I was just reading the mini-motorsport website and it says the ecu upgrade provides "a very safe 13/1 air/fuel ratio at redline." Isn't the ideal 12?? Max 12.5?? Thanks guys.

 

A/F Ratio is dependent on the situation. Are you saying that the Idea AF ratio for the mini is 12-12.5 or in general. YOu need to remember that the mini is supercharged, and that requires more fuel because of the boost

Thanks
Dan

 

>>A/F Ratio is dependent on the situation. Are you saying that the Idea AF ratio for the mini is 12-12.5 or in general. YOu need to remember that the mini is supercharged, and that requires more fuel because of the boost
>>
>>Thanks
>>Dan

I was just speaking specifically for the mini, but isn't 13/1 still rich? I wanna learn more on this subject, there's hardly talk on it. Maybe it's google time!

 

>>I was just speaking specifically for the mini, but isn't 13/1 still rich? I wanna learn more on this subject, there's hardly talk on it. Maybe it's google time! >>


I think 13/1 might still be a tad rich or at least whatever the new ECU upgrade runs is still a little on the rich side. But a little rich is better than lean in terms of reliability. I wouldn't want to run the risk of burning out the valves or something.

 

I think that with the supercharger, and the people that are doing pulley upgrade, they try to stay on the safe side. THe mini even from the factory runs rath rich, they lean it out a bit with the chips, but not too much to still allow people to increase boost with out problems.

 

According to Bosch, maximum power is generated between A/F of 12.5 and 14.0. With forced induction engines, mixtures richer than that are sometimes used since the extra fuel cools the combustion chamber and helps to prevent detonation. The MCS engine runs VERY, VERY rich from the factory (richer than 10:1 in some cases). Sleepless posted a good chart showing his A/F mostly stock (only an intake), then with the pulley, then with the pulley and Evotech:

 

>>
>>I was just speaking specifically for the mini, but isn't 13/1 still rich? I wanna
learn more on this subject, there's hardly talk on it. Maybe it's google time!

No it is not rich, it is more on the lean side of things. Remeber it Air TO Fuel ratio. 10:1 is pig rich, 14.1 is way lean.

Ken

 

14:1 is NOT lean. Stoichiometric ratio is when all of the fuel and all of the air are combined with neither being left over. That ratio is about 14.7:1 for gasoline and air. Any number higher than 14.7 is considered "lean" and any number lower than 14.7 is considered rich.

 

Usually this measurement is taken at the end of the tailpipe. The ideal 14.7 is before it goes into the cylinder and your primary O2 sensor should be sensing (converting Voltage to info), before the cat, how much oxygen is left over over. The secondary after/in cat O2 sensor should be sensing less. When tuning on a Dyno, 13:1 is typically chosen as the tuning rule of thumb as the ideal ratio for the extremely downstream dyno's O2 sensor. If you tune it to that theorhetical 14.7 you will see a lot of knocking and ignition retardation before seeing redline because in actuality what you are getting is a very very lean 16-17:1 A/F ratio going into the cylinders.

 

I have a bung welded upstream of the cat and have done quite a bit of logging with a wideband A/F meter. You are correct, there is a difference between a tailpipe sniffer and measurements taken upstream of the cat. Here's a dyno on my car completely stock, and with a pulley:

 

>>I have a bung welded upstream of the cat and have done quite a bit of logging with a wideband A/F meter. You are correct, there is a difference between a tailpipe sniffer and measurements taken upstream of the cat. Here's a dyno on my car completely stock, and with a pulley:
>>

Andy,
So in other words as the MINI gets stronger in its higher RPM band, the air/fuel misture with the stock ECU gets richer and richer? Further away from the optimum performance levels??? That's odd. I guess an ECU upgrade with a leaner A/F mixture -esp towards redline - really should make a big difference!

 

While your are correct as to the AF ratio required to reach Stoich, the complete burn of all fuel and by products, you are incorrect on what is and is not lean.....especially for a forced induction motor.

You tune your supercharged motor to a 14.7 to 1 air fuel ratio and I promise you that your motor wouldnt make it for very long before you made a sacrifice to the gods of aluminum.

Supercharged motors create a lot of heat, the added fuel is needed to keep the heat down and prevent pre ignition.

14.7 to 1 is way lean.

 

I always thought the mix at WOT, high rpm should be around 12-13:1 for a blown motor. I don't think the Mini is there yet...Andy's chart shows the A/F passing 12 at 4500 and getting way richer.

 

Maximum combustion chamber pressure is centered around 12.5:1, keeping in the range of 12-13:1 will yield the most power, all other things being equal. What surprises me is how rich these cars run from the factory! The chips in my Porsche turbo are currently running in the 10-11:1 range and I've left a stream of unburnt fuel behind my car on the floor of the dyno facility! I don't have a cat on that car, but you guys with cat's will be destroying them with alot of hard motoring...

 

Punisher wrote:Please read (or re-read) the other posts I made in this thread.