Scrapman1959

Fuel Injection or Carbs on my next CT?

66 posts in this topic

Since I don't have any first hand experience with the iS engine I have some questions for someone that has one.

 Ok, I get that carbs are arcane and require balancing, servicing, overhaul etc and that the floats have been an issue lately. That said, I'm told you have to buy a diagnostic tool for about $1000 (?) in order to download warnings and error messages etc? This has to be at least as complex as fiddling with carbs so where is the advantage?

The ignition modules seem to require attention as regards cooling ?

I get that FI provides better economics with fuel burn but apart from that how do you consider it to be 'a more efficient engine"?

Looking at the performance figures comparing the 912 ULS versus the iS engine I figure overall the iS is more efficient by about 1 gph. If I could magically put the iS into my RV-12 I'd get about 45-55 minutes more range based on power setting with my 19.8 capacity tank. Today I flew on a hot day (95F) here in SC with a DA of 2700'. I had to climb to 6500-7500' to find cooler and smoother air. Once there I flew 2.4 hours at 5000 rpm as I'm trying to fiddle with k-factor to get a more accurate fuel flow on the SkyView. Hey I'm retired Ive got time to burn too!

  Result was 4.5 mph in cruise with 112 KTAS. The tank takes 19.8 and I probably can squeeze a bit more in so say its 20.0. Back at hangar I loaded 11 gals and its completely full. My hangar neighbor's newly built RV-12 gets about 4.4 mph at his preferred 5000 rpm. I get 4.8gph at my preferred 5300 rpm and plan on 5 mph for about 3hrs 40 min range leaving a 4 gallon reserve. There isn't really unuseable fuel in RV-12 but there is a caution not to take off, climb steeply or go-around with less than 4 galls so I use that as my personal reserve.

The iS is more expensive and heavier so there's a cost penalty as well as an empty weight penalty. In return I'm told its a much more efficient and reliable engine hence my questions. I figure it would take a lot of flight time to break even on reduced fuel burn to offset the several thousand $$ extra acquisition cost. Presumably the extra complexity of the diagnostics, the tool required adds to maintenance cost too?

Vans is now offering the iS engine option to their RV-12 kits as well as the SLSA (which I have, ULS version). They've had to modify the cowl for better iS engine cooling.

 So...genuine question...what exactly does the iS engine offer me IF I was ever to consider trading up the RV-12 to the iS powered version? Not actually considering it but mulling over the pros and cons.

Thanks for any input!

 

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18 minutes ago, Top Cat said:

Since I don't have any first hand experience with the iS engine I have some questions for someone that has one.

Ok, I get that carbs are arcane and require balancing, servicing, overhaul etc and that the floats have been an issue lately. That said, I'm told you have to buy a diagnostic tool for about $1000 (?) in order to download warnings and error messages etc? This has to be at least as complex as fiddling with carbs so where is the advantage?

The ignition modules seem to require attention as regards cooling ?

I get that FI provides better economics with fuel burn but apart from that how do you consider it to be 'a more efficient engine"?

Looking at the performance figures comparing the 912 ULS versus the iS engine I figure overall the iS is more efficient by about 1 gph. If I could magically put the iS into my RV-12 I'd get about 45-55 minutes more range based on power setting with my 19.8 capacity tank. Today I flew on a hot day (95F) here in SC with a DA of 2700'. I had to climb to 6500-7500' to find cooler and smoother air. Once there I flew 2.4 hours at 5000 rpm as I'm trying to fiddle with k-factor to get a more accurate fuel flow on the SkyView. Hey I'm retired Ive got time to burn too!

  Result was 4.5 mph in cruise with 112 KTAS. The tank takes 19.8 and I probably can squeeze a bit more in so say its 20.0. Back at hangar I loaded 11 gals and its completely full. My hangar neighbor's newly built RV-12 gets about 4.4 mph at his preferred 5000 rpm. I get 4.8gph at my preferred 5300 rpm and plan on 5 mph for about 3hrs 40 min range leaving a 4 gallon reserve. There isn't really unuseable fuel in RV-12 but there is a caution not to take off, climb steeply or go-around with less than 4 galls so I use that as my personal reserve.

The iS is more expensive and heavier so there's a cost penalty as well as an empty weight penalty. In return I'm told its a much more efficient and reliable engine hence my questions. I figure it would take a lot of flight time to break even on reduced fuel burn to offset the several thousand $$ extra acquisition cost. Presumably the extra complexity of the diagnostics, the tool required adds to maintenance cost too?

Vans is now offering the iS engine option to their RV-12 kits as well as the SLSA (which I have, ULS version). They've had to modify the cowl for better iS engine cooling.

