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Correct me if I'm wrong, but the Titan X340 engine is about 100 lbs heavier and burns about twice as much fuel as a 914. At sea level, you get some payoff for the weight and the fuel burn by having a lot more punch. But if high-altitude ops are what you're after, the turbo seems the more efficient route from a weight and fuel perspective.
Nyrikki
I am not ready for w/b but I know it will need a little weight in the back. I’ve added a few pounds in tailwheel and shock spring and a 3/8” steel plate for a battery hold down. Making a cowl and hope to shed a few pounds there as well.
In a few weeks I should have a w/b answer
Yeah its heavier and burns more gas. For what it’s worth, I don’t see anyone spending 6 hours in a Kitfox seat...
Just to be clear, I'm not criticizing jmodguy's engine choice. His build is beautiful and the engine/cowl look mean. I'm just saying that at high elevation the tradeoffs are different, and probably favor the turbo.Quote:
Originally Posted by jmodguy
I can pretend that I have some rational reason like the 914's non-linear power delivery in the boost range or that it's fuel burn is also higher in the takeoff power range (~9gph) but I'll just be honest that I prefer the sound of a slow turning aircraft engine :)Quote:
Originally Posted by aviator79
If my primary mission altitude was lower a 912uls or is sport would be fine...but because of the sound and lie I tell myself about the costs of the 5 year rubber replacement on the rotax justifies 100# of increase weight.
Outside of the increased induced drag from the extra weight the fuel burn at cruise may not be that large under typical slow and low flying.
The x340 burns 5.5 gph at 75%, but I would be running slower than that as my friends will be in super cubs.
Please correct my assumptions if they are wrong but here is a poster with a O-290D 2200RPM for 118mph @ 5.1gph
At the high 90's speed range, assuming a 4700 rpm for a 914 at that speed (which is very approximate and shouldn't be used for any planing) it appears to be burning about 4.5 GPH based on the rotax spec sheet.
So my assumption, which is biased and most likely inaccurate with a lower cruise speed, EFI and CDI I should be somewhere within 10 to 15% of the fuel burn between the two engines at those slower speeds. This is still significant though as 15% of a conservative 20 gallons of usable after takeoff and climb is ~3 US gallons.
I am sure there is complex variables like flame front speed and other considerations that my wag isn't capturing and obviously there are losses from fuel vaporization from air cooled heads on the titan but the 914 is also lacks an intercooler which will also have an impact that may make that a wash. CDI, electronic timing and mixture control will have a big boost on efficiency too while also increasing weight due to high pressure pumps return lines etc..
It is all mission based and I have more data to collect before I decide but outside of the extra weight and resulting drag it seems to be a wash if you consider I will probably just equip less gear and reduce my baggage but fly both engines at just below max legal take off weight.
If anyone does have reliable numbers especially with the FlyEFII systems it would be appreciated as I am sure confirmation bias and my desire to get that direct drive sound are coloring my assumptions but a ~2100 rpm cruise seems like it wouldn't be a massive change in consumption but with the benefit of a massive increase in reserve power.
Very informed and honest assesment.
OK I found more data in my docs and wanted to ask for more input.
The 914 at 75 % continuous performance is 5.4 gph and continuous HP is ~98.5 HP and 75 % ~74 HP. Rated at 80 HP continuous the x340 burns 5.5gph at 75%.
From the Rotax 914 user manual PDF
While the docs are harder to find now that conti owns the titan brand the earlier EFI/CDI claim was 5gph at 80hp but 5.5 seems more typical from guesses from the carbon cub users and some loose math.
Obviously not having a continuous prop and the induced drag from the extra weight will impact this significantly but the burn rate is pretty close at lower power ratios may actually slightly favor the 340 at lower airspeeds. Purely because of timing + efi.
Feedback would be appreciated as my soon to be home airport is 7142' MSL and the 914 is only normalized for takeoff to 8000' I am concerned that some work will be in the less than linear part of the power band as the ECU will be limiting boost on fairly typical days DA and obviously I will be past that value at pattern altitude.
Hopefully someone will talk some sense into me to shoot for that useful load, but it is unclear if that 8000' limit is just a limitation of the turbo and power drops in a predictable fashion or if the ECU is limited on pressure differential and power could drop by a large fraction when the waist gate is opened.
I can deal with the implications of DA but with the reported non-linearity of the boosted power band of the 914 and the fact that I will hit their normalized limit when getting to pattern altitude even under standard conditions it is a bit concerning.
