuel stop, I have found that the other RV's are just clearing the pumps when I roll up.

Another truth is, I wonder what 160 HP in a light -9 would feel like.

Oh, and a light plane flies better than a heavy one. Just remember, build the plane you want, not the plane others want you to build.
 
Something that is missing is max range (no wind) is at L/D max. Much slower than most would be willing to fly. As such a RV would (unless very high) be flying at partial throttle (different MP for different displacements).

All things equal except displacement and MP due to partial throttle, I would not expect the BSFC to be exactly the same for an O-290, O-320, O-360, O-390, etc.
 
Really?

I never flown partial throttle in my IO360 yet..Once I get high enough to run 24" mp there is no reason to run partial throttle..Thats inefficient due to the pumping losses...Power is limited with the mixture and RPM knob (assuming C/S)

I severly doubt my Hartz C/S prop comes in at 80 lbs either.

By the list provided above the 360 is 19 lbs heavier than the o290...If you both have the same prop and you both pay the going rate for the engine I would suggest there is hardly any difference....except resaleability and you can get out of short and high strips.

Again why would't you go for the 360?

Now sure, if you get a screaming deal on a small motor then it will fly way better than any spam can.

But the idea your going to save a ton of money on gas is really not substantiated I think.
Frank
 
Exactly why I have an O-290-D2 on the front of my -9.

I just bought an O-320-E2A for $2000... couldn't pass that one up. I have no idea what I'm going to do with it yet...maybe the next rocket will be really light with a wood prop...
 
Do you know of one?

Having more power in an emergency can be a plane and LIFE saver.

Pilots are miss judging conditions and flying when and where they shouldn't all the time, but I don't think that I have ever heard of and RV that got into trouble over lack of power.

Do you know of any instances where a RV could have been kept out of trouble if they only had 12% more power?

I think knowing the theory we can easily be misled into forming a requirement.

Kent
 
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