FI vs. Carb--O-320 Pros, cons

RScott · #1 03-26-2007, 09:16 PM

I'm building a 9A and some folks with more experience than I have are urging me to go with FI rather than a carb.

I will probably go with an Aerosport engine. FI costs $1,000 more from them. But then there is also the high pressure fuel pump, which I understand runs about $750. And a high pressure filter at some unknown cost. And what else?

How much more is it likely to cost me to go with FI? Will I get it back in resale value? How much fuel will it save? I have heard claims of 1/2 gph and others who should know say a lot less than that. Any other pros, cons other than no carb ice with FI?

Thanks.

Richard Scott
RV-9A Wings almost done, p/u QB fuselage tomorrow!

dan · #2 03-26-2007, 09:58 PM

If you buy your engine from AeroSport Power, and if you go with Airflow Performance fuel injection, you will most likely get the AFP pump & filter kit WITH your engine. At least that's what happened to me when I bought my IO-360 from AeroSport. I had already coughed up the bucks for the AFP pump/filter/install kit from Van's, so when I got the dupe pump/filter from AeroSport Power I just sold it to another builder.

Anyway, something to keep in mind...that the cost of FI will probably include the cost of pump+filter...if you go with AFP injection (which I sure would!).

rv9builder · #3 03-26-2007, 10:19 PM

Quote:

Originally Posted by dan

If you buy your engine from AeroSport Power, and if you go with Airflow Performance fuel injection, you will most likely get the AFP pump & filter kit WITH your engine.

I'm not sure this is still true. On the AeroSport Power web site, under 320/360 Engine Options, the AFP Electric Boost Pump and Filter Assembly is listed as a $520.00 option.

http://www.aerosportpower.com/Prices.htm#Options

petehowell · #4 03-26-2007, 11:13 PM

I built an O-320 from an ECi kit and opted for a carb. It is simple, cheap and I can overhaul it myself. I have dual electronic ignition as well and have done a lot of experimenting with LOP/ROP. I have certain points where my mixture distribution is very even (4 EGT Peaks within .1GPH) and I can run LOP. One is a low power setting 16in/2200 @ 3500 ft where I can fly around on 3.8GPH at 125-130 mph in my RV-9A. The other useful one is a high cruise. WOT/2300 rpm at 8000ft. Here by using carb heat and cracking the throttle, I can balance the EGT peaks to within .1-.2GPH. Here the RV-9A will do 170-175 mph TAS on 5.9-6.1 GPH

For the ultimate in saving gas, FI is the way to go. You can control the fuel flows very easily. The system is more complex, but not overly so.

Pros and cons to both systems. If I build again, it would be a toss up. I'd probably go FI to try something different and to go after better fuel economy.

I do think Elect Ign is the key to frugal engine operations. YMMV

gmcjetpilot · #5 03-26-2007, 11:38 PM

In the past it was more of a toss up, but fuel prices have gone down and FI systems have actually got cheaper relative to Carbs which have not dropped in price, keeping up with inflation. My guess is competition of FI systems between Bendix/Precision, AFP and now ECI keeps price competitive.

Carbs are a single source Marvel Schibler (now Precision Airmotive Corp). Also rumor has it (I think there is some truth to it) the long run of Carbs and lawsuit liability with so many Carbs in service, today's price has to cover and absorb the liability. AFP's and ECI's have not been around or sued enough.

The big selling point of the FI is fuel economy, specifically in cruise. With gas prices that is even more valuable. In the past FI would not pay for its self very fast when gas was cheap, but now saving 0.50 gal an hour or what ever adds up. espcially at +$4.00/gal

HOWEVER, you have to be a clever operator and move that red knob very judicially to get max advantage out of FI. If you are lazy and not attentive to power and mixture optimization (minimization of fuel flow) it does not matter what fuel delivery system you have, you will burn more fuel than your hanger neighbor with a carb, who watches mixture and power like a hawk.

However if you can dial-in the FI (even flow out of injectors), using the FI system to its most advantageous AND you cruise a lot, you can save $$$$. However cost, build time will be higher initially, and even maintenance may be higher initially, but with the fuel prices pay back time will be much shorter. Depending on how you fly that might be 5 years or 20 years?

Don't worry about resale, but FI is thought to be a nice selling point or feature. I think HP and constant speed prop are higher up on the want to have list.

