Consistency between Air-Fuel #s on dyno & in-car wideband O2?

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Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by BradH » Fri Oct 27, 2017 2:31 pm

We had a full day of flogging my BB Mopar engine on the dyno last Saturday. I'm happy with the power it made, but none of the carbs tested (most of which are ones I assembled & baseline-tuned) were set up currently where they'd run clean on the street.

The dyno data includes the calculated air-fuel ratios from the air turbine & fuel-flow readings. We also had a wideband 02 tapped into one header collector for checking idle & part-throttle readings separately from the dyno software, but no recorded data from that.

I'll be using an in-car wideband O2 meter for the first time when the engine's back in the car. I'm not expecting the actual #s from the multiple air-fuel readings (dyno software, dyno wideband, in-car wideband) to be the same due to all the variables that can come into play. However, is it reasonable to expect that the trends seen during the dyno session will be the same with the in-car meter? I have in mind readings such as a consistent part-throttle richness, lean or rich swings in the air-fuel ratio at different RPM ranges depending upon the carb, etc.

I'm hoping the in-car wideband will help refine the street tune, but still expect the track will be where the full-power settings are dialed in.

Thanks - Brad

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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by nickpohlaandp » Fri Oct 27, 2017 2:45 pm

How different was the dyno setup vs. what you'll be running in the car? Same sensor? Same headers? I've been taught that the dyno AFR readings are usually more accurate than the widebands that you buy and install in your car, but if you're using the same type of sensor I imagine you'll get the same type of readings, assuming the headers are the same too.
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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by BradH » Fri Oct 27, 2017 2:49 pm

Headers are different (the headers that fit the car don't fit the dyno, and vice versa), although similar dimensions for average primary length, same collector diameter & length. Both are passenger car headers, rather than a dyno header and a car header.

Not sure if the O2 sensors are the same, but I'm fairly sure the dyno wideband was a F.A.S.T. unit and my in-car is an Innovate.

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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by nickpohlaandp » Fri Oct 27, 2017 2:57 pm

BradH wrote:
Fri Oct 27, 2017 2:49 pm
Headers are different (the headers that fit the car don't fit the dyno, and vice versa).

Not sure if the O2 sensors are the same, but I'm fairly sure the dyno wideband was a F.A.S.T. unit and my in-car is an Innovate.
FAST and Innovate are who market the "sensor/gauge" set, but they don't make the sensors. The most common one I've seen is the Bosch UGEO sensor.
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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by BradH » Fri Oct 27, 2017 5:45 pm

I'm 99% sure my Innovate unit has a Bosch, and it's possible the F.A.S.T. used w/ the dyno does also.

Someone posted -- then deleted -- that the sensors would need to be in the same location of the exhaust to be able to show the same readings . That won't be the case, since the in-car sensor will be 6-8" farther back in the exhaust system than where the sensor was fitted to the headers used on the dyno.

The expression: "Close, but no cigar" sounds like it applies here. I wouldn't expect the #s to match, but I do hope to see the same general behavior from one setup to the other all else being the same.

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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by PRH » Fri Oct 27, 2017 9:08 pm

Brad, the o2 set up for the dyno is from Daytona Sensors.
Somewhat handy with a die grinder.

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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by BradH » Fri Oct 27, 2017 9:59 pm

PRH wrote:
Fri Oct 27, 2017 9:08 pm
Brad, the o2 set up for the dyno is from Daytona Sensors.
I'm an idiot... you even showed it to me.

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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by Belgian1979 » Sat Oct 28, 2017 4:35 am

BradH wrote:
Fri Oct 27, 2017 2:31 pm
We had a full day of flogging my BB Mopar engine on the dyno last Saturday. I'm happy with the power it made, but none of the carbs tested (most of which are ones I assembled & baseline-tuned) were set up currently where they'd run clean on the street.

