AFR Strictly for Mileage

[Scott Novak]

The Air Fuel ratio for maximum horsepower for gasoline appears to be about 12.7:1. The AFR for maximum mileage appears to be approximately 18:1.

What percentage of horsepower is lost using an 18:1 AFR versus a 12.7 AFR?

I'm wondering if it is even practical to jet a carb for 18:1 AFR throughout it's entire range for maximum fuel mileage.

What are your thoughts?

Scott Novak

[Northof49]

I think there is a reason fuel injection became the standard. It's the only practical way to maintain accurate fuel to air ratios across a variety of loads, rpms, operating temperatures and atmospheric conditions.
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1958 Karmann Ghia owner

[andy198712]

i don't think something that lean would be safe for the engine at WOT under load.... ?
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[Eaallred]

When you jet a set of carbs to high 12's at wide open throttle, the air fuel ratio at cruise leans itself out, usually around 16-17:1

No engine runs a constant a/f ratio through the rpm and load range.
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Eric Allred

You have to remember something: Everybody pities the weak; Jealousy you have to earn.

[Juanito84]

[Juanito84]

[kamesama980]

A lot of it depends on combustion chamber design and other engine characteristics. If the engine runs bad/misfires and can't make enough power to maintain speed, you'll get worse fuel economy. Remember that the engine is a system of components working together, not a set of individual components that can be adjusted independently. When running that lean, you also can/should run a lot more ignition advance. lean mixtures are harder to light and burn slower.

Because of some head changes (precombustion chamber, extra valve, convoluted intake tract), Honda CVCC engines in the 80s ran much leaner than that without a cat. Again with the 1st gen insights and HX civics (with more conventional headwork and EFI sytsems), combustion chamber and valve timing tricks let them easily run somewhere around 24:1 AFR.
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-Russell
"You don't get to blame me for how I fix what you break"
1970 VW Beetle
1994 Pontiac Firebird
2001 BMW R1200C
2003 Subaru Baja

[Juanito84]

[Scott Novak]

A good inductive ignition system is not a problem. I've been using Jacobs ignition systems for systems for over 14 years.

Currently I'm using a Jacobs Mileage Master with their Ultra Torquer coil. It's an inductive based multiple spark ignition computer. It has air gapped coil with only about 1:60 turns ratio which is capable of higher spark currents and is able to fire wet fouled spark plugs as well as very lean mixtures.

I had been using it in my last engine using 0.070" spark plug gaps and spark plugs two heat ranges colder than stock. At present I've only been using 0.060" gaps until I get the jetting corrected. Then I'll open the spark plug gaps until I see a loss of mileage or performance.

I'm also using a modified Mallory Comp 9000 SVDA distributor that is capable of as much as 62,000 V without crossfiring.

Scott Novak

[miniman82]

I'll give you a hint: this is a lot harder to pull off with a carb/dizzy than it is with distributorless/EFI, no matter how many lightning bolts you throw into your spark plugs. ACCURACY is the name of the game, a lean engine won't tolerate any variance if it's already on the edge of surging. Many engines will take a very lean mixture and still run well but again, it takes timing curves that you simply cannot get with a distributor unless there's a programmable box between it and the coil.
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[MURZI]

Lean mixtures sell parts.........

And just remember Stioch on e10 isn't 14.7 it's a lot lower. So.......unless you have adjustable timing control and EGT's you are flirting with disaster pushing the AFR's up.

13.2-4 AFR with a 009 and E10 is lean for me. Lean enough.
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62 vert
2276
Tim’s welded heads
45 Dells
A1 sidewinder
Fk44 cam

[miniman82]

Lean where, and how lean is lean? Lean at full power sure, but very lean mixtures at part throttle are seldom dangerous. I have run my 1915 at 18:1 before, but it's a low compression turbo engine so much leaner than that and it starts surging. My 2.5L turbo dodge is running at 15:1 right now, right up till around 3 lbs boost where it gets rich again back down to 13:1. Different engine, but same rules apply. It will tell you what it will and will not tolerate, all you need to do is listen.
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[Scott Novak]

I had my Beetle on the freeway today. It felt like I was still running on the progression circuit at 70 MPH and the mains hadn't come into play yet. But I haven't pulled the jet stacks yet so I don't know for sure. That's on hold until I freshen the brakes.

But the big question that nobody has answered yet is whether there would still be enough power running on a lean (16:1 - 18:1) mixture.

I'm still dealing with some computer issues, like I need to add USB to serial port adapter to use the LM-1 and LMA-3 Aux box.

Scott Novak

[Juanito84]

[[email protected]]

It depends. For example, you will get a lot more power (air) at 20% throttle (progression) with a set of 48 IDFs, over 44 IDFs, compared to 40 IDFs or 34 ICTs.

Or a stock carb.

So what speed you can lean-cruise at, depends on the engine and what carbs you have. If you have a stock carb it peters out at around 40mph.

[Scott Novak]

[[email protected]]

this is one reason you can get better MPG with duals.

With all of the carbs we use on our engines you can tune the progression lean, and main rich. Works best with vac advance.

But with duals you can get so much more air to the engine while still in progression, the car speed will be higher.

It's not wise to try to tune the main to 17:1 IMO. But I suppose you could if you wanted to, I've never tried that. You'd probably give up 30-40% of your power but it would yield high MPG for sure.

The beauty of our carbs is you can tune for both cruise and WOT conditions differently, if you want to and know how.
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[Juanito84]

Try looking up info on flying airplanes lean. For an example: EGT/CHT and Leaning Technique | Black Belt Aviator - 800-707-4071

According to that article running 12.5:1 produces the most power, and lowers the EGT by 75-100°, running 15:1 produces the cleanest exhaust, but the highest EGTs, and running 16:1 produces the best fuel economy with EGTs being 30-70° lower. Of course the leaner you go from 16:1 the lower your EGTs will be.

Some pilots take off richer than peak EGT (ROP) for power, and then leave the throttle wide open and lean back leaner than peak EGT (LOP) to regulate power and lower CHT temps. A lot pilots promote cruising lean of peak (LOP) since it gives lower CHTs. However, in order to run full throttle LOP it seems necessary to have good instrumentation in order to inspect EGTs and CHTs on each cylinder. General Aviation Modifications, Inc.

So hence the idea of running lean pilot jets and rich mains is really a good idea. The engine will handle peak EGTs (stochiometric) at part throttle just fine. It's at full throttle where just one cylinder running at peak EGTs will sooner or later fry itself, even if the rest are running LOP.

Another factor to mention is timing. Of course airplane pilots get considerably better fuel economy simply running LOP even with fixed ignition timing. But in order to fully utilize lean burn it would be best to have a timing curve optimized to your particular engine since the lean burn will burn slower.
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If a water cooled engine cools its water with air, isn't it just an overcomplicated air cooled engine?

[FreeBug]

I think leaning it out beyond 16:1, you will hit the law of diminishing returns, as efficiency drops off even faster than fuel, past that point.

Also, that what you might gain through lean burning, you will lose from having to open the throttle more.

How well is the car set up for mileage has almost as much impact as jetting, maybe more. I'm curious as to your results, I'm also chasing MPG...

[[email protected]]

this stuff is all covered in the wideband tuning thread.

and understand that you'd have to run 50% more throttle at 18:1 than at 12:1 for the fuel flow to be the same.

Yes you have to run more throttle, but you are kidding yourself if you think that you will get close to the same MPG at 13:1.
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