posted on May, 19 2010 @ 12:31 PM
So I have to respond to this thread. Like several other of you that are gear-heads, I've spent hundreds of hours under the hoods of cars, or on my
motorcycle. More so I've spent a huge amount of focus on EFI tuning for both N/A and F/I cars, performance orientated of course.
Now in the OP there is a statement made that, to paraphrase, basically states that because the modern EFI system is so sophisticated, there is no need
for it to waste fuel, and that the mandate of a catalytic converter is just a waste of gas.
While I can't say for sure there isn't a conspiracy involved with them, the OP is missing a lot of information that goes into how the tuning, OEM or
aftermarket, that goes into these modern ECU's work.
To understand the fueling output on a modern vehicles one needs to understand that most ECU's use a combination of "open loop" and "closed loop"
During start up, and most normal driving conditions, most vehicles refer to their "Open Loop" tables. In Open Loop mode, vehicles ignore the
majority of their input from MAF sensors, speed density sensors, and 02 sensors.
They instead refer to pre-programmed tables that engineers have filled with fuel, timing, and ignition values based on their calculated needs for the
This is a very logical process and they are generally very good at it, but understand that even if they use multiple tables for the same values, they
are still "estimating" the values. Ie: they can only take so much into account.
Generally they pick values that will work between sea-level and 5,000 feet, at temperatures between 0 and 100 degrees. Because of those relatively
broad values, a car is never "perfectly" tuned for it's location from the factory.
Variances in temperature, humidity, elevation all have an effect on the vehicle and how much fuel it's adding. Vehicles running at the lower and
upper edge of these tables can be running more, or less fuel, than optimal. While the car will reference their other sensor input to make minute
changes, in Open Loop, it's almost 95% based on it's preset tables.
In "Closed Loop" mode, the car uses the full input available to it by the o2 sensor, MAF or MAS, and then uses that data referenced against pre-set
tables to determine optimal A/F ratios.
Closed Loop activation triggers vary from manufacturer to manufacturer, but typically only occurs during more than 80% throttle position, or often
times by a combination of Throttle Position (TP), Load, and RPM.
Once in Closed Loop, the car primarily relies on it's 02 sensor, reading to see the value of unburnt fuel left in the vehicles exhaust stream.
Depending on the value, it either adds fuel, or lessens fuel, up to pre-set maximum/minimum values in the appropriate Closed Loop tables.
So, we now know that the pre-set tables for standard driving are not 100% precise. We also know that under WOT applications (wide open throttle) the
car is better able to precisely monitor A/F ratios (air/fuel).
The next thing to understand is that while the ideal/stoichiometric A/F ratio is 14.7:1, manufacturers almost NEVER aim to hit that. Car's produce
more torque, and run more reliably, under slightly rich conditions. Rich refers to an excess of fuel to air. Many manufacturers target around
13-13.5:1 for naturally aspirated cars, and as low as 12:1 for forced induction applications.
Running rich also helps prevent premature detonation or "knock." Because of this, almost all table values where the throttle is open more than 5%
are richer than they technically need to be.
So now that we know that the cars are purposely built to run rich, and that running rich is also a "safety-net" against knock. We know that almost
all the tables on a car will be slightly rich, so the car is always using excess fuel.
Continued in second post...
[edit on 19-5-2010 by UnmitigatedDisaster]