I'm trying to understand the general theory behind AFC tuning. From what I understand, the stock Evo's run fairly rich and most people tune the AFC to run leaner. I think because I don't yet understand all the variables involved, I don't quite understand why this leads to better performance (except in terms of fuel economy).

It was my understanding that running rich will decrease the chances of detonation, which means that the ECU can advance timing more aggressively. It seems like this would be preferable to running lean which would lead the ECU to retard timing.

Is the reason that running leaner yields better performance because with a higher A/F ratio, you can cram more air into the engine? Or is there some emissions sensor that's reducing performance when the engine runs really rich?



Any explanation would be greatly appreciated.

 

Well first look up what combustion engines are... I'll try and answer your question though.

Yes, running with the correct amount of gasoline will keep you from detonating. No, that does not mean the ecu can increase timing more because you run richer. No, running with less gasoline will not make the ecu retard the timing.

Yes, running a high A/F means you have more air than gas. No, there is not a sensor that reduces performance because you are running with more gas.

Ok, all that being said let me explain...

Running rich means having more fuel than you can burn at any time (means more fuel than air). The timing is totally based on what the ecu was programmed to believe what is a good situation to have a certian amount of timing in. It varies from one design to another.

You are not making more power by running leaner... You are making more power because you are balancing the amount of fuel for air to enable the largest bang to happen.

Too much gas blows out the fire and can possibly unseat your piston rings.

Too little gas and there is very little fire. You can also have the gasoline prematurely (exploding before the spark) explode (pinging) or have the gas explode in two+ points (detonation).


It's basically the fight for getting just the right amount of fuel to air for the biggest possible bang... If you go too much one way you will be harming the engine or making less power (less of a bang).



bryan

 

Good explaination, bryans2k

To simplify, messing around with A/F will increase performance. If you archived the best A/F ratio, you get the most combustion. Combustion is what creates the car HP.

 

Let's get educate.

Detonation :-

Detonation is occurs when air is more abundant than fuel. Oxygen is an essential on combustion. If you lean out your fuel, you will have more air/O2. While the piston compressed this A/F mixture, the abundant of O2 might combust before the spark ignites the air/fuel mixture. When the spark ignites, the A/F mixture already combust thus only portion of un-combust mixture remains will combust.

To simplify, your car performance will decrease dramatically. It's also very harmful to your car.

 

Is this also "Random Misfire in Cylinders". I had this cel twice already.

 

I wouldn't bother trying to tune an S-AFC unless it is on a dyno with at least a wideband o2 sensor. This will provide somewhat accurate monitoring.

Best regards,

Mike
NorthEast Auto Design
www.ForPerformance.com
www.NorthEastAutoDesign.com
1.800.296.6560 ext. 202