Calling AFR experts

[Norm01]

I read that the AFR reading should be taken under load. Since it's winter here and that can't be done does anyone know if there is a huge difference between AFR readings at any given RPM under load and just reving at no load.

[tirwin]

That is a "it depends" answer.

Ideally, you want a richer AFR at WOT for max power. You want closer to stoich at idle for better emissions. If you look at modern EFI systems as an example, the tables will vary the AFR under different conditions -- idle, tip in, cruising, WOT, etc.

What fuel system are we talking about here? Carbs? CIS? Motronic? Each one has a different way of getting there.

[TimT]

Pray tell why can't the AFR be measured in winter? You can load the engine to 100% or any increment you wish..

[DRACO A5OG]

Originally Posted by tirwin

That is a "it depends" answer.

Ideally, you want a richer AFR at WOT for max power. You want closer to stoich at idle for better emissions. If you look at modern EFI systems as an example, the tables will vary the AFR under different conditions -- idle, tip in, cruising, WOT, etc.

What fuel system are we talking about here? Carbs? CIS? Motronic? Each one has a different way of getting there.

No expert here, but I like to attach my LM2 on my car after she is adjusted at idle hot then drive her around to see WOT, part and cruising readings are.

You could take her to a dyno test shop, I am sure then can turn the heater on for you.

[Norm01]

Originally Posted by TimT

Pray tell why can't the AFR be measured in winter? You can load the engine to 100% or any increment you wish..

Maybe I wasn't clear in my post but what I meant by engine load was referring to driving or loading the engine. I believe reving the engine in the garage with no load to be different

[Norm01]

Originally Posted by tirwin

That is a "it depends" answer.

Ideally, you want a richer AFR at WOT for max power. You want closer to stoich at idle for better emissions. If you look at modern EFI systems as an example, the tables will vary the AFR under different conditions -- idle, tip in, cruising, WOT, etc.

What fuel system are we talking about here? Carbs? CIS? Motronic? Each one has a different way of getting there.

Weber carbs on a 68 2.0S. Bought a AEM x-wifi with the exhaust tip adapter. I know it's not ideal but I like it and it has really helped me dial in my idle. I'm a little rich now at 12.5 but car idles really nice.

[Norm01]

I will rephrase my question. If I were to dial in my carbs with my AFR in the garage with a free reving engine would I or should I expect same readings at same RPM's on the street.

[DRACO A5OG]

It should be close but better verify on the road.

[ischmitz]

Let's think this through (and you will answer your question).

If you run the engine in the garage up to let's say 5000 RPM you will open the throttle maybe 10% - 20%. So now you have one AFR reading.

If you run the engine up to 5000 RPM on the highway in 4th or 5th going up a slope you will
a) be going above the speed limit
b) probably having to put the pedal to the metal (running WOT)

So the AFR at 5000 RPM under load is most likely very different from the AFR at 5000RPM and zero load because the carburetor isn't an ideal device. And you should be more concerned of the loaded readings.

Ingo

[mysocal911]

Originally Posted by tirwin

You want closer to stoich at idle for better emissions.

And while cruising under a light load also for emissions.

Originally Posted by tirwin

What fuel system are we talking about here? Carbs? CIS? Motronic? Each one has a different way of getting there.

They all vary the same parameter, i.e. the fuel quantity, to vary the AFR ratios under differing conditions:

1. Carbs use different jets for variations in fuel quantity.
2. CIS uses movements/adjustments to the sensor plate for variations in fuel quantity.
3. EFI, e.g. Motronic, uses air flow thru a sensor (AFM) or air mass flow thru a sensor (MAFS).

[mysocal911]

Originally Posted by Norm01

I will rephrase my question. If I were to dial in my carbs with my AFR in the garage with a free reving engine would I or should I expect same readings at same RPM's on the street.

No, as expressed in post #9.

[tirwin]

Originally Posted by mysocal911

And while cruising under a light load also for emissions.



They all vary the same parameter, i.e. the fuel quantity, to vary the AFR ratios under differing conditions:

1. Carbs use different jets for variations in fuel quantity.
2. CIS uses movements/adjustments to the sensor plate for variations in fuel quantity.
3. EFI, e.g. Motronic, uses air flow thru a sensor (AFM) or air mass flow thru a sensor (MAFS).

Yes, I understand that. It wasn't clear what the problem was or what type of fuel system the car was. The OP has clarified and I believe has his answer.

[Rawknees'Turbo]

As others have mentioned, there is no useful AFR readings to be had while operating the engine under no load (except idle AFRs) - almost no fuel is required to spin the engine in a no load situation.

[T77911S]

I have driven my car at steady speeds and checked the AFR, then revd it to the same RPM in the garage and got the same AFR's.
I still do a final check at speed but will use the parked checks for quick testing.

[tobluforu]

I have done both, logged data at idle which is great and is what the OP can do to set idle. I have an mfi motor, apples to oranges here. I have revved the motor, kept it steady at certain rpms, adjusted the mfi. Took it out on the road, readings were way off as it needed more fuel. I have done this numerous times in the past 10 years but I'm no expert.
Took 4 hours of tuning the ecu on another turbo car I have with 2 yahoos logging data and making on the fly adjustments while putting it through its paces on the road. After that was done, they adjusted the idle maps, done.
Steady speeds is one thing, wot is another and that is where you burn pistons up.

[T77911S]

there is a difference between steady speeds and accelerating loads as far as how the fuel needs change.
if the OP has CIS there is nothing he can do as far as accelerating loads.

[Norm01]

Originally Posted by ischmitz

Let's think this through (and you will answer your question).

If you run the engine in the garage up to let's say 5000 RPM you will open the throttle maybe 10% - 20%. So now you have one AFR reading.

If you run the engine up to 5000 RPM on the highway in 4th or 5th going up a slope you will
a) be going above the speed limit
b) probably having to put the pedal to the metal (running WOT)

So the AFR at 5000 RPM under load is most likely very different from the AFR at 5000RPM and zero load because the carburetor isn't an ideal device. And you should be more concerned of the loaded readings.

Ingo

Thanks, that answered my question.

[Walt Fricke]

Agreed about WOT vs idle. Except for CIS. Control pressures are always checked at idle, and if the mixture adjustment screw is adjusted for CO there, the WOT and acceleration adjustments are fixed. Not like EFI with a three dimensional map, or how carbs do things with idle jets vs main jets and acceleration pumps.

[dicklague]

You need a bung for the wideband sensor in the exhaust system as close to the head as you can get.

The tail pipe sensor is contaminated by airflow around the pipe. I have never been able to make one work above idle with the car moving.



My AFR is permanently installed on my 1973 MFI 2.7. Bung is at the 2 into one joint.

[Walt Fricke]

Dick - EGT probes get installed maybe an inch down from the port. But all the AFR sensors I have seen are installed after the collector, which isn't really close to the head as such things go, is it? I think that you don't want the really hot exhaust hitting them.

But you make a good point about contaminated flow. The dyno shop I use gets fine results with the tailpipe sensor, but the car is just sitting there.

It isn't hard to weld in a bung.

What brand do you have?