Lambda vs MPG & HP

[2000StreetRod]

My 1991 copy of "Bosch Fuel Injection & Engine Management" shows on a graph that maximum power occurs when lambda is 0.86 (A/F=12.6:1) and minimum fuel consumption occurs when lambda is 1.05 (A/F=15.4:1). Most of the O2 sensor plots I've seen on this forum show a normal fluctuation between 0.1 volts (lambda = 1.02) and 0.9 volts (lambda = 0.98) with a mean of 0.5 volts (lambda = 1.005). This means that best fuel economy is being slightly sacrificed [(1.05-1.005)/1.05 = 4.3%] to reduce NOx emissions which is greatest at lambda = 1.03. This is because three-way catalytic converters are only effective in suppressing NOx emissions when lambda is less than or equal to 1.0.
Has anyone used a custom tune that leans the mixture to lambda = 1.05 resulting in a mean O2 sensor voltage of less than 0.1 volts? If so, how did it affect mpg?

 

[2000StreetRod]

A/R @ max throttle for stock SOHC?

Is there anyone with a wideband A/F meter still driving a stock SOHC? If so, please post the A/F ratio at max throttle so I can compare it to mine. Thanks!

 

[2000StreetRod]

Max throttle A/F ratio?

Is there anyone on the forum driving a stock Gen 2 SOHC who knows their A/F ratio at full throttle?

 

[Jakee]

I can give you the 2001 stock air/fuel for an explorer sport.

The top is lambada; the bottom is converted to air/fuel

 

[2000StreetRod]

Very helpful!

Thanks so much for the table Jakee! I have one question. What is the meaning of column B that varies from 700 to 100? I assume that it relates to throttle position but I can't get the correlation. It might be CFM but I thought a normally aspirated SOHC has a maximum of around 550 CFM. My scanner measures throttle position as absolute and it varies from 18.8% to I believe 92% which corresponds to a TPS voltage of 0.97 to 4.7 volts.

 

[Lignite]

streetrod, I have an a/f gauge on my Ranger (with 1997 expo 5.0). It runs a mean a/f of about 15.0 until you get into it, or at least it used to, but thats another problem. This is on a custom PCM tune. Of course I can't compare fuel mileage, but in my own opinion its quite good with the large mud and lift on this rig; about 16-18mph if you drive decent.

 

[2000StreetRod]

thanks!

Thanks for the reply Lignite! I've read that max power is theoretically achieved with an A/F of about 12.5:1. I also run around 14.7:1 during normal driving which is controlled by the closed loop using the O2 sensors. Currently with my Lightning 90mm MAF sensor and associated amplifier, if I stomp the accelerator my A/F ratio drops below 10:1 (the lower limit of my meter) and the engine stumbles. The A/F is OK to 6,000 rpm if I don't stomp the accelerator. During the next few days I will adjust the amplifier for a richer mixture at idle and a leaner mixture above idle to improve the throttle change response. I was looking for someone's actual WOT ratio to compare to theoretical.

 

[Jakee]

You're way rich.

It's throttle position. It doesn't quite fit the range but also notice once you get above 550 TP the Air/Fuel is the same. The trigger in the stock tune for open loop is around 550 TP. Basically, once you go WOT you want to get rich quick to prevent pinging and other damage.

Here's what it looks like..

RPM TP
16383.75 550
4000 550
3500 500
0 500
0 500
0 500
0 500


The table I posted is what the truck is supposed to do when in open loop. (The O2's are not giving the PCM feed back). When in closed loop, the stoich fuel for gas is 14.6 and that is what the PCM will continue to keep fuel at. If a wideband says the air/fuel is not at stoich when in closed loop then you'll more than likely already of thrown a code or your MAF is out of whack.

Most people with the 4.0 SOHC has had better luck with a WOT air/fuel at around 11.7. Of course, it's not just about air/fuel but also timing at the commanded air/fuel ratio. Sometimes you'll get more power by running a little more timing and a richer air/fuel. The tricky part is there is no two engines the same. The only way to find the optimal combo is with some sort of precise measuring tool.......a dyno, or many hours of analyzing data from logs with timed track runs.

 

[2000StreetRod]

Change in TPS?

You're way rich.

It's throttle position. It doesn't quite fit the range but also notice once you get above 550 TP the Air/Fuel is the same. The trigger in the stock tune for open loop is around 550 TP. Basically, once you go WOT you want to get rich quick to prevent pinging and other damage. . .

The table I posted is what the truck is supposed to do when in open loop. (The O2's are not giving the PCM feed back). When in closed loop, the stoich fuel for gas is 14.6 and that is what the PCM will continue to keep fuel at. If a wideband says the air/fuel is not at stoich when in closed loop then you'll more than likely already of thrown a code or your MAF is out of whack.

Most people with the 4.0 SOHC has had better luck with a WOT air/fuel at around 11.7. . .

I think I'm starting to understand the table and what you're telling me. Thanks for your patience.

My Bosch Fuel Injection & Engine Management book states that most stock engines reach maximum volumetric efficiency around 3500 to 4000 rpm. At higher engine speeds the airflow into the engine per stroke actually decreases. Fuel delivery becomes a function of engine rpm rather than MAFS measured air flow. That might explain why a MAFS that's pegged doesn't necessarily result in a lean mixture.

It also states that when the accelerator is rapidly depressed the ECU (PCM for Ford) measures the TPS change rate and increases the pulse width accordingly. I have read - and you have confirmed - that if the TPS change exceeds some value the fuel control reverts to open loop. I'm guessing that the "throttle position" in the table you have kindly provided is some parameter that indicates change in TPS vs time rather than the actual throttle position.

As far as setting DTCs is concerned, with my MAFS amplifier and associated pillar pod mounted richness control I can easily vary the A/F ratio over a large range when in open loop. I have varied the A/F ratio from 10:1 to 16:1 (my meter limits for gas) and not set a DTC. In closed loop DTCs are more easily set and the range is decreased.

Thank you for providing the WOT best power A/F ratio (11.7:1) for an SOHC. That is exactly the information I needed!