Posted: Tue Sep 30, 2008 10:08 am
It’s not really that simple, not all motors are the same and because of the shape of combustion chambers, Sparkplugs and piston crowns there are no perfect AFR for any motor.
OK lets start at the beginning the best overall combustion AFR is 14.7:1 this AFR completely burns the available air and fuel (This is atmosphere dependant) but a N/a car normally runs 13:1 or 14:1 AFR and a turbo is normally around 11:1 and 12:1 this is because we are using the fuel to cool the combustion process and to slow down the flame front. Why are we doing this? Because we want to make the motor resistant to detonation and the amount of heat generated at 14.7:1 AFR.
Without getting into a long drawn-out discussion the best over all AFR for reliability would be a Rich around 10.7 to 11.8 on boost, but there are costs to running this AFR your performance will suffer so will you fuel bill and the motor could suffer from some bad choking over time without water injection.
Now for best performance look at around 12:1 to 13.5:1 , this is allot leaner allowing an increase in combustion chamber heat and pressure and an overall increase in power the down side to this the motor is way less resistant to detonation even with water injection also in both cases you overall timing will need to be looked at.
So you have to compromise what I have seen from my access to the big tuner maps is they run lean low down to increase the EGT and so the energy available to the turbo change then run slightly rich on boost around the 11:8 to 12:5 range
OK lets start at the beginning the best overall combustion AFR is 14.7:1 this AFR completely burns the available air and fuel (This is atmosphere dependant) but a N/a car normally runs 13:1 or 14:1 AFR and a turbo is normally around 11:1 and 12:1 this is because we are using the fuel to cool the combustion process and to slow down the flame front. Why are we doing this? Because we want to make the motor resistant to detonation and the amount of heat generated at 14.7:1 AFR.
Without getting into a long drawn-out discussion the best over all AFR for reliability would be a Rich around 10.7 to 11.8 on boost, but there are costs to running this AFR your performance will suffer so will you fuel bill and the motor could suffer from some bad choking over time without water injection.
Now for best performance look at around 12:1 to 13.5:1 , this is allot leaner allowing an increase in combustion chamber heat and pressure and an overall increase in power the down side to this the motor is way less resistant to detonation even with water injection also in both cases you overall timing will need to be looked at.
So you have to compromise what I have seen from my access to the big tuner maps is they run lean low down to increase the EGT and so the energy available to the turbo change then run slightly rich on boost around the 11:8 to 12:5 range