Hey there, just wanted to know what the difference is between running rich vs. lean?
...A little off topic but i was told by a fellow rx-7 owner at a car event that sometimes when running rich, a flame can shoot out via the exhaust...is that possible for both lean and rich?

 

rich means more fuel which means flames. lean means less fuel, less flame. if your engine runs lean, you will gain little more power but hotter temperatures and less engine life.

 

In order for combustion to happen you need a few things. Fuel, oxygen and heat (ignition source). In an engine we use a spark plug to provide the ignition source, generally gasoline for fuel and air to provide oxygen. In a gasoline engine you need a ratio of 14.7 to 1 of oxygen to fuel (ideally), however if you were to do that under load (go pedal down) the engine will quickly blow up. In order to prevent detonation from happening and to ensure that there is enough fuel to react with the oxygen you inject a little bit more fuel into the mixture therefore lowering the ratio (11.5 to 1 being a common number), this is known as running rich. since this fuel cannot burn because there is no o2 it goes into the exhaust, and if there are no cats, air pump or flame arrestors in the exhaust (most racers eliminate it) it will continue all the way to the atmosphere where it will encounter oxygen. Since this fuel is already very hot from being in the combustion chamber it already has enough energy to ignite itself when it mixes with the oxygen and create a flame very close to the exhaust tip. Rotaries are known for running rich.

Lean is the exact opposite of this condition although in a rotary you can hear people say that a car is running lean at 12.5 or 13 to 1 ratio. this is saying that its running leaner than usual rotary cars. Normal terminology is anything under 14.7 is rich, 14.7 is known as stoich, and over that is lean. Lean conditions should not spit flames out of the exhaust. Normal driving (cruising) will usually be run close to stoich (14.7) for fuel economy, it is around this number that your engine will be most efficient in fuel consumption. Id like to point out that a car running rich will not look like a flamethrower in the back, it will be a quick flame when the right conditions present themselves it can be reasonably long but usually lasts for less than a second. Hope this helps

 

So for longevity a little rich is good or should u shoot for 14.7

 

If you were using a piston engine I would say shoot for as close to 14.7, but for rotary I believe it's general practice to run a little rich to ensure the life of the apex seals. However like luism said it shouldn't be spewing flame like a dragon, just little pops.
To summarize for it's a little different depending of engine and set up but generally richer means longevity and lean means more efficient.

 

Only shoot for 14.7 during light cruising and maybe (if you can) idling. Ive tried to idle my car at 14.7 but it becomes undriveable. Im sure it can be done but it is a little bit harder. with the stock ecu my FD idles around 12.3 when warmed up, then goes to 14.7 in light cruise and when you get on the throttle it starts getting richer and richer depending on rpm and boost (load) all the way down to 10.5.

 

So the little pops your referring to are backfires right, and that's a good thing right.

 

The pops are a byproduct of running rich; so in a sense they're good. but you can tune for the pops, in England they do it, but I would not recommend it. An experienced tuner will decide whats best for your engine. As a rule of thumb, rich is is safer up to a point; too rich will flood your engine and foul your plugs. An experienced tuner (preferably experienced with rotaries) would decide whats best for your engine. here are several books about tuning that are very helpful, they are not specific about rotaries but internal combustion is internal combustion. Rotaries are a little bit more sensitive so usually we run a little bit richer, but from what i've seen its not that much.

 

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