Air fuel ratios

[nissan240sxmn]

Can someone explain to me pls what "A/F base%" and "A/F Base SL%" are i can't seem to find a understandable definition of what these percentages represent and what are the optimal values.

Thanks in advance

[carmo]

Air fuel ratio is how many parts of air to 1 part gas. So stoichiometric is 14.7. That means 14.7 parts of air to 1 part of gas. Stoichiometric is the most optimum "bang for the buck" for standard gasolines. Methanol and other race fuels are probably different. So when people tune cars (fuel injection) they tend to tune on the richer side i.e. more fuel per parts of air. So the tune will be more like 13 to 1. More fuel reduces combustion temps. which reduces your chances of preignition or detonation. On regular engines its best to run closer to stoich. because your not pushing the motor to capacity and fuel economy is a plus. With turbo upgrades or bolt on kits you start pushing the motor beyond what it was intended for and heat and cylinder pressures build quickly. If the car goes lean for some reason, say 16 to 1 A/F ratio, while your at peak boost. Ok, heres the scenario. Your fuel tank sloshes in a sweeper while your at 25psi of boost and the pump fails to supply the injectors with enough fuel. A couple of cylinders see 16 to 1 or 17 to 1 A/F. The cylinder is being crammed w/ 25psi while the piston is going to go for 8.8 to 1 compression and now you just drizzled a very little bit of finely misted gas in there and the "timed" explosion has just occured well before your piston rod had a chance to come close to top dead center. That lack of fuel just arm bared your lower end. So lean is mean. On Nissans I guess the percentages you are refering to are Air fuel alpha. The computers ideal range is between 90% and 110%. 90 being rich side and 110 being lean side. But what these are are computer compensations. The computer will look at the front O2 sensor and compensate for what it sees. So if the front O2 sees the motor is running a little rich it will back off on the injector pulse and you see that in the form of Air Fuel alpha %s. If your airflow meter (MAF) goes bad and the car starts running super lean, based off of the bad info the MAF sent to the ECM, the control unit may show A/F alpha %s as high as 135% or more. This is the computer trying to compensate by telling the injectors we need 35% more effort than the usual 100%. And dont mistake these percentages for injector duty cycle (also show in %s sometimes or maybe all the time idk), these %s are just reference values that the ECM understands. Wow! I hope this helped. Sorry for writing a friggin book and I hope somewhere in there is your answer.

[nissan240sxmn]

thanks for the reply, i understand it a little bi more now.