I have the opportunity to participate in a dyno day, but I think the shop uses the tailpipe probe for its wideband sensor. The problems I can think of with this are that I'll have to compensate for the time it'll take the gas to reach the sensor, and also the exhaust will cool off during its travel affecting A/F so it may not be as accurate. Is that correct? Would the wideband controller's temperature compensation negate my second point? Please share some advice for tuning with these things.

 

Don't all chime in at once.

 

Stop obsessing.
The acccuracy deficit cause by sticking the probe up the car's tailpipe is insignificant compared to tuning with no idea about the AFRs. If you are that worried, add enough fuel to increase the reported AFRs a tenth or two.

 

Don't tune on the dyno day... just see what you're running... borrow or buy a wideband, then tune...

I wouldn't tune on tailpipe... alone...

 

I tested my car with a tailpipe sensor and later with bung sensor placed just ahead of the cat and the readings were within a few tenths of each other.

The runs were on two separate days, so the small difference could have been due to different conditions during the runs.

 

If you have a top line AF meter like an Autronic A or B model for example it will read most accuratley at the tail pipe with a probe.

Sensors accuracy are effected by pressure, too much and they throw out readings, NO WB equipment accounts for pressure at the sensor and hence if its mounted in the DP your readings will be WRONG ! (can have up to 5 or 8 psi back pressure or MORE in the DP)

Good meters will read right from 300deg C to 770deg C and temp corrections will be carried out by the units software or programming.

When I use my WB I only ever use it at the end of the exhaust system or close to the end

The other BIG DISADVANTAGE to running in the DP is the HEAT ! This drasticaly reduces sensor life, only real problem with running @ the tail pipe is clearance on some cars and can be difficult to mount on RICE BOY mufflers !

 

I just want to clarify Rice's statement that good meters read right from 330 to 770 degrees C. I am certain he means that the sensors are designed to read right between those temps. All sensors actually require fully heated cells before they produce accurate voltages. Most sensors self heat to over 700C. The meter itself merely interprets the voltage sent to it from the sensor.

 

Okay well, if you're getting 5-8 PSI in the DP, the you have a clogged Cat... a new cat produces 3 Psi, or somewhere around there... 3.5 etc.. Psi of backpressure, so if you're getting more than 5 up to 12 PSI count that against your boost and you're making 3 to -2 PSI of boost... (BAD)

and if you've stuck a EGT in the dp, you know that it idles at around 800deg F (your readings are in C I know) and peaks out maybe as high as 1400 -1600deg F and I know the stock (Bosch / NGK) O2 can handle that and from what I've seen from my FJO it doesn't seem to mind it just turns the heater off sooner (I assume) ...

-DC

 

DC if you drill and tap a port into your DP and measure the pressure @ WOT you will find the figure is quite high !

You will measure the pressure of the DP, the cat/midpipe, the pipe work @ the rear muffler, all of these elements ADD UP to give total system pressures that are quite high !

If you take the measurement 4 to 5" after the turbine you will get a result like I indicated, the further back you take the pressure reading the lower the indicated system pressure. Obviously as there are less things to provide resistance and hence pressure build up.

There are quite a few tuners here who put the sensor in the mid section of the pipe work or at the rear of the system in a test pipe, it is the most accurate location and is the easiest on the sensor in regards to heat stress.

On my Autronic unit when set to most sensitive update seting as soon as you press the throttle you get a change in the A/F, so there are no issues with delayed responce so long as you make a good test pipe.