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Lean Idle and cruise AFR's

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Hey guys, just after some advice on leaning out my cars idle and cruise AFR's to save some fuel in traffic and on long highway drives.

I'm just after some ideas on what a reasonably safe AFR I could target at idle and at cruise (under about 3000 rpm and off boost), engine is an RB25DET NEO.

Thanks in advance.

You should really be targeting stoichiometric 14.7, lambda 1 on normal pump fuel for cruise and idle. Running any leaner will only generate heat and cost you power. What AFR's are you running just now?

Take into account what Chris has said above but also consider the fact that when at cruise you only need a small portion of the power an engine can produce. On (Australian) Delco ECUs it is not uncommon to run (on petrol) about 15-1 (1.03 or so Lambda) at cruise. It is possible to adjust them to run as lean as 17-1 when a couple of conditions are met.

With regards to idle I just stick with 14.7-1. Unless you are in stop start city traffic where you sit at traffic lights alot I don't see a good reason to go outside of stoichimetric.

It's currently running e85 and is tuned to be smack on lambda 1 while idling and cruising, idle doesn't concern me too much but I'd like to squeeze some more efficiency out of it while on the highway while not increasing risk too much.

In almost all cases you want a idle and cruise at lambda 1 or 14.7, just like everyone said before me. I have done a few cars for a few customers who have a far commute daily that wanted to get the most MPG. Have a 2jz powered Tacoma that runs 16-16.5:1 at cruising that we have gotten 25MPG on a 1000 mile trip.

This an interesting article in regards to knock, detonation and fuel mixtures.

It may help clear up some myths in regards to a leaner engine running hotter than stoich.


As you lean out the cruise and idle mixture, you will find that the engine's torque production drops off. Since we need a certain amount of engine torque to maintain a steady cruise speed on the road you reach a point where a leaner AFR reduces torque so far that you need to open the throttle further to maintain speed and hence you end up using more fuel anyway - The whole situation becomes counter productive.

You can achieve a small improvement in fuel economy by targeting a slightly leaner than stoichiometric AFR - Around 15.0-15.5:1 would be about as lean as I'd suggest. You may be able to tune leaner than this but you're unlikely to see any improvement in economy. You may also find that at some point you're limited by the engine beginning to lean misfire. This in my experience is more prevalent on small displacement engines and those that are heavily modified, particularly with large cams offering a lot of overlap.

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