AFR/Lambda readings on a stock bosch FI ?

AFR/Lambda readings on a stock bosch FI ?

Joined: August 6th, 2005, 6:03 pm

June 11th, 2008, 2:13 am #1


Hey guys, has anyone monitored Air/Fuel ratio on a stock bosch setup ? Yeah I know it should be "about 14.7:1" but had anoyone measured it ?

Today I was monitoring the stock L-jetronic with a seperate wideband (not connected at all to the Bosch) and I saw the bosch running a bit richer than I would have expected. Any insight as to how rich it should be running ?

From the glass lined tanks of old Bertone
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John Allen
John Allen

June 11th, 2008, 3:25 am #2

all narrow band O2 sensors are set to 'switch over' at 14.7:1 and the Bosch ECU will richen or lean out the mixture based on what the voltage reads.

If you mounted your wide-band in the bung that the narow band was in AND didn't connect it back to the ECU, the "low voltage" signal the ECU recieves (since no O2 sensor is connected) will tell the ECU to richen up. The amount of correction the stock ECU provides I suspect is pretty small. Your wide-band reading of 'rich' is probably correct given the ECU's correction for no O2 sensor input.

Also, when are you reading the AFR? At idle and full throttle the O2 correction is switched off (by way of the TPS) and intentionally runs rich at full throttle and can e made to run rich at idle via the air bypass in the AFM.
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Joined: August 6th, 2005, 6:03 pm

June 11th, 2008, 4:05 am #3

A new narrow band was installed to feed the L-jet (only) and a seperate wideband was installed further downstream just before the cat. WB was only used for loging, no feedback to the ECU. My comment was about AFR during cruise (10-15"Hg). Mixture at idle and decel were where I'd expect them.

Idle was 13.7-14, cruise at 14.1-14.5, and closed throttle decel was 20+. At cruise, the AFR dithered about every 0.6 seconds. Datalogs were taken, but in a queer format.

Fuel pressure was verified to be in spec, I couldn't think of what else to check. Narrow band and wide band were mounted about 6" apart. NB grounded through it's housing, WB grounded to the valve cover (using #10 copper, on the same stud as the EFI ground)

Thoughts, oh wise one ?



From the glass lined tanks of old Bertone
Last edited by Damonfg on June 11th, 2008, 4:06 am, edited 1 time in total.
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John Allen
John Allen

June 11th, 2008, 6:17 am #4

Ok, no one is answering so I will take a shot...

One thing to remember is that the wide bands do need to be calibrated, you probably have done that. 14.1:1~14.5:1 at cruise isn't that far off of stoich. If your narrow band is old or not hot enough it could give errors in that small range.

The cycling you are noticing is exactly how the Bosch system works. It will run slightly lean then correct on the rich side, then back. Averaging between these swings wil be stoich. Remeber that narrow bands can't read stoich, only swing from one side to the other.

MegaSquirt (actually all FI closed loop systems) works the same way. The ECU will start to compensate until the mixture gets too rich, then it will start a cyle of leaning the mixture until it swings below stoich and starts all over again. The Bosch system is pretty slow, as you notes - about 1/2 second swings - but MS can be setup to react very quickly. Mine is set up to adjust every 16 igition cycles.

I think you are OK, the 'rich' condition you see are not bad at all. The cat should be able to cean up the exaust at those AFRs.
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Joined: August 29th, 2005, 1:53 am

June 11th, 2008, 2:23 pm #5

A new narrow band was installed to feed the L-jet (only) and a seperate wideband was installed further downstream just before the cat. WB was only used for loging, no feedback to the ECU. My comment was about AFR during cruise (10-15"Hg). Mixture at idle and decel were where I'd expect them.

Idle was 13.7-14, cruise at 14.1-14.5, and closed throttle decel was 20+. At cruise, the AFR dithered about every 0.6 seconds. Datalogs were taken, but in a queer format.

