Quote:
Originally Posted by blue62
IKE
Most people tend to think that the O2 sensor signal drives the DME/ECU.
But what really happens is the DME/ECU drives the O2 signal.
The O2 signal on these cars operates between a low of around .1 millivolt to a high of about .950 millivolts. Stoichiometric or 14.7-1 is at .450 millivolts.
So the DME/ECU goes a little rich to drive voltage down then goes a little lean to drive volt up. Constantly. The DME/ECU uses the response from the O2 sensor as feedback (closed loop) info to determine fuel requirements or fuel trim to drive the O2 sensor signal.. Which in the end is about Catalytic Converter performance.
Stoichiometeric or 14.7 lbs. of air to 1 lb. of fuel is the ratio at which there is exactly enough molecules of air to combust ever molecule of gas.
So 14.7-1 produces an exhaust that is most easily cleaned by the Catalytic converter.
To rich (excess gas molecules) Cats plug up. to lean(excess air molecules) Cats burn up.
But 14.7-1 it is not the ratio at which fuel air burns the coolest or produces the most power.
That is why you see a richer mix at WOT or high load and leaner mix at idle or very little load.
Engine performance needs vrs Cat converter needs.
Most of the time cars are operated in the lower end to middle of the cruise range as far as engine load goes. That's why you see the air fuel ratio closest to 14.7-1 in that range. 14.7-1 meets engine needs and Cat needs equally in that range.
The shutting off of fuel and or spark at off throttle conditions does save fuel but it is more about pollution.
With the constrains put on automobile manufactures by the EPA everything (as far is engine performance goes) is about pollution control. Performance (power) or fuel mileage is secondary.
EPA restrictions are the biggest reason you can get more power and sometimes fuel economy from an after market tune. It is also why after market tunes are illegal in some states.
|
Blue, thanks for the knowledge brother. Always more to learn about this stuff. I've been working through an automotive textbook to get a better understanding of different systems and this convo definitely put the fuel chapter next on my list.
I will say that the last drive I took (15 miles home) the afr stayed 14.5-14.9 the whole time during closed loop operation. I guess the stfts really did play into that. Which I think is interesting because it implies that the dme is programmed to run lean until it can dial in 14.7 more precisely, or that this setup provides more air than the stock 996 setup (less likely since the MAF should be able to dial this in precisely without the help of the fuel trims). Thinking this through though it may just represent a difference in MAF housings since I am using a 3.25" diameter honeycombed pipe from a BMW V8 intake - same diameter as the 996 but maybe with fewer curves in the piping I am flowing air more efficiently.
I'll have the battery unhooked while replacing the plug tubes so I'll be curious if it goes back lean for the first drive after that.
Sent from my POCOPHONE F1 using Tapatalk