How accurate can the A/F ratio be if the sniffer is used after the cat? I noticed that some dyno tuners use this method. I read that its not accurate for a wideband so why do tuners use sniffers after the cat? Is there some sort of correction for it?

 

If the cat is still intact, the reading at the tail pipe my differ a half of a percent, but it's always leaner at the tail pipe... In a catless exhaust the readings are pretty much dead on at both locations...

 

All fuel burning in the exhaust would have been done BEFORE hitting an O2 bung?

 

it doesn't work that way - based on your logic, you would get no reading at the tail pipe... I'm speaking from practical experience,
I'm sure any tuner on this site would validate my comments.

 

I am not a professional, but from what I've observed is that the AFR tailpipe sniffer can be QUITE accurate if it is calibrated properly. The only unknown for me is I'm not sure how they do the calibration, but I'd assume they do it with an external wideband that is already calbrated

I compared a calibrated AFR tailpipe sniffer to the readings from cipher on the factory widebands, and it was very very close.

 

The differences in between the cats and tail pipe may be off .1 or .2 of an AF point at most. We however 99% of the time monitor the AF as close to the manifolds as we can get..

 

IIRC, Julian measured at the tailpipe AND at the cats when he tuned my car. I guess he went with an average of the two?

 

Actually I tuned with the O2 closer to the manifols/cats.The tail pipe is usually to back up and for the dyno read out.

 

my AEM UEGO read about .5 richer than SGP's sniffer at the tail pipe.

 

i have cats on my car and my uego bung is before the cats. when vinny ten tuned my car we used the sniffer in that bung. It was very close to my aem display.

 

I don't think you understand what I'm saying then...

Fuel obviously burns in hot exhaust pipes, correct? I'm not saying that ALL FUEL in the exhaust is burnt before leaving the tail pipes, I was asking, since you stated that there is almost no difference between the AFR reading at an early O2 bung or a sniffer at the tail pipes, if all fuel that IS burned in the exhaust (notice, not ALL FUEL IN THE EXHAUST), does burn before that first O2 bung, meaning NO fuel is burned between O2 sensor and tail pipe...?

Basically what you made it out to seem is that there is the exact same amount of fuel in the exhaust from the header to the tail pipe and I was questioning that.

Lastly, what if someone is using stock cats? Do you take out a factory O2 sensor from one of those bungs or do you just resort to sniffer? (IE, no extra bung to plug in for reading AFR)

 

That's exactly what I meant to say - in a catless system, the readings are pretty much the same.. in a catted system, the readings are less than a half percent leaner at the tail pipe.

Your understanding of how fuel burns seems to be of the novice level at best.. it takes a lot to make fuel burn… having some gas touch a very hot surface isn't going to make it burn, hence why we have spark plugs in our motor - to ignite the flame...

If gas touching a hot surface would cause it to immediate burn there would be no way to control pre-ignition and/or detonation. Think about it for a second, the intake valves open and fuel is dumped into the extremely hot cylinder (by your logic it would instantly burn, not the case), its the spark from the spark plugs that ignite the fuel and causes the controlled explosion...

 

Gentlemen, keep in mind that the sensors are measuring oxygen, not fuel. My understanding is that an oxygen sensor is constantly making a comparison between the oxygen inside the exhaust manifold and air outside the engine. If this comparison shows little or no oxygen in the exhaust manifold, a voltage is generated.

A catalytic converter takes unburnt fuel and CONVERTS it into carbon dioxide and water. It acts as a catalyst and lowers the activation energy required for this transformation (i.e. no spark required, but a great deal of heat is still required to allow the reaction to occur). So there IS LESS FUEL at the tail pipe with a cat in place.

The number of oxygen molecules should remain relatively the same, however, following the burn in the cylinder.

 

Ture, but that information is used to determine the amount of fuel in the exhuast gasses - hence why they call it a Air / Fuel meter and not just an Air/oxygen meter...

 

Now that you mention that Philthy, I realize my post above is only half true (it is true for narrow band sensors). For widebands in a rich condition, a measured amount of O2 is supplied to react with unburnt hydrocarbons until the O2 level climbs (which is still what is detected). So fuel is being measured (as you pointed out) albeit indirectly. In this case, in rich conditions, a wideband should have significant loss of accuracy after the catalytic converter. In lean conditions, it should be spot on. Theory only - based on my understanding - u guys saying that ain't so though, so... unless your observations are at idle?

 

I'm not certain what you're confused about...? In a catless system, the tail pipe readings are pretty much dead on. In a catted system, the tail pipe readings are aprx .2-.5 leaner than a pre cat reading...

If a tuner is using the tail pipe sniffer in a catted system, they just factor the variance into their targets...

 

Philthy speaks da truft..

 

ha ha - no biggie - just wondering why it's not a bigger difference in rich conditions with a cat in place...