[iminhell]

OK I ran a dyno the other day and they did AFR with it, but I have a high flow cat on the car (AFR was taken from the exit of the exhaust after the cat).
Does it even pay to take the AFR into account in that case?
I'd imagine all it really shows if if the cat is working well and not anything usable for tuning purposes. Guy running the dyno had commented that I looked lean near the top, I neglected to mention the cat.


Reference:

 

[jetrinka]

Honestly AF ratios outta the tailpipe aren't accurate at all. The most accurate place is basically right in the header collector. The farther back you go, the less accurate the reading is going to be. The cat shouldn't make a difference. If you have the ability to adjust WOT parameters through your tune/chip you could add 2% more fuel at WOT but honestly I wouldn't really worry about it. It is probably reading much leaner than it actually is and right at 7000 RPM it reads around 14.7 which is pretty darn good for a NA car. I would be worried if it was an FI car but its not.

I wouldn't worry about it.

 

[02SVTsilverstreak]

Honestly AF ratios outta the tailpipe aren't accurate at all. The most accurate place is basically right in the header collector. The farther back you go, the less accurate the reading is going to be. The cat shouldn't make a difference. If you have the ability to adjust WOT parameters through your tune/chip you could add 2% more fuel at WOT but honestly I wouldn't really worry about it. It is probably reading much leaner than it actually is and right at 7000 RPM it reads around 14.7 which is pretty darn good for a NA car. I would be worried if it was an FI car but its not.

I wouldn't worry about it.

I second that its not accurate. The thing is we have two O2 sensors for a reason. Why measure behind the last one where everything has already been monitored. That defeats the purpose.

The best thing to do is measure at the first sensor, which is the header collector, Bank 1, Sensor 1. That way you're reading actual AFR instead of a "ECU Corrected" AFR because if you measure at the back, you may be Stoich/Rich/Lean, but you dont know what the O2 sensors are really reading from Bank 1, Sensor 1.

If I'm correct, Bank 1 Sensor 1 reads the AFR, and Bank 1, Sensor 2 verifies the first reading to let the ECU know what the fuel mixture is, and how to compensate to either add more fuel/ or not add as much.

Fumes evaporate, this is why you measure at a more direct location as stated above.

[strongman]

 

[saps]

I second that its not accurate. The thing is we have two O2 sensors for a reason. Why measure behind the last one where everything has already been monitored. That defeats the purpose.

The best thing to do is measure at the first sensor, which is the header collector, Bank 1, Sensor 1. That way you're reading actual AFR instead of a "ECU Corrected" AFR because if you measure at the back, you may be Stoich/Rich/Lean, but you dont know what the O2 sensors are really reading from Bank 1, Sensor 1.

If I'm correct, Bank 1 Sensor 1 reads the AFR, and Bank 1, Sensor 2 verifies the first reading to let the ECU know what the fuel mixture is, and how to compensate to either add more fuel/ or not add as much.

Fumes evaporate, this is why you measure at a more direct location as stated above.

[strongman]

Nope O2 sensors read nothing more than oxygen content in the exhaust gas, and generates a cycling DC voltage, an average of 450 mv is 14.7:1. The second sensor ONLY tracks catalyst efficiency. The first sensor is solely responsible for fuel trims, not the second.

 

[02SVTsilverstreak]

Nope O2 sensors read nothing more than oxygen content in the exhaust gas, and generates a cycling DC voltage, an average of 450 mv is 14.7:1. The second sensor ONLY tracks catalyst efficiency. The first sensor is solely responsible for fuel trims, not the second.

Shit. I guess I was wrong, or atleast trying to find the right track. I always figured thats what it was seeing how when my car was running so rich, i had sensor 2 go bad, and i had black smoke and crap all over my rear bumper.

 

[03OrangeSVT]

I absolutly DISAGREE with you guys... I know many, many shops that use the O2 in the tailpipe to tune.... even TOM uses it. I feel completely confident in tuning a car using the O2 sensor in the tailpipe.

Also as stated earlier the 2nd O2 is ONLY used for Cat efficiency readings.

 

[iminhell]

I'm just going to state again that I do have a cat on the car and these readings where taken after said cat.

I guess the answer I was looking for is, no you can not tune any car via WBO2 after the cat because it's the intended purpose of a catalytic converter to "burn" spent hydrocarbons and return the AFR to a Stoch ratio. SO any reading over the Stioch mark would be either a poor performing cat or just the it was overwhelmed and could not perform it's duty.



Oh, and unless there are exhaust leaks location really does not matter. Nothing will evaporate of perform any type of chemical change (with the exception of a catalytic converted, which is a secondary heat source). The only thing that is affected it reaction time.