Moto GP Racer
Join Date: Feb 2009
Location: Canada, bitches!
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Yes! Technical time.
To run an engine lean is to have an A/F ratio that is Air-biased, so anywhere from 51/49 and ascending (whereas Air is the primary). A lean mixture can yield a more OR less powerful combustion, depending on how lean you are running. Lean engines typically run hotter, burn A/F mixtures more thoroughly, leave less carbon deposits and expell less emissions. Technically, a lean engine will run MUCH smoother than a rich engine but it will also overheat MUCH faster. However, the downside is that it is almost impossible to detect cylinder head temperature changes without a manifold-mounted EGT sensor. If you don't detect changes in temperature and immediately correct the problem then you'll run into major problems, mainly consisting of burning pistons (almost instantly), rings, cylinder and even the cylinder head, if you leave a lean engine running for too long.
To run an engine rich is to have an A/F ratio that is Fuel-biased, so anywhere from 49/51 and descending (whereas Air is still the primary). Too rich of a mixture will lead to premature carbon deposits (which will directly affect ignition function), sluggish power delivery and, in worst case scenarios, cylinder wall scorching. However, with today's EFI systems, the worst that will generally happen is carbon deposits, sluggish power delivery and progressive cylinder/cylinder wall washing.
Anyways, for a typical engine (one that doesn't get rebuilt often) a good A/F curve is one that is slightly rich while at idle, slightly lean throughout the power band and then again to the slightly rich side just before peak power is made up until rotational limits of the engine's design. The variance should usually be no more than +/- 3-7% between the two A/F ratio extremes, depending on your style of driving.
Now, you can get away with running a lean engine ALL the time if you have a proper cooling system and proper electronics to ensure your engine can vary A/F ratios depending on EGT at the manifold at any given moment. But I wouldn't really suggest looking into it.