The dyno data includes the calculated air-fuel ratios from the air turbine & fuel-flow readings. We also had a wideband 02 tapped into one header collector for checking idle & part-throttle readings separately from the dyno software, but no recorded data from that.

I'll be using an in-car wideband O2 meter for the first time when the engine's back in the car. I'm not expecting the actual #s from the multiple air-fuel readings (dyno software, dyno wideband, in-car wideband) to be the same due to all the variables that can come into play. However, is it reasonable to expect that the trends seen during the dyno session will be the same with the in-car meter? I have in mind readings such as a consistent part-throttle richness, lean or rich swings in the air-fuel ratio at different RPM ranges depending upon the carb, etc.

I'm hoping the in-car wideband will help refine the street tune, but still expect the track will be where the full-power settings are dialed in.

Thanks - Brad
A dyno is different from real world conditions. For one, when driving you have air pressure building up in the engine compartment, which alters the way the engine reacts. Also air humidity and temp change. Alle these have an effect on the fuel metering of your carb. There is not much use in compare numbers here without taking everything in consideration. Furthermore I don't see much use of a wideband O2 on an engine without FI which is supposed to correct minute differences from ideal in actual working conditions.

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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by RCJ » Sat Oct 28, 2017 8:28 am

I don't try to get the a/f perfect on the dyno.I watch the curve as rpm goes up and after every change we adjust it if needed.I try to keep it " 2 jet sizes on the rich side" by reading the plugs.When it goes in the car with air filters,headers that fit car,air pressure difference's, temps, etc...There will be some change but this is close enough that there is never any problems.While I an guilty of chasing a number on the dyno there is really no reason to lean on it that hard on the dyno, I try to stay consistent with my jetting.

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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by andyf » Sat Oct 28, 2017 11:46 am

AFR calculations based on fuel turbine and air flow can be a lot different than what a wide band says. I'm sure I don't know all the reasons why that happens but in my experience the two numbers rarely match up. Part of it is due to the fact that the fuel turbine is upstream of the carb bowls so the fuel passing the turbine isn't the same fuel going thru the jets. During steady state the numbers should match but when the engine is accelerating during a dyno pull the engine can be pulling fuel from the bowl rather than the fuel turbine.

Wide band in the car should be close to wide band on the dyno but there are lots of reasons why the numbers will change. Did you watch the wide band on the dyno when you changed the engine timing? If the AFR changes when you change timing then you know you have another factor in the equation. I think what it means when the AFR changes with timing is that the combustion isn't efficient. When that happens you can chase your tail tuning the carb when the problem is elsewhere. I've seen the wide band AFR change by a full point when I adjust the timing on an engine. It is a little spooky when that happens since then you don't know if the AFR readings are correct.

By all means put the wide band in the car and watch it. I just don't think I believe a wide band more than reading the plugs or what the time slip says. There is a lot of funky chemistry going on inside of a wideband in a rich burn (AFR of 12.5) so I wouldn't necessarily trust what it tells you.
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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by David Redszus » Sat Oct 28, 2017 12:50 pm

An air turbine and fuel flow meter used on a dyno will not produce the same readings as a broadband O2 sensor.
The turbine/flow meter indicates the volume of air and fuel going into the engine. It does not measure how much is trapped in the chamber and available for combustion, nor indicate any misfire effects. Additionally, combustion is based on the mass of air and fuel, not volume.

Mounting a broadband sensor along with the turbine/meter will indicate the difference between the measurement systems. The broadband indicates the effects of actual combustion and not pre-conditions.

Using the same broadband in-car at the track will again produce a difference in readings because underhood conditions are far different than dyno conditions. A measure of inlet air temp and pressure will confirm the density differences.

Gas temperature differences will sometimes produce a different O2 reading. Hold the engine at a steady rpm and load and note any change in Lambda vs exhaust gas temperature. Using x-y data plots of lambda vs rpm, throttle position and EGT provides a useful insight to the effect of operating variables.