I may be overthinking this as the SS7 with the 912ULS is set to a service ceiling of 16K which I assume is when climb drops below 300fpm and the 914 ceiling has been reported at 25,000' so it is really concerns about an abrupt change in behavior by the waste gate being opened.
To be honest the 912ULS may be fine but without hard climb numbers at around 10K DA I am planning on erroring on the expensive/heavy side.
Greg - sorry to be picking on you today. The o-290 at 5.1 gph at 2200 is very believable but I doubt you could get anywhere near that with the x340 at 75% which you suggest is 5.5 gph. Would love to see data on that.Quote:
The x340 burns 5.5 gph at 75%, but I would be running slower than that as my friends will be in super cubs.
Please correct my assumptions if they are wrong but here is a poster with a O-290D 2200RPM for 118mph @ 5.1gph
A bone stock 340 (not Titan as they have not released HP curves that I know of) normally aspirated, magnetos, fixed pitch, specs at .46 lb/hr at BEST economy which is at 125 bhp which equals 9.58 gph. I know the Titan is better and especially if injected and with electronic ignition and a constant speed prop - but 5.5 gph would be a fantastic accomplishment at 75% power.
YMMV, Greg
Ha ha! No I dont think you are criticizing. I’ve been told I can come across a bit gruff 😬Quote:
Originally Posted by aviator79
Turbo 340??? Hmmm 🤔
And thanks tor the kind words
Yep, the other was my mistake and don't feel bad about it. I am far happier to find I am in error and personally don't consider myself omnipotent.Quote:
Originally Posted by Danzer1
The problem is that engine manufacturers play tricks and the consumption on the rotax is not based on peak but on continuous rating. In the case of an engine like the x340 you need to think about what it's cruise output is and not think 85% of the 180hp but that it has a large amount of reserve power and a lower continuous in this application.
To avoid fractions or decimal points I'm going to mix units of measure here but a general wag number you can use on traditional air cooled, magitoed engines is that they will burn 27 litres of gas an hour to produce 100hp.
If you look at the spec sheet for the carbon cub you will see they claim 5-6 GPH at 80hp on the carb version of their engine.
http://cubcrafters.com/carboncub/ss
If you look at the 912 user manual
And look at page 5-6 you will see the max continuous is 90HP and 10 in reserve. Their 75% cruise is at 71.84 HP and not 75 HP.
If you look at a similar 80HP output from cubcrafters they are burning about 6 gallons per hour too looking based on chart from the previous page on the rotax doc.
The rotax can be lighter because it spins faster but that gear train also has losses too.
Historically simple aircraft engines have timing and mixture problems that dramatically reduce their efficiency at lower RPMs and power levels and even with CDI and EFI you do lose some efficacy but it just doesn't change the fact that there are ~115,000 BTU/gal of gasoline.
The power to weight ratio of the Rotax is far better than a Lycoming based engine but what is actually delivered per volume of fuel is incremental and as I mentioned gear reduction is not zero cost. Do to the increased weight of the x340 you will typically have a higher angle of attack which will impact things slightly but when loaded to similar weights and CG's it should be expected that a 912 and a x340 would burn similar amounts of fuel at the same airspeed.
Don't make the mistake of assuming 75% of 180hp when you will be using less than half that number for any cruise speed where range is important. You have to consider similar airspeeds to compair burn rates especially in draggy airplanes like a cub or a kitfox as the energy required to go faster is exponential in nature.
Full liquid cooling and variable valve timing is probably what would be required to gain a significant amount of efficacy past EFI and electronic ignition as fuel vaporization is a large limiter on efficiency of an air cooled engine.
Greg, I hear what you are saying but you stated:So at what rpm and HP is that 75% at - on an x340?Quote:
The x340 burns 5.5 gph at 75%
Per your own equation:which equals 7.13 GPH, so I can only conclude that to be at 5.5 GPH on a x340, the HP must be somewhere less than 100 which would certainly be less than 75%.Quote:
they will burn 27 litres of gas an hour to produce 100hp
Further the Cubcrafters data states "at optimal conditions" but nowhere states what those are or any percntage power associated. But knowing 320's and 360's (340 being in between) 5.5 GPH would likely be at 50 to 55% at best.
Not arguing if 5.5 gph can be achieved, but not anywhere near 75% as stated, no matter how you slice it! IMHO
Greg