Fuel pressure was verified to be in spec, I couldn't think of what else to check. Narrow band and wide band were mounted about 6" apart. NB grounded through it's housing, WB grounded to the valve cover (using #10 copper, on the same stud as the EFI ground)

Thoughts, oh wise one ?



From the glass lined tanks of old Bertone
John has accurately described how the L-Jet (and most other) systems work.

<<< Idle was 13.7-14, cruise at 14.1-14.5, and closed throttle decel was 20+. At cruise, the AFR dithered about every 0.6 seconds. Datalogs were taken, but in a queer format. >>>

Those numbers actually look pretty good, considering these data points:

<<< Narrow band and wide band were mounted about 6" apart. NB grounded through it's housing, >>>

John is right, the wide band does need to be calibrated. The further down stream from the OE location, the more error you will see.

I have done hundreds of dyno runs using wide band O2 sensors to monitor A/F ratio along with exhaust gas temperature monitoring. It should be noted that 14.7 is indeed stoich and where you want to be for best mileage and emissions. However, in the typical X SOHC, best power is made at about 13:1 A/F ratio.

Another very important factor regarding A/F ratio is that ignition timing is nearly as important to A/F ratio as the amount of fuel. So before you go screwing around with the mixture screw, be sure the ignition timing is correct.

Steve
1x5
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Joined: August 6th, 2005, 6:03 pm

June 11th, 2008, 2:47 pm #6

Ok, no one is answering so I will take a shot...

One thing to remember is that the wide bands do need to be calibrated, you probably have done that. 14.1:1~14.5:1 at cruise isn't that far off of stoich. If your narrow band is old or not hot enough it could give errors in that small range.

The cycling you are noticing is exactly how the Bosch system works. It will run slightly lean then correct on the rich side, then back. Averaging between these swings wil be stoich. Remeber that narrow bands can't read stoich, only swing from one side to the other.

MegaSquirt (actually all FI closed loop systems) works the same way. The ECU will start to compensate until the mixture gets too rich, then it will start a cyle of leaning the mixture until it swings below stoich and starts all over again. The Bosch system is pretty slow, as you notes - about 1/2 second swings - but MS can be setup to react very quickly. Mine is set up to adjust every 16 igition cycles.

I think you are OK, the 'rich' condition you see are not bad at all. The cat should be able to cean up the exaust at those AFRs.
It seems the only way possible for a L-jet to work is to dither around 14.7:1. The performance was exactly as I expected, but the AFR value wasn't what I would have figured.

There are roughly 100 miles on the narrowband Bosch sensor. It is potentially in a cooler location than stock - mounted immediately downstream of the turbo. EGTs (just upstream of the turbo) were 1250F, rising to 1500F under boost.

The wideband Bosch has 10 miles on it now, freshly calibrated and it reads 20.8%O2 in free air. It is mounted about 8" downstream of the narrowband and 6" ahead of the cat.

There is no fuel enrichment during NA operation.

Attached are two shots of the log:

Idle, open loop:


Cruise, closed loop:


Histogram of the session:



From the glass lined tanks of old Bertone
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Joined: August 6th, 2005, 6:03 pm

June 11th, 2008, 3:28 pm #7

John has accurately described how the L-Jet (and most other) systems work.

<<< Idle was 13.7-14, cruise at 14.1-14.5, and closed throttle decel was 20+. At cruise, the AFR dithered about every 0.6 seconds. Datalogs were taken, but in a queer format. >>>

Those numbers actually look pretty good, considering these data points:

<<< Narrow band and wide band were mounted about 6" apart. NB grounded through it's housing, >>>

John is right, the wide band does need to be calibrated. The further down stream from the OE location, the more error you will see.

I have done hundreds of dyno runs using wide band O2 sensors to monitor A/F ratio along with exhaust gas temperature monitoring. It should be noted that 14.7 is indeed stoich and where you want to be for best mileage and emissions. However, in the typical X SOHC, best power is made at about 13:1 A/F ratio.

Another very important factor regarding A/F ratio is that ignition timing is nearly as important to A/F ratio as the amount of fuel. So before you go screwing around with the mixture screw, be sure the ignition timing is correct.

