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Permalink Reply by Mark Pensenstadler on October 21, 2020 at 6:58pm

As a follow up, I read this online and it seems to make a good case fro the lower cost and simplicity of a carb!

"You need to do a business case. It's possibly not worth the price. The Bendix fuel injection system used on the Lyc is a fairly crude (by car standards) mechanical constant flow system, and the specific fuel consumption is perhaps 5-10% lower, maybe, at best, .4 lb/hp/hr vs .44 or so for a carb (cars are in the high .3s, diesels in the low .3s). Lets be generous and say the fuel burn is 9 gal/hr instead of 10. At 100 hrs a year that's a $500-550 saving. If you spend 5000 extra for the injected engine, it's 10 years just to break even. Hopefully the fuel injected option is a lot cheaper than that, or you do enough flying to recoup the extra price much faster.

You prime by pressurizing the system with the fuel pump (using the mixture) to inject fuel pretty much where the primer would be on a carbureted engine; so far so good, but it's a somewhat more finicky procedure and easy to flood.

There's a little bit of a pucker factor issue because you have a high pressure fuel distribution system snaking around the engine (yes there are primer lines on a carb engine but they aren't pressurized) so there is a higher fire risk due to a fuel leak (it's the main reason carbureted engines are always updraft - nobody puts the carb on top).

When the engine is hot and sitting on hot day, you can get vapor lock in the distribution manifold and lines, sizzling away like a pan of bacon in the plenum of the cowling, making the engine really hard to start. If you're on floats drifting away from a dock after pushing off and frantically trying to start the dang thing before you drift into the rocks, it becomes a big deal.

My personal opinion is the fuel burn reduction is not worth the costs, extra failure modes, and loss of simplicity and I prefer carburetors, for simple, low speed airplanes at least."

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Permalink Reply by Daniel Niendorff on October 21, 2020 at 7:13pm

I went with fuel injection mainly because of the lack of carb icing potential.  Lower risk, and you can skip all carb heat procedures.  Most, but not all injection systems require return lines.  Mine did not, so easy choice for me.  The basic  system described is one option, but it’s experimental so you can also use the more advanced systems that meter the injectors electronically.  Just depends how far you want to take it.  

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Permalink Reply by Mark Pensenstadler on October 21, 2020 at 7:17pm

Thanks Daniel. Injected engines can still get induction icing in the intake filters I hear, so they require an alternate air source. I wonder if that's any more complex than adding carb heat?

Also I was thinking...when I watch videos from Trent Palmer and others who fly the back country, it seems that a lot of the time they land, stay for 10 minutes, and then they are off to someplace else. I could see me doing that type of exploring and flying (that's why I'm building the SD!) and with multiple (hot) starts, I'm thinking the carb might be the better option?

I'm interested in hearing what everyone says...

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Permalink Reply by Daniel Niendorff on October 21, 2020 at 7:25pm

Induction ice (or other blockage) is still possible,  but the absence of a Venturi pressure drop makes it far less likely.  Alternate air source is designed exactly like carb heat,  but gets used much less often.  

I have electronically controlled injectors, and for me hot starts are exactly the same as regular starts.   They are a non-issue with my setup.  But in the more basic systems I think they are an issue that has to be dealt with.  I can’t say to much on that,.....  doesn’t happen to me.

I hope you find a set that meets your needs.

Dan

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Permalink Reply by Clint L Gosch on October 21, 2020 at 8:57pm

Hi Mark,

The Mike Busch crowd likes injection, seems easier to achieve smooth and consistent lean of peak operation when you can get all your fuel/air mixtures balanced by adjusting the injectors.  I have a carb on mine and really like the simplicity, I just lean to roughness during cruise and enjoy the view. The mixture distribution on my engine doesn't seem to allow for aggressive leaning. Flying 70 hours/year, I can't imagine I would save very much on fuel anyway.  For my type of flying, I'd choose a carb again.

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Permalink Reply by Mark Pensenstadler on October 22, 2020 at 8:46am

Thanks Clint-simplicity is what I'm after I think. Especially since I could be by myself in the back-country. I like the idea of not having to rely on pumps and batteries to make everything work.

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Permalink Reply by John Austin on October 22, 2020 at 8:10am

I have an AeroInjector carb (confusing name - it is a throttle-body carb and NOT fuel injection!) on my Jab 3300 and love the simplicity.  I think for the recreational/experimental flyer, one can never go wrong with the KISS (Keep It Simple, Stupid!) philosophy. (That's also why I like the Jab - air-cooled, direct-drive, put gas and oil in it and it once you start it, it'll keep running even with no battery!)  

I also had years and thousands of hours ownership experience with a IO-360 Continental in a 172XP and a IO-540 Lycoming in a C206H Stationair.  The fuel injection systems were never problematic and hot-starts were easy once you learn how to do it:  Crack the throttle slightly open, pull the mixture to idle/cutoff, crank the engine and then advance the mixture until it starts.  Worked each and every time!  :>)

All that being said, I like my carb, I understand my carb, and I can actually work on it  - which is rarely needed. 

John

N750A

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Permalink Reply by Mark Pensenstadler on October 22, 2020 at 8:49am

Thanks for the insight John. At this point I'm leaning towards the carb. On this airplane I'm after simplicity, lightness, and well, cheapness! It looks like good fuel injection systems are fairly pricy. My Mooney had a carb and I've never had any issues with it. 

I also like the idea of a much simpler fuel system too. 

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Permalink Reply by Mark Pensenstadler on October 22, 2020 at 11:18am

If anyone else is interested, I found this short article written by Stein Bruch on the VAF website. Click on the link and scroll about half way down...

I think I'm sold on the carb.

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Permalink Reply by David Krakowsky on February 3, 2021 at 4:02pm

The simpler it is the less that can go wrong.

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Permalink Reply by Arnie Minear on September 19, 2021 at 2:16pm

I am looking at the fuel injection as my best option.  The system I am getting doesnt have the hot start problem since it only injects on the cylinder that is getting ready to fire.  I believe the recirc line will remove vapor from the hot engine compartment through recirculation by running the pump a little bit before starting.  No carb heat, no mix adjustments, improved performance and eff.

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