Lean low-load AIR/FUEL for more MPG?

[MeatHead]

A car I'm currently working on AEM EMS (programmable engine management system) and I was wondering if any of you had success with running 19+ air fuel ratios on light load "cruising" area of map? Since no catalytic converters are required where I live it seems like this leaner air fuep ratio can help when not in boost and in light-load areas of map. Anyone tried it?

[JasonG]

A few of the Hondas here have lean-burn.
I believe the timing also needs to be retarded to prevent detonation.
I would reccomend studying their maps closely before melting a hole in you're pistons.

[SVOboy]

Yeah, most engines just aren't designed to suddenly go from 14.7 to 19+. I would test your car's AFR in increments to find where you can feasibly run it based on engine performance.

[MeatHead]

But retarding timing increases egt (exhaust gas temp) no?

[E4ODnut]

How lean you can run your engine will depend on the engine. Also, one thing to consider is that your wide band may not be 100% accurate. It's not uncommon to have a significant difference between makes, 0.5 or more. That's of no major consequence because you'll likely just be tuning with one and it's the relative readings that are important to document what your engine likes.

You will definitely get better mileage with leaner mix. I have fuel burn read outs with my systems and they confirm it. If you don't already have one the MPG instrument offered on this site would likely be a good investment so you can track your changes.

I've found with all of my engines, I can get between 17 and 18:1 indicated before I get any sign of a lean stumble. I've run as much as 19:1 but that was on a very light load, and my engines are almost never at that power setting.

I've also found that is beneficial to add quite a bit of timing with the lean mixtures as they burn a lot slower than a richer mixture will.

It takes a lot of experimenting, and data logs are your best friend

[Ladogaboy]

Leaning out the AFR will increase the EGT. The timing wouldn't play much of a role in that; but retarding the timing might prevent you from blowing up a piston. Also, I'm not sure that exceeding ~16:1 AFR will give you much in the way of fuel economy gains. 14.7:1 is stoich (theoretically, a perfect burn off), so you'll probably see diminishing returns by any increase beyond that point.

[E4ODnut]

To the best of my knowledge, EGTs are maximum at stoich and decrease either side of. I have EGT instruments on my engines. The ones on the boat are accurate to 4 degrees F. My observations don't show any increase in EGT with lean mix.

At light loads your timing would probably have to be advanced to the point where your power would fall off before you would be anywhere near any danger of detonation and holing a piston. The engine simply isn't making enough power to hurt itself at those settings. I've experimented quite a bit with timing and that was my observation. You can retard the timing if you like, but all that will do will decrease your efficiency and raise the EGT a bit.

As I mentioned before, my instruments confirm better economy at mixtures leaner than stoich, but, in all fairness, these are on my engines with my tuning methods. Your's might not agree.

It's entirely possible that mixtures leaner than ~16:1 wouldn't get any better economy. I've found it almost impossible to set things that fine over a reasonable cruising range and the indicated mix ranges from about 15.5 to about 18 or so. It's beyond my capability to determine exactly where optimum is and maintain it.

[Ladogaboy]

That sounds reasonable. Most of what I know about these modifications is done for performance purposes; I guess the fine line is where performance, efficiency, and economy meet. Under low loads, these modifications (timing and AFR) probably shouldn't cause much damage.

You're absolutely right, though... None of this can be established without logging and tracking the modifications and data.

[E4ODnut]

You are right. Most programmable EFI is used by the go fast boys. I've been using mine for almost 7 years now, mainly tuning for economy. I keep searching for others who may be doing something similar to compare notes and bounce ideas off. The interest is practically non existent. Some will post theories, which is fine, but little if any real world experimenting out there, or so it would seem.

[Ladogaboy]

Well, I hope to be adding to that collective: I'm picking up an OBD-2 cable that I can use in conjunction with an open-source program that will allow me to adjust these settings and log the results. My car kind of lives in both worlds (being turbocharged), so I'm looking to increase fuel economy without major concessions on performance.