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Fuel economy AFR

Understanding AFR

Discussion and questions related to the course Understand AFR

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Hello to all ,

I have 2 questions ,

Woundering if I can target lean AFR like 13 and retard some timing at wot , does this combination benefit economy ?

If I increase spark timing at partial throttle (low-mediuem load ) this will increase fuel economy ?


If you don't have to pass a drive cycle multigas emissions test and your cams aren't too wild lambda 1.05 to 1.1 will probably give you best cruise economy. Full load is way too engine dependant to know what might work but distegarding thermal damage or knock more retarded timing will lose efficency. If you look at the page grab from a paper Raymond posted in the other thread it should give you an idea.

Don't try to get fuel economy at WOT, try to keep the engine from breaking. Any area where the spark isn't knock limited, advancing the spark doesn't get you any fuel economy benefit. If you are driving at 2000 rpm and very light throttle advancing the spark doesn't get you anything. If you are accelerating up a hill but not at WOT, that kind of load could benefit from more spark for better fuel economy. At WOT fuel economy COULD be your priority but SHOULD it be? The risk of breaking something outweighs the financial benefits of saving fuel.

Targeting lean afr than 1 lambda is good idea for fuel economy but that usually works good at standalone ecuĀ“s , I am stuck with reflashing oem ecu's and normally closed loop tables don't give effective conrol at target lambda cruising or idling !!

Raymond , some NA vehicles targeting rich afr at wot , my idea is to to target leaner afr in those to gain fuel economy .

Sometimes the real world differ , I tuned several NA cars increasing timing spark at light load and it really benefit economy . My understanding that more spark =more torque = less throttling = less injection. I may be wrong but I followed it and successfully sometimes .


Under WOT you often can see an improvement in fuel economy as a result of leaning out a factory tune, however this would be as a result of the factory calibration being excessively rich to begin with (which is quite common). Your aims with WOT tuning should be to provide the AFR that the engine is happy at rather than one that provides best economy, unless you're running an endurance race car. Leaning out the AFR will increase combustion temperatures which can end up damaging engine components and can also make the engine more prone to knock.

Teams who run endurance races and want to achieve a fuel economy benefit account for the extra combustion chamber heat in the engine design and clearances.

Thanks andre ,

Got it ,

Ok , how about increasing spark timing around cruising areas would save some fuel ?

Light through medium throttle and/or lower rpm you can reduce the fuelling with a degree of safety (where manifold pressure is in the 'vacuum range') with 15.5 to 16.5:1 being the normal 'lean best' ratio - IIRC, Andre has a video on 'lean best torque' or something similar.

With the ignition timing, it is expected that the optimum will vary with the cylinder pressures, so expect to use more timing as rpm is increased and/or the load on the engine is reduced - this is the primary difference between 2D and 3D timing maps.