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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by joe 90 » Sat Oct 28, 2017 6:51 pm

With a proper dyno cell you'll have mass flow meters for both air and fuel.
You should also have a 5 gas exhaust analyser.
They'll usually give very close numbers.

Then when you add the WB it's not close to the other 2
They never are.
Over rated.
But if you get a good one you should be able to custom calibrate it so all 3 devices give similar readings.

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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by jmarkaudio » Sun Oct 29, 2017 10:32 pm

andyf wrote:
Sat Oct 28, 2017 11:46 am
AFR calculations based on fuel turbine and air flow can be a lot different than what a wide band says. I'm sure I don't know all the reasons why that happens but in my experience the two numbers rarely match up. Part of it is due to the fact that the fuel turbine is upstream of the carb bowls so the fuel passing the turbine isn't the same fuel going thru the jets. During steady state the numbers should match but when the engine is accelerating during a dyno pull the engine can be pulling fuel from the bowl rather than the fuel turbine.

Wide band in the car should be close to wide band on the dyno but there are lots of reasons why the numbers will change. Did you watch the wide band on the dyno when you changed the engine timing? If the AFR changes when you change timing then you know you have another factor in the equation. I think what it means when the AFR changes with timing is that the combustion isn't efficient. When that happens you can chase your tail tuning the carb when the problem is elsewhere. I've seen the wide band AFR change by a full point when I adjust the timing on an engine. It is a little spooky when that happens since then you don't know if the AFR readings are correct.

By all means put the wide band in the car and watch it. I just don't think I believe a wide band more than reading the plugs or what the time slip says. There is a lot of funky chemistry going on inside of a wideband in a rich burn (AFR of 12.5) so I wouldn't necessarily trust what it tells you.
The air and fuel flow turbines measure the amount of air and fuel used. A Lambda sensor, air/fuel meter like the Innovate, Daytona, and others measure the available oxygen after combustion. EVERYTHING affect this, quality of combustion being primary, as well as different exhaust restrictions, and changes in airflow/pressure at the top of the carb. G-forces, fuel pressure, attitude of the vehicle at launch... the list is long. I would use your dyno info as a guide, if your O2's stray too far from it in the vehicle look to figure out why. Cowl hoods or scoops that cause air to flow across the vent tubes are notorious for causing AFR to go lean down track. Excessive fuel pressure causes fuel to foam in the bowls. Incorrect timing causes poor combustion and will skew the AFR readings. More or less restrictive exhaust will change the pressure at combustion. I could be here quite a while...
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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by joe 90 » Mon Oct 30, 2017 12:19 am

Only on the lean side of lambda 1 there's oxygen left.....they're accurate there.
They're accurate at lambda 1.
They're also accurate in free air because that's where the calibration is done.

On the rich side of lambda 1 the pump cell works in the opposite direction, there's no excess of oxygen, there's an excess of combustibles....that's where they're the most inaccurate.
A WB sensor is more accurately known as a lambda sensor because it actually senses lambda, not oxygen.

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Re: Consistency between Air-Fuel #s on dyno & in-car wideband O2?

Post by andyf » Mon Oct 30, 2017 11:20 am

Yeah, the rich side of things (which is where most performance engines live) is the wacky world for widebands. The wideband uses an internal charge pump to try and figure out what the AFR is. I'm not sure how good it works since I'm not a chemistry major, but I do know that you can fool a wideband pretty easily. Air leaks are probably the most common problem with performance exhaust systems and a small air leak will produce false readings. The widebands also seem to be temp sensitive so you have to watch that.

I don't fully understand why widebands tell you the AFR changes when you change timing. It must be due to the way that the wideband interprets the chemical makeup of the exhaust. I've tried to find this answer before but have never gotten very far. I wish there was a wideband expert from Bosch who would come on SpeedTalk and give us a short seminar. I'd love to hear from the guys who design these things how they work and why they do what they do.
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