 So...genuine question...what exactly does the iS engine offer me IF I was ever to consider trading up the RV-12 to the iS powered version? Not actually considering it but mulling over the pros and cons.

For the same reasons you drive a car that's fuel injected . . . rather than a car that's carbureted.

Trouble-free operation and better fuel economy.

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At $1000 the diagnostic cable cost per hour is only 50 cents per hour based on a 2000 hour TBO. If you pay a shop rate that is more than $50 per hour, and spend 1 hour of maintenance time on carbs every 100 hours it is a break even. I think both the $5o per hour shop rate and the estimate of only 1 hour maintenance per 100 hours of operation are both on the light side.

 

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1 hour ago, WmInce said:

For the same reasons you drive a car that's fuel injected . . . rather than a car that's carbureted.

Trouble-free operation and better fuel economy.

Bill

I have a trouble free operation NOW with the carbureted engine and pretty good fuel economy at 4.4 to 4.8 gph on E10 93 octane which is about $2.36 per gallon here in SC with my grocery store discount card.

How is the economy improved if I have to spend several thousand $$$ to acquire the engine and fly many hours over several years of typical sport flying before I break even?

Just saying!

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13 minutes ago, Tom Baker said:

At $1000 the diagnostic cable cost per hour is only 50 cents per hour based on a 2000 hour TBO. If you pay a shop rate that is more than $50 per hour, and spend 1 hour of maintenance time on carbs every 100 hours it is a break even. I think both the $5o per hour shop rate and the estimate of only 1 hour maintenance per 100 hours of operation are both on the light side.

 

Thanks Tom. My Lockwood trained Rotax mechanic charges $57 per hour. He just did the annual, inspected carbs and balanced them as part of his $330 fee for the annual.

At 200 hours the carbs were overhauled but the cost wasn't that much as I remember. He is trained on the iS engine but hasn't seen too many. He says they're more time consuming, in his experience, and therefore labor costs would be more. I'm not sure, hence the questions.

I'm not saying FI isn't the preferred way to go as Rotax refines its engines. I guess im evaluating the additional option of the iS engine from Vans ( apologies to FD folks but the emphasis is on the engine rather than the airframe in my questions). In the case of the RV-12 the empty weight increases, the cost increases, the complexity increases somewhat, but the fuel efficiency increases. With a slightly increased fuel capacity ( 20.2 versus 19.8 gallons) so the range is increased by maybe an hour.

So is it worth it?

Is the iS engine more reliable?

Appreciate the thoughtful input guys.

 

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1 hour ago, Top Cat said:

Bill

I have a trouble free operation NOW with the carbureted engine and pretty good fuel economy at 4.4 to 4.8 gph on E10 93 octane which is about $2.36 per gallon here in SC with my grocery store discount card.

How is the economy improved if I have to spend several thousand $$$ to acquire the engine and fly many hours over several years of typical sport flying before I break even?

Just saying!

Point well taken . . . that's why I fly a CTSW. I have never had any carb issues yet.

That withstanding, there are carburetor issues splattered all over this forum. Just do a search.  

Regarding fuel economy, it is 4.5/gph vs. about 3.8/gph. In an RV-12 (20 gal tank) that equates to about 50 more minutes of fuel endurance. Go figure.

There are some Rotax 912is operators who claim even better fuel economy than that.

Just sayin'.:)

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12 minutes ago, WmInce said:

Point well taken . . . that's why I fly a CTSW. I have never had any carb issues yet.

That withstanding, there are carburetor issues splattered all over this forum. Just do a search.  

Regarding fuel economy, it is 4.5/gph vs. about 3.8/gph. In an RV-12 (20 gal tank) that equates to about 50 more minutes of fuel endurance. Go figure.

There are some Rotax 912is operators who claim even better fuel economy than that.

Just sayin'.:)

Thanks and agree! 

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2 hours ago, Top Cat said:

Bill

I have a trouble free operation NOW with the carbureted engine and pretty good fuel economy at 4.4 to 4.8 gph on E10 93 octane which is about $2.36 per gallon here in SC with my grocery store discount card.

How is the economy improved if I have to spend several thousand $$$ to acquire the engine and fly many hours over several years of typical sport flying before I break even?

Just saying!

This is exactly how I've been looking at this.  There are certainly some advantages, but they don't seem to be worth the additional cost and weight to me.

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Not sure the economy is any better on the FI engine versus the carbs, unless you pull power back from your standard cruise power? I have been contemplating the Tecnam P2008 with 914 and they tell me their is no difference in fuel economy on the 914 at 35" s of manifold wether it's carbed  or FI. You will only see a better economy in lower manifold pressure settings. This may not be true for a 912, which I have no knowledge of carparisons. I was also told the bings carbs only compensate to 3500 ft msl, so when you climb above that altitude, you are using the same amount of fuel per stroke, even though its way to rich for the available air. Maybe you high altitude guys out west can chime in and educate us on that statement? Are you guys using different needles in your carbs to offset the lack of air? I remember having my CTLS at 13,000 over the sierra Madres in Mexico and it had no power but was still burning 5 gph at WOT and only making 5000 rpms, with it set at 5500 at sea level. 

