How accurate is a tailpipe sniffer for a/f?

[svt03cobra10th]

Did the search didn't get much info.

Just wanted some input on how accurate these things are, in general. Dynojet dyno -- the closest one to me. Doing extensive mods next week and don't want to screw up.

Thanx

Don

 

[Shadowgray03]

Ive heard differences range from as little as .5 to as much as 1.5 which is why a bung for the wideband O2 is so important in my opinion.

 

[marcseidler]

As long as you get a decent long pull they are extremely accurate. I have a LM1 that I use and it is within .1 to the lean side using a tailpipe sniffer vs the pre cat bung. It takes a few seconds (5-10) for it to settle down. I usually run it on the street with a long 3rd or 4th gear pull from 2000 - 6500 rpm. It settles down at around 4000. Mine shows almost 12.0 flat from 4-6500 with the new RWTD tune that fixs the ford rich dump at 5500 from one of the temp sensors (I am told). If anything using a tailpipe sniffer after the cats reads to the lean side so the readings are safe to go by.

 

[jordanvraptor]

I saw a thread awhile back where they did back to back runs hooked to a bung in midpipe before cats and then a sniffer. There was only a slight difference in the low rpms and hardly any difference at WOT. If you look at how it detects air to fuel ratio and the effect of cats, the sniffer would likely give you a leaner reading since the cats take out some of the fuel from the exhaust. So, I believe that if you have a safe A/F ratio from a sniffer, it is safe with a little insurance padded into it. The mixing of extra air coming back through the tailpipe only seems to be a factor at low rpms because the pressure of WOT ensures all gas being metered is exhaust gas. This is just my opinion, I might be wrong as I'm not a rocket scientist....

 

[PhillyCobra]

Agree with prior posters that at high exhaust flow rates it doesn't make much difference.

People assume the convertor only does oxidation-- combining extra O2 with unburnt fuel and carbon monoxide, resulting in less O2 after convertor and richer reading. However, convertors also do chemical reduction, breaking NO down to N2 and O2. So the net effect, while variable dependent on conditions, is that oxygen content of exhaust doesn't change much before and after cat.

 

[svt03cobra10th]

Thanx guys, for the info