Engine Lean, Normal, Rich

Lean, Normal, Rich

Well i hear you all talking about turbo's a lot. I also here you talking about how your A/F ratio is either rich or lean or what not. i was just wondering why it bad if it is lean. or if it is bad both ways. ....basically i guess i need a run down on the whole thing lol.

brad

If you do a search on Google or something for engine tuning you will find a lot of good info.

Basically, a "normal" air/fuel ratio is 14.7:1, which is called stiometric. Stiometric is not the best ratio for power, but is the best for emissions. The best ratio for power is *about* 12:1 with a turbo, but that changes with a lot of things.

Now, if the a/f ration get's too lean, the fuel will burn too fast and get too hot, resulting in detonation, as well as other problems. Detonation = engine kaboom. That's the problem with just strapping a turbo onto an engine. The stock computer will not provide enough fuel, so the a/f gets too lean, and bam, you just blew a hole in a piston.

To fix this, an aftermarket computer has to be used to richen the a/f ratio. (The a/f ratio is measured by an 02 sensor, a wideband 02 must be used for tuning, since the stock one just tells you whether or not you're at stiometric). The a/f can be richened basically three ways.

1. Widen the pulse width. The fuel injector stay open longer, and more fuel is sprayed in. An aftermarket computer is needed to do this. This is the best option.

2. Larger injectors. The fuel injector stays open for the same amount of time, but sprays more fuel because of the bigger orifice. Larger injectors are a MUST with high boost turbo applications.

3. High fuel pressure. This is why people use FMUs. They increase the fuel pressure relative to the amount of boost, and therefore the injector sprays more fuel for the same pulsewidth. This method is not very reliable and cannot be tuned as well as using an aftermarket computer.

Keep in mind that an overly rich mixture is bad too, it will foul the spark plugs and you could nuke the cat, if you have one. However, in general, a rich mixture is preferred as opposed to a lean one.

Those are the basics, anyway.

Ideal A/F ratio is 14.7 oxygen molecules to 1 fuel molecule. That is called stoichometric burn and it gives the least emissions - with a normal ratio, the fuel is almost entirely burned without any leftover and little noxious emissions.

With a lean mix (too much air or not enough fuel), the combustion temperature spikes high. Ideally, exhaust gas temperatures shouldn't spike above 1800ºF for N/A and shouldn't go above 1600º for FI. Lean mixture causes EGT's to rise - that's bad for the catalytic convertor, exhaust valves and emissions and strains the cooling system, burns spark plugs, etc. Nitrogenous (NOx) emissions rise with high EGT's (this is the purpose for exhaust gas recirculation - cools combustion down but allows leaner mixtures and higher gas mileage).

Rich mixtures are the opposite. Hydrocarbon and CO emissions go up (unburned fuel). Rich mix is better for power to a point (13.0:1). If an engine runs rich constantly over any length of time, it will cause gasoline to dissolve in the oil, which thins it out and causes excessive wear. It also leaves a lot of carbon residue b/c the combustion temperature doesn't get hot enough to burn it off - which means fouled spark plugs (your spark grounds out without arcing), carbon deposits on the pistons and valves. Unburned fuel also collects in the catalytic convertor and can overheat it - possible cat conv fire (bad). That's why a lot of vehicles put out a slightly black smoke from the exhaust when @ full throttle. Dark black smoke or lots of soot out the exhaust is a sign of a rich mixture. You WANT a rich mix @ WOT - that's good to a point. Adding extra fuel is good b/c it cools combustion down and gives a bit more power.

Ideally you'd like it to run rich @ WOT and idle, but slightly lean at low throttle for good highway gas mileage.

Late,
Trav

HAHA! funny - I think we agree on most everything. I think we were on that post at the same time, JDLangevin.

Trav

Right on, brother.

YEAH - I about forgot about a lean mix causing detonation. That's the big thing about a lean mixture that will bring your engine to an untimely death VERY quickly.

If you ever have the choice between a lean or a rich mixture, definitely choose a rich mixture. Running rich is erring on the side of caution.

Late,
Trav

very informative guys!

thanks guys!