tuning for economy: where and how lean can i go ?

[mops]

Hello.

I'm driving my machine and fuel economy is not bad, i.e. comparable to stock ECU. However I'm sure I can lean out there and there. I got LC-1 WBO

Basically I'm trying to tune my engine for really good economy up to 3krpm and for absolute max power over that.....

My question is.... (this only applies to rpm below 3krpm):

is it ok to run long time lean, by lean I mean 15 to 16 afr's... my car might run lower in some cases, is it fine to lean it out to 17 on cruise ? consider that daily i drive gently, so this engine will be driven daily on lean, so basically 90% of milage will be done lean.

I guess my question is whether it might damage/reduce lifespan of the engine ?

Ideally I'd like to tun over 16 afr's on cruise, around ~15 for light acceleration (up to 75kpa), then for WOT I run 12.5-13afr's...... I think with some good, fine tuning this can be achieved, I'm just concerned whether it is actually healthy for the engine to run that for extensive periods of time.....

I think with good tune, more power than stock ecu can be produced AND fuel consumption might be reducted significantly (up to maybe 20%)

[jakobsladderz]

You shouldn't need to run richer than 13.5:1 at full power, unless you have a turbo (then 12-12.5 is ~right). Basically the problem with running lean is exhaust temperature combined with the ability of the exhaust to convect heat to the exhaust valve, etc. at light throttle but lean, the exhaust will be hot but there won't be much mass-flow so the exhaust valve can cool adequately by conducting heat through the valve seat. At heavy throttle settings, there is much more flow so it's important not to run too lean or the valves may be damaged.
As for the rest of the combustion chamber, I think it's fine to run lean at light load. Keep in mind that best economy will be found close to 15.5:1 on petrol. Leaner than this and you start to have more unburned hydrocarbons going out the tailpipe and economy again suffers (basically, the pockets of lean mixture in the chamber that won't/can't ignite get biger and bigger till eventually you will get noticeable misfire). This may not apply is special cases (high turbulence motors, GDI) but is generally the case for 'normal' engines.

[wes kiser]

The general rule is go until you get a lean miss, and then richen it up a little. In my case I am in the 16-17 range, and still no lean miss. I am averaging 29mpg on highway cruise. Something that also really helps, is when in "lean" cruise, the engine will need more timing (lean mix burns slower). In my case, in the regions I am lean cruising (2000-3200 rpm, 45-75 kpa) I have between 40 and 44 degress of timing. Your results will vary, but add a small amount of timing when you are lean cruising.