Pay back in two to never years
Steve, Not to muddy the water, but I think many carb engines can be run LOP if they have dual EI with advance. It requires some futzing, but that becomes second nature after you do it for a bit.
OK I'll bite.
What does the EI w/ advance have to do with it?
What kind of futzing did you do to make it work?
Not doubting just asking. I do think its possible for some with Carbs to run LOP and for some, never possible. Individual and normal variation from cyl to cyl can cause this inability to run LOP. Ignition in my opinion is not a player or a very minor one, its all fuel and volumetric issues. A carb just blows a wet fuel mix in a hole and you hope the cylinders suck-up the needed fuel/air mix to run LOP, all about the same time the other cylinders are running LOP. I don't think ignition has much say so and you really can't vary the ignition to "tune" for LOP. Balanced cylinders makes an engine more accepting of LOP ops, which is mechanical and hard to change. I also say LOP is WAY OVER BLOWN. You can run LAMAYCG (lean as much as you can get) just fine and save lots of fuel. FI has the ability to compensate for unbalanced cylinders may not even balanced flow but imbalanced flow to compensate.
One Gent whose post I read on VAF could not get his engine to run LOP before a rebuild to save his life, but he could run LOP after he had the engine rebuilt. He had his cylinders custom flow balanced. If you think about it, a Lyc is like 4 separate one cylinder engines that share a crank an induction plenum. Each jug can vary. Unless you can "tune" your fuel flow to each cylinder's individual needs accurately, you will not run LOP with a carb. It's not about even fuel distribution but individual tuning to each cylinders needs. However if you balance the cylinders on the bench, that can be half the battle. FI has more ability to customize and tune fuel and compensate for variations between cylinders, whether they are flowed or not.
Also LOP has taken on mythical importance. LOP has been around forever. Lyc says lean to rough and than enrichen to regain smooth ops. What do you think that is? Well it may not be LOP but peak just slightly ROP. The difference in LOP and PEAK is not that much fuel savings. LOP loses power at an ever increasing rate, meaning slower airspeed. You really want to SAVE fuel? Pull the black knob back and fly slower; fly closer to long range L/D speed. I know Avweb is persuasive with their gospel, but my gosh, they make it sound like they discovered the holy grail of aviation.
How do you tune a Carb and induction of LOP?
So you fly and see one cylinder is rich, one is super lean and stumbling and the other two are about the same and in between. WHAT TO DO?
There should be some way to bias the fuel to/away from individual cylinders even with a Carb, by futzing with the plenum or induction runners? Any IDEAS? With the C-182 you can move the carb heat around and play with the fuel flow and get LOP ops. Of course LOP with hot air is less power. Again just pull the throttle back and fly slower with "LAMAYCG".
LOP 1 to 2 gal/hour less? No
As far as fuel savings
I'm not buying 1 to 2 Gal and hour, sorry AlexPeterson. May be in a IO540 you might get +1 gal less per hour. The real savings is about 0.5 or 0.75 gal per hour at best. Lets not forget you can LEAN with a Carb right up to as lean as you can get for smooth operations.
The difference between LOP and LAMAYCG - LEAN AS MUCH AS YOU CAN GET is not 1 to 2 gal hour. May be lean from full rich to LOP is 1 to 2 gal an hour. Also what people don't count on with LOP is lower power = slower airspeed, so you add TIME to your flight and more fuel burn.
There's no free lunch, but you can LEAN and fly economically with out LOP. Avweb has made this a big deal, however this is OLD NEWS. If you are below 75% power and all temps are in the green, you can fly at PEAK, per the Lyc manual. All the happy talk about HOW WOUNDERFUL LOP is and how COOL your engine will run is great but semi-trivial; the reason its cool is because you're LOSING POWER. All this techno talk about combustion pressure is fascinating but you want to lower your combustion pressures and FF, throttle back, lean the heck out of it as much as you can get and FLY at slower air speeds. As far as efficiency, LOP is goodness IF YOU CAN GET IT. I like to date supermodels and own a winning Westminster purebred show dog, but you see what my dog looks like. You should see my girlfriend looks like. You might as well get a constant speed prop and drop the nose gear if you are into efficiency, both of which have impact on efficiency as much or more than FI.
If you want to save fuel, Carb or FI, throttle way-way-way-back and fly at say 120 mph TAS, and lean as much as you can for smooth ops (provide a tail wind, flying faster with a headwind is better right). PER LYC: Lean to rough and than enrichen to regain smooth ops. That is anything from LOP to near peak. No engine monitor is required. So this LOP stuff is NOT NEW, just over hyped. It's still important and its cool to balance your FI's injectors, no doubt. If you have FI its almost a free lunch, why not do it. But it should not be the only reason to go with FI. There are other reasons to go FI today.
PAY BACK MATH?
As was said too many variables; still the math is easy. Now how much do you fly fully leaned in the magical LOP zone? Well mostly in cruise. Your ability to flight-plan trips, pick altitudes, throttle back and lean aggressively save's as much or more than FI can. If you're lazy and fly WOT at full-rich, it doesn't matter what's blowing fuel into your engine. From my estimate, assuming $5/gal, 150 hours a year, 50% cruise w/ lean-O-peak (0.75 gal less than peak) is about $280 a year, less if you just putz around local doing touch-n-goes. Assume 100% of your 150 hours/yr is LOP ops, than $560/year. So it could pay for itself in who knows? I don't think you need an Excel spread sheet to calculate pay back. In theory it will pay back at some point. If gas goes to $10/gal, YEA think of the money you will save! (joke, sad but true).
MY reason I'm suggesting FI today
The temporary (I hope) suspension of Carb production by Precision Airmotive Inc. throws a monkey wrench in the works. If I was buying today I'd go FI, even though I love Carbs. Some people are afraid of FI because its an unknown to them, like tail-draggers or constant speed props. FI set-up is a little more involved and there are operational "procedures" or techniques with FI, but they're easily understood & learned. I'd get FI if buying today, simply because its in production from three different companies, and the one manufacture of Carbs has stopped making parts because of product liability insurance cost. Yes, Kelly Aerospace makes parts, but I don't think they can make all new assemblies outright, as far as I know, just OH parts. If a used engine with a Carb fell in my lap, I'd use it with out hesitation. You can convert to FI but that is expensive. If you're buying a new engine, now is the time to go FI. Forget the cost of the PUMP FUEL savings. Your spending $60k on your plane a $600 fuel pump is a show stopper? FI can't get Carb ice which is nice (can get snow / sleet blockage). FI is "better" technically from a fuel metering standpoint, and the acquisition cost difference is not that great any more.