Dyno Tunning & The Effect On A/f Ratio By Cat

[R31Heaven]

I was talking to a Mech yesterday on getting my car dyno tuned and he said you cant do it properly if you place the probe into the end of the exhaust, as when the cat warms up and is at operating temp it shows an incorrect ratio. He said the only way to do this is to drill a hole and weld a nut in the pipe in front of the CAT and place the probe there. Has any one done this? Also does the probe have a screw in adaptor if so what size nut would be needed?

Cheers

[Guilt-Toy]

What a load of crap! I have never heard of this before and EVERY tune shop i have ever been too puts the probe in the exaust pipe at the rear of the car.

If you put the probe in the dump pipe in the standard oxy sensor position it can get very very hot and then you start to get weird readings....

You can get a fitting that you weld into your exhaust from just about any exhaust shop. sure the reading may be a little more accurate in high HP applications but anything within 250rkw i wouldnt worry about it...

[Robo's]

Ive been told about this too by a few experienced dyno operators. They prefer to punch a hole before the cat and read the measures from there. I think it would give a more accurate result but still i cant complain about mine being measured from the rear.

[Beer Baron]

it does make quite a difference. from my personal experience with this it seems the readings taken post cat are on average richer than those taken without a cat. i couldn't say if it's always like this, but that was my observation with my own car.

[yogi000]

its a fact you'll get different reading pre cat and post cat.....most tuners know this and know how much the cat usually affects this so they tune accordingly.

[wrxkilla]

exactly.

although you can also be more acurate with tuning before the cat.

mick

[RB30-POWER]

At high load and rich mixtures the cat wont be able to make any impact on the readings. So provided you allow for the slight lag of the gas reaching the rear of the exhaust for measurement it will be fine when tuning.

At light load lean mictures and around stoich mixtures, the readings at the rear of the exhaust or after cat will not be entirely correct. But then again these mixtures aren't real critical anyway.

Sometimes tuners just prefer the quicker response of the lamda sensors placed in the turbo dump or pre-cat positions so its easier to tune.

It will only cost a few bucks to get a 18x1.5 bung put into the downpipe, so just get it done if thats what the tuner prefers.

I have a wideband that actual has a simulated narrow band output as well, so i basically remove the factory sensor, screw in the wideband, and just use the narrowband output to connect to the ecu and the wideband output to the laptop for accurate readout.

Makes so very easy.

[R31Heaven]

I think I will get a wide band sensor and gauge and add it into the pipe, just before the cat. How do they go with the heat from a Turbo?