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28 minutes ago, Scrapman1959 said:

Not sure the economy is any better on the FI engine versus the carbs, unless you pull power back from your standard cruise power? I have been contemplating the Tecnam P2008 with 914 and they tell me their is no difference in fuel economy on the 914 at 35" s of manifold wether it's carbed  or FI. You will only see a better economy in lower manifold pressure settings. This may not be true for a 912, which I have no knowledge of carparisons. I was also told the bings carbs only compensate to 3500 ft msl, so when you climb above that altitude, you are using the same amount of fuel per stroke, even though its way to rich for the available air. Maybe you high altitude guys out west can chime in and educate us on that statement? Are you guys using different needles in your carbs to offset the lack of air? I remember having my CTLS at 13,000 over the sierra Madres in Mexico and it had no power but was still burning 5 gph at WOT and only making 5000 rpms, with it set at 5500 at sea level. 

I routinely fly at 13,000' and my opinion is that the bing carb leaning does work poorly and that I run rich at altitude.  The economy is good enough to live with it I guess.

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Ed, u really need a 914. The 914 can make great power at 13K. What field elevation do u set your prop at so it runs 5500/5600 WOT? You must be much higher than us flatlanders, or you would be hurting for RPMs up there. 

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That would make a huge difference at 13K! What happens when u go down near sealevel? Or even to say, 4000? What is the performance like if you change nothing and hen decend to much lower altitudes?

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3 minutes ago, Scrapman1959 said:

That would make a huge difference at 13K! What happens when u go down near sealevel? Or even to say, 4000? What is the performance like if you change nothing and hen decend to much lower altitudes?

In my opinion I don't suffer at all at lower altitudes.  For the most part I'm descending in or climbing out and the flat pitch helps not hurts on the climb out.

Cruising at low altitudes is mostly done at lower power settings (CT pilots most often refer to cruise power settings as 5,200RPM. That may not be a preceise power setting but we know its a retarded throttle and partial power) prolly cause of noise level.  

I always keep up just fine.

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I wouldn't want the fuel injected motor. There's less than 500 out there. ULS is 50,000. Assuming I'm comfortable with an engine that's got a couple hundred hours, (I am) I can do a full swap for under 10k. Lots of mechanics out there that are comfortable working on them. Cheaper replacement parts too. 

Also interesting to see people say the fact that there's lots of carb related threads on this forum. Oddly that's an asset not a liability. The "issues" which EVERY ENGINE WILL HAVE are well quantified, tracked, discussed and thus easily identified and remedied in the ULS. The fuel injected motors aren't that well known, and their "issues" are still being discovered. There might not be a thread that's all about the problem you are facing. You might be the first. "Better the devil you know than the devil you don't" 

i also really liked the comment about the fuel efficiency benefit being not so relevant when we consider we are already at 5 gal/hr with $2.90/gal pump gas. I think they call that the marginal benefit curve. I guess I don't think the marginal benefit is worth the marginal cost either. It's already so dang cheap.....

Then again maybe my bias is because I just can't afford one! 

On another note I just passed 130 CTSW hours in 9 months of ownership. Runs like a top and I'm loving every minute!

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13 hours ago, Ed Cesnalis said:

In my opinion I don't suffer at all at lower altitudes.  For the most part I'm descending in or climbing out and the flat pitch helps not hurts on the climb out.

Cruising at low altitudes is mostly done at lower power settings (CT pilots most often refer to cruise power settings as 5,200RPM. That may not be a preceise power setting but we know its a retarded throttle and partial power) prolly cause of noise level.  

I always keep up just fine.

I agree.  These engines seem to benefit by being on the flat side more than the coarse side.  As a flat-lander, I have one of the flatter-pitched props out there.  My takeoff RPM is 5100-5200rpm, which is pretty high, and my WOT rpm at ~3000ft MSL is about 5750-5800.  As I have stated several times, I see TAS in the 125kt range at 5300rpm.  In good conditions I climb 1000fpm+, and have seen 1250fpm in the winter.  And I like that if I fly somewhere with higher elevations, like the flight out to Page, I don't feel like I'm over-propped for the conditions.    I had no trouble climbing up to 11,500ft over the Grand Canyon.

I do have the standard, non-tundra gear and that is probably worth a few knots, but I'm guessing the prop pitch is the primary reason for my good performance. 

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