lean burn tuning ideas
 

For my car I have programmed the ECU to set 16.2:1 AFR as lambda 1.0. this means every time it's targeting 14.7 afr it's actually doing 16.2. I have confirmed this with wideband. Ignition timing wise i could not add much more than 2-4 degrees depending on load/throttle before onset of knock. I also experimented with going leaner to 18:1 but I experienced minor (but persistent) knocking or detonation at cruise. So is it worth going any leaner at the cost of ignition timing? So far consumption for last 1000 kilometers have improved by 9% with 16.5:1 AFR.

Are you sure it's detonation at 18:1? If it's a popping sound, that's probably misfiring not detonation. Detonation above 16:1 is very unlikely because you have a lower flame temperature.

I've written about my experiences doing this on my Scion FR-S (a Subie FA20 motor). It doesn't like idling at 17:1 no matter what, and doesn't let you tweak cam timing at idle to reduce EGR. Lean burn only works when the engine is able to consistently burn the mix quickly.

good post:thumbup:

Would more octane help?

So is it worth going any leaner at the cost of ignition timing? Usually not because a leaner A/F ratio will require more ignition advance because of a slower flame speed. If you decrease your ignition advance this will increase your BSFC and hurt FE.

yes im sure its knock, the knock sensors are pretty reliable. At cruising 2500 rpm if i back off 2-3* the pinging stops at 17-18:1 AFR. idle i can go 15.6:1 with no issues due to AVLS low lift mode. but i like smooth buttery idle at 14.7:1. anyway i can run 16.2:1 afr at very advanced timing but anything leaner requires timing retard.

currently 16:1 afr at 2500 rpm or 60mph at 40* timing with no EGR. with egr i can add 5-6* without knock.

Don't add timing then! Modern engines are typically a little knock limited but not always. I recall the new Subie FA20 turbo engines for example have very high tumble flow heads that allow the timing to usually be <20 degrees BTDC. On the other hand you have the Mazda Skyactiv engines which will go to 70 or 80 degrees BTDC at the lowest loads.

I would think EGR is inferior to a leaner burn, but if it enables more timing it might work. I think it's worth remembering that there's diminishing returns to having the mix burning earlier in the compression cycle, if you added timing from stock already maybe you overdid it. Why not test it out?

adding timing definately helped with torque and throttle response. I experimented with this on the dyno but only limited to enriched WOT. just adding 2* timing added about 7-10whp. at part load i managed to add a lot more timing over stock, something like 4-8*. this could definately be felt in the throttle response ans how much smooth and quieter it cruises now. however now that im doing leanburn i found i cannot advance the timing any further. reading that leaner burn requires advanced timing i thought this might be a problem. however so far i have not noticed any downsides. The EGR is disabled for now because i noticed spark plugs get really white at the tip with this combo i think due to very advanced timing ~47-50* cruise. i also think advanced timing is bad in cold weather for plug life.

Oh I just remembered, knock is definitely possible at 17:1 since that's still close to stoich flame temperature. That's why lean burn engines from years past would jump straight to 20+ AFRs.

That makes lean burn retrofit an interesting problem then. Do you have electronic control over an external EGR system? The EJ motors have that right? Reducing the oxygen content in the charge would reduce knock, but the margin is probably very thin before you lose efficiency to poor combustion. You can hijack the factory EGR pipe with an EGR cooler from another car, piping coolant to it, and then hopefully opening it up at cruise somehow.

This mod would make sense because if you trick the engine into running lean, you're also reducing EGR because there's less manifold vacuum. Increasing the fraction of exhaust gas to fresh air would just be bringing it back to normal.

thanks for response. i never ventured foing leaner than 17 afr fearing missfires. but from my experience i cant rly advance timing much but the consumption reductions are still significant despite lack of much advance. the only issue is the car feels weak overall in this mode. the throttle opening requirement is significantly higher almost 90% of time. the throttle response is completely gone and sometimes the car goes very rich. this is all because of full time open loop condition and its very hard to get the maf 100% scaled. so far im running 14.7 afr with more timing. 50* vs 44* in leanburn knock free. its still very efficient, usually 30mpg highway at 70mph vs 27 mpg stock. with wgr disabled i have to retard 5-7* to prevent knock but it seems egr helps with efficiency on highway.