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Fri Aug 21, 2015 10:34 am

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

Fri Aug 21, 2015 11:14 am

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.

Fri Aug 21, 2015 11:20 am

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

Fri Aug 21, 2015 11:39 am

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.

Fri Aug 21, 2015 1:49 pm

Eaallred wrote: 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.

Exactly. Jet it lean at part throttle and rich at wide open throttle and enjoy both best fuel mileage and best power.

I do believe that 18:1 is leaner than best fuel mileage. I think that's around 16:1. But 18:1 will run cooler as long as you don't get misfires. It will also give you better longevity as a result.

Sat Aug 22, 2015 2:51 pm

andy198712 wrote: i don't think something that lean would be safe for the engine at WOT under load.... ?

18:1 might do alright at WOT. The hottest flame is just lean of stoichiometric (14.7:1) at around 15:1 to 16:1. It's also where you get your best fuel mileage. That's because you're burning every last drop off fuel and oxygen available, and the greater the temperature difference the greater the efficiency. Going richer or leaner than that will cause a cooler flame. The reason is that there'll be more stuff passing through that doesn't get burned. When richer the unburned fuel vaporizes which draws out a lot of heat. This is also why stoichiometric doesn't always end up being the hottest since there'll still be some unburned fuel since the engine isn't perfect. It's also why at around 13.5:1 you get your best power since the expanding unburned fuel acts like a steam engine as it literally boils in the cylinders.

Going leaner also cools the flame since the excess air, without fuel, is inert. It's much like having an EGR valve. The difference is that it'll still run fairly efficient due to the fact that you're still burning every last drop of fuel. Many pilots try to run lean of peak EGT since they get cooler heads and better fuel economy plus less carbon build up. Still they usually try to not run at WOT very long lean of peak EGT especially at low altitudes. However if a person were to run leaner than 17:1 he might be able to get by running at WOT. The leaner you go the cooler your EGT will be. I've been told that it's possible to run 17:1 ask day long at WOT. So 18:1 is of course cooler.

But in the end why not run 16-17:1 at low loads and 13:1 at full throttle and get the best of both worlds? If you truly want the best AFR for fuel mileage at full throttle I'd say run a 15.5:1 AFR along with water injection.

Mon Aug 24, 2015 11:04 am

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.

Mon Aug 24, 2015 11:49 am

kamesama980 wrote: 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.

Yes! Normally a carbureted engine can't run much leaner than 18:1. Honda was able to run extremely lean (20's) by means of creating a stratified charge where part of the charge was richer and placed closer to the spark plug. But that's not possible to do in our engines as far as I know.

In order to run lean (16-18:1) in our air-cooled engines it would be a good idea to get a good inductive spark ignition system (CDI doesn't work well with lean burn). Seems like a Toyota 1.5 ohm coil with a Ford TFI module would be a good choice. Dual spark would also help. A good quench and higher compression ratio would help. Maybe some ACN Super Squishies. And good air/fuel mixing would also be important. The better the air and fuel are mixed the leaner it will be able to run.

Mon Aug 24, 2015 6:05 pm

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

Tue Aug 25, 2015 5:34 pm

:roll:

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.

Tue Aug 25, 2015 5:45 pm

Lean mixtures sell parts......... :lol:

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.

Tue Aug 25, 2015 6:03 pm

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.

Tue Aug 25, 2015 8:03 pm

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

Wed Aug 26, 2015 7:34 am

Scott Novak wrote: 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.

Well, yes. You should have enough power to drive it at highway speeds. The amount of energy in the AFR per volume comparitively is as follows:

12:1 = Let's compare the rest to this one.
13:2 = has 92% of the energy 12:1 has.
14:1:= 86%
15:1 = 80%
16:1 = 75%
17:1 = 71%
18:1 = 67%

Of course this doesn't account for efficiency. 12:1 has more energy in the fuel but since there's not enough oxygen to burn it all the engine converts less of it into power. So in reality, 18:1 is going to give more than 67% of the power 12:1 does (perhaps closer to 75%-80%).

Since it takes around 20 HP to push a Beetle down the highway at part throttle, and the maximum power a stock Beetle puts out at the wheels is around 40-45 HP then with an 18:1 you'll still have enough power to cruise along at part throttle at 60mph.

At 18:1 you could likely run full throttle at that AFR without killing the engine. Cruising asking with a 18:1 AFR will give you cooler heads than at a 13:1 AFR.

Wed Aug 26, 2015 8:53 am

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 wrote: 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

Wed Aug 26, 2015 9:13 am

[email protected] wrote: 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.

THat would agree with my perception of driving with the 34PICT. It seemed like my throttle was closer to WOT than with he 40IDFs. It didn't seem like my throttle was very far open at 70 mph with the 40 IDFs.

I am going to try it with both a lean progression, lean main., and a lean progression circuits and a rich main. If I'm still in the lean area at 70MPH I'll go with lean progression rich main.

Juanito84,

That's the information that I was looking for.

Scott Novak

Wed Aug 26, 2015 10:10 am

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.

Wed Aug 26, 2015 12:02 pm

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.

Wed Aug 26, 2015 12:35 pm

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...

Wed Aug 26, 2015 1:11 pm

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.