I've spoken with people from 2 reputable shops, but never got a clear answer.

When dynoing the car they check afr with a wideband sniffer up the muffler. How is this in any way accurate if the car is equipped with a cat? Cats store oxygen when momentarily lean, and use this to burn excess fuel when momentarily rich. I hardly believe that a cats effectiveness is linear across the entire rpm band. How can they safely tune a car with tampered a/f samples?

Thanks

 

i was wondering the same thing... dynoflash uses a sniffer for their street tune...

 

+2

I also wondered the same

especially since the placement of an egt probe is so crucial

makes me think the same would apply with an a/f sensor/sniffer

 

Even if they take out the 2nd 02 and used like a turbo xs a/f meter that is still after the cat so sniffer or not same reading my 2 cents

 

hmmm.... Good question!

 

No they are not as accurate. Do you want to pay to have a bung installed to use just for tuning? They use the tailpipe sniffer because it is fast and easy. Most people dont have a bung in their DP so the tuner cant use that, so they use the tailpipe.

 

The further you are down steam the less acurate the AFR's will be

 

Less accurate AFR's, but the cats dont affect it enough, dont ask me how I know, I was just at a dyno day in miami today and I asked like 10 different people working there, and one of them gave me this long complex answer, so I just took his word for it.

 

why wud u want to take ur car for a dyno tune with a cat on in the first place. why not just put a test pipe on (which is cheap) which will help with the flow a lil better too?

 

Ask that to someone who lives in Cali and I bet they could explain to you.

 

Well normally wideband in tailpipe is around .5 off wideband in the downpipe, this is with no cat, but not sure how much a cat would affect o2 readings.

 

^very true but the sniffer method is very accurate enough to see change improvements to the fuel and ignition maps...i have a 2nd o2 bung for tuning purposes and then plug it up with a plug when im done...it takes only 5 mins to plug and unplug the sensor!!!

 

I think it's time for me to get a wideband installed. I need the peace of mind after my sniffer tune.

 

Hi

The cat will only work when the mixture is stoic, thats 14.7 for petrol, and the car is running well out of that when on boost, so the method is pretty acurate.

MB

 

Using a sniffer pipe will yeald a difference of up to 0,6AFR towards the rich compared to a bung in the pipe just after the turbo. This makes the tune more safe. IE - if you registeruing 11,6 on the tail after a tune at high RPM then your actual AFR is closer to 11:1 at the turbo. Th cat makes a difference though it tends to interfear more at low revs than at high.

I never tune with a sniffer pipe as this defeats the purpose of a clean power tune, but for the mass market - tune-and-go shops this is afast and costeffective method.

A