Steve
1x5
I am not really trying to do or modify anything, just trying to get a grasp on what I am reading.

Timing is 10°BTDC at 900 RPM. Well, that 10° is only as accurate as the mark on the crank pulley. Someone (you?) posted here about measuring the timing on some cam pulleys to get the best advance on a stock pulley, so I thought it was worth checking crank pulleys too. The 5 crank pulleys that I measured seemed to vary 1 to 2° (as measured between the tickmark and the keyway on the pulley)

If best power is obtained at 13:1, how do you get that rich with an L-jet? Were you able to get there with the mixture screw only?


Just trying to understand what is happening, and why. I'm stashing away all of this baseline data as a benchmark for later. MS & EDIS will be happening shortly and this way I can at least compare before and after.



From the glass lined tanks of old Bertone
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Joined: August 29th, 2005, 1:53 am

June 11th, 2008, 4:37 pm #8

<<< Someone (you?) posted here about measuring the timing on some cam pulleys to get the best advance on a stock pulley, so I thought it was worth checking crank pulleys too. >>>

Yep. The front crank pulley is "adjustable". The pointer's mounts are slotted. I recommend you index it to the mark on the flywheel.

Using the cam pulley isn't always accurate either as it is subject to changes in deck height and doubles your manufacturing tollerance error.

<<< If best power is obtained at 13:1, how do you get that rich with an L-jet? Were you able to get there with the mixture screw only? >>>

No. And if you tried to get there with the mixture screw you wouldn't get a linear increase in A/F. I used a fully programmable Digital Fuel Injection engine management system and a custom designed, tuned runner, slide throttle intake manifold and header.


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The system does use a Bosch Fuel pump (although not the OE pump) and the OE injectors and pressure regulator.

Steve
1x5
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Pete Whitstone
Pete Whitstone

June 11th, 2008, 4:38 pm #9

Ok, no one is answering so I will take a shot...

One thing to remember is that the wide bands do need to be calibrated, you probably have done that. 14.1:1~14.5:1 at cruise isn't that far off of stoich. If your narrow band is old or not hot enough it could give errors in that small range.

The cycling you are noticing is exactly how the Bosch system works. It will run slightly lean then correct on the rich side, then back. Averaging between these swings wil be stoich. Remeber that narrow bands can't read stoich, only swing from one side to the other.

MegaSquirt (actually all FI closed loop systems) works the same way. The ECU will start to compensate until the mixture gets too rich, then it will start a cyle of leaning the mixture until it swings below stoich and starts all over again. The Bosch system is pretty slow, as you notes - about 1/2 second swings - but MS can be setup to react very quickly. Mine is set up to adjust every 16 igition cycles.

I think you are OK, the 'rich' condition you see are not bad at all. The cat should be able to cean up the exaust at those AFRs.
How does one calibrate a WB sensor? I am just starting to play with my PLX Devices WB A/F monitor, and I thought the instructions said it did not need calibration. Do all WB sensors need calibration? If so, what are the general steps, or are they specific to the individual sensor/device combination?

Thanks,
Pete
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Joined: August 6th, 2005, 6:03 pm

June 11th, 2008, 4:50 pm #10


I got stiffed by PLX to the tune of $700...... Very long story, but I did get plenty of knowledge of their units and what goes on under the hood though.


PLX does say that their sensor does not need calibration. They do this by using generic data in their firmware. Meaning the data you see will be close*, but likely not exact. If you change sensors later on, there will be an apparent shift in the AFR (but no REAL shift, just an apparation of the firmware being overly general.

On a wideband O2, a typical cal will baseline the O2 content of free air. Typically ~20.8% on planet earth. Some wideband controls also calibrate the performance of the heater element in the WBO2. This allows for the sensor to be brought up to operating temperature quicker.



* "close" as used by PLX means country-frikin-mile. Honest to god I was holding a steady 10PSI boost and their gauge displayed it as 30inHg.


From the glass lined tanks of old Bertone
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