Don't worry what everyone else has. A 150HP, Carb, fixed wood prop RV is a rocket and fun fun fun. Fuel Injection does not make the smile much wider. The economy aspect will take many years of flying to pay for itself in gas, but still its a good thing and gets better with worse fuel prices. With that said if buying a new engine today I'd buy FI just because it CAN meter fuel a little more precisely.

Bottom line if buying a NEW engine get FI, because the price is not much greater. If buying a used engine with a Carb, stick with that. Also don't go broke building you plane. There are things you can add later, like going from fixed to C/S prop. However FI is something you want, you should try to go with from the get go. Converting is expensive and selling a used carb is hit and miss.

dmeloche · #6 03-29-2007, 04:27 PM

I bought my 0-320D1A from Bart and he talked me OUT of FI. It was a while ago but if I remember the gist of the conversation he seemed to think that if you weren't planning to do a lot of flying around upside down he didn't think it was worth it. I wouldn't hestiate to talk to him about it. In my experience he won't try to push an option on you that you don't need.

Having said all that If I did it again I would seriously consider FI due to the increased economy and the ability to obtain even temperatures between cylinders.

Doug
RV6

Steve Sampson · #7 03-30-2007, 02:07 AM

With a clean sheet I was planning FI but then switched to a carb., for a variety of reasons. I found an article by Stein Bruch quite helpful. There is a link to it here.

Scroll down to you come to the Note added 4/10/06. There is a link there.

elippse · #8 03-30-2007, 08:32 PM

With an FI you no longer have the 1.2"-1.5" MAP drop across the venturi, so your manifold pressure at full throttle will be almost static + dynamic. That will give about 3% more power and 1% more speed. Carbs are simpler, FIs more complex! An even charge-distribution isn't a guarantee, since all cylinders may not get the same amount of air. That's why there are the GAMI injectors! I recently had two people try out my original two-blade prop on their planes, one an RV-6 and the other an RV-9A. Nothing outstanding has so far shown up, but one thing that did show up is that the TAS calculated from IAS was 9-11 mph greater than the TAS from GPS. One of them used the pop-rivet static port, and the other had the Cessna flat-disc style. Another RV-9 pilot who was there said that his was doing the same, about 10 mph high on TAS. Here's the way to fix this. Start adding layers of Scotch tape right behind the static hole to increase the static pressure to lower IAS, or add layers in front of the hole to reduce static pressure to raise the IAS. Use the two-way GPS runs into and with the forecast winds to obtain avg. GS/TAS. If you are venting static to the cabin, then IAS-TAS are just whimsical numbers with little association to reality, but if they make you feel good, well, what the H**l!

RV6_flyer · #9 04-01-2007, 01:26 PM

Quote:

Originally Posted by elippse

With an FI you no longer have the 1.2"-1.5" MAP drop across the venturi, so your manifold pressure at full throttle will be almost static + dynamic. That will give about 3% more power and 1% more speed.

Where can I find DATA that supports the above statement? (some or all of it)

If there is hard factual data to support this statement, I will convert to FI.

An increase in MAP would mean more horsepower. Informaiton published by Lycoming shows no difference in horsepower output of a fuel injected engine and one that has a carb.

I have 1,988 hobbs hours on flying O-320 RV-6. I have September / October 2007 scheduled for engine work and can put my carb overhaul money toward FI.

elippse · #10 04-01-2007, 09:19 PM

Quote:

Originally Posted by RV6_flyer

Where can I find DATA that supports the above statement? (some or all of it)

If there is hard factual data to support this statement, I will convert to FI.

An increase in MAP would mean more horsepower. Informaiton published by Lycoming shows no difference in horsepower output of a fuel injected engine and one that has a carb.

I have 1,988 hobbs hours on flying O-320 RV-6. I have September / October 2007 scheduled for engine work and can put my carb overhaul money toward FI.

Lycoming spec. 2213-C, O-320B, curve 11260A, abs. dry manifold pressure, in Hg. at sea level, 2700 rpm, 28.3". Spec 2283-G, O-320D, curve 13381, 28.7". Spec. 2499-G, O235-L, curve 13265-B, 28.4". I know since someone is flying with the prototype EFI I designed and that's what he's experiencing; his MAP is within 0.1"-0.2" of static pressure on run up. Check your MAP at full-throttle run-up against the MAP with the engine off.