Lean low-load AIR/FUEL for more MPG?
 

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?

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.

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.

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

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

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.

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.

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.

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.

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.

Tygen1 has been doing some experimenting with lean burn and IIRC he says that mildly lean will cause increased EGT, but as you get away from stoich, the temps go back down.

Great input from everyone! My plan is to buy a good EGT sensor and rent a load bearing dyno for a day testing these settings. Lets keep the discussion going, good stuff.

Now if only I could try pgfpro's fuel tuning spreadsheet to help me in my findings. I posted in his thread, but no response yet. Soes anyone have and wouldn't mind me trying it? I'm happy to post all my findings and fuel/ignition maps publicly to the community here too.

There's no reason why you can't get the best of both worlds from a boosted engine. It just takes more time tuning and you have to be especially careful when tuning under boost, that's all.

Some people advocate tuning with EGT. I'm not so sure about it. I do know that EGTs can be very handy if there is lots of documentation on what's acceptable and what's not, like on an aircraft engine. However, when you're experimenting on your own, there is none of this information to base your readings on. The temperatures can vary quite a bit from engine to engine, and just where the probe is placed. Having said that, any information is good information because you can document the temperatures under different conditions to compare it to other conditions.

For tuning power under boost I suspect it would be safer to reference exhaust gas oxygen. Start with a rich mixture and work from there because things are more predicable and the sensor response is very fast. This is just an opinion. I have no experience with boosted engines.

Tuning for economy is much safer because if you screw up you aren't likely to hurt anything. It just takes lots of driving under lots of conditions, data logging, and making small changes at a time.

Probably the ideal situation would be to do dyno tuning first, then do the final stuff on the road under real world conditions. I don't have access to a dyno, so it's not an option for me.

I've been thinking about buying both an EGT and a wideband for my car to try tuning the lean-burn area, as my car has very good knock resistance at low loads. I've run from 16:1 to 22:1 at times.

Also catless. As long as I pass emissions, I don't need one.

Will be following this thread with interest.

Yeah, the dyno tune is done on my end. Under high-load conditions, the best I do is roughly 12:1 AFR. Low-load adjustments will have to be done with that in mind. It seems like the transition points between no boost/boost and low-load timing/high-load timing will be the hairiest. :eek:

I've tuned a few 4 cylinder honda cars (turbo + na), a turbo nissan v6, and now my car (inline 6, turbo motor). My plan was to apply lean-burn to only vacuum loads, not positive boost load range. Under boost, It will be optimum for power safety (typically around 11:1 afr). Besides, when I press WOT condition I want speed :P!

I've read about some tuners "turning cylinders off" during light load cruising range as well. Does this mean they turn off the injector and spark to certain cylinders during each complete engine cycle? If yes, how feasible is it to combine this with lean burn? By cruising condition, I had in mind those road trips at constant speed/load (typically 60mph range)

"Turning off cylinders" is getting beyond what I am comfortable tuning with. I would think that simply cutting off fuel won't do a great deal of good unless you can also eliminate the pumping losses with some sort of inlet and/or exhaust valve control.

The engines I work with are relatively low tech with stock configuration. My philosophy is one to just try maximize what I've got, tuning to the degree in which I am able to do in real world situations. I just go with the theory that any throttle setting that results in about 80 KPA of manifold pressure or less is "cruise" and I'll run as lean as I can without any sign of lean surge (idle range excepted). For my engines this is about 17:1 indicated. After about 80 KPA I feel that I am requiring some serious power and ramp up to about 12:1 at 100 KPA at max HP RPM. This is just my view on things and what I feel comfortable with. There may be a better way.

The main reason I could see for shutting off cylinders isn't for efficiency but rather to occasionally cycle cooler air through the cylinders. Doing so could prevent potential damage to engine components, valve seats, etc.

A local guy who I respect and trust told me he played with a lean burn B18 honda engine. We compared notes, and arrived at the following recipe

Pretty high compression - he was 12:1.
Do NOT be afraid to advance the timing. A bunch. Like 13 degrees.
Have a knock or det sensor, and respect it.
Do NOT use lean burn over about 20 percent TPS.

He said he got 45-48 mpg out of the vehicle, and running it stoich, it only got 30 or so.

FWIW, From Stoich, if you lean it out a little bit, EGT's will go up for a while, then they will start plumetting.

This is incorrect information.

I remember reading that EGT is not the poblem with running lean but the excess of oxygen with high temperature of combustion tend to oxydize the piston.