986 Forum - The Community for Porsche Boxster & Cayman Owners - View Single Post

blue62 · 05-18-2021, 10:10 AM

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.

05-18-2021, 10:10 AM	#36
blue62 Registered User Join Date: Aug 2018 Location: Woodland Wa Posts: 1,309	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.