Exhaust back pressure? What exhaust back pressure? Aren't these race vehicles with unrestricted, open exhausts? Unless the sensors are pre-turbo, you don't really need to worry about exhaust back pressure unless you've got some kind of restriction which I'm guessing most race vehicles won't have. Now, if going pre-turbo, you'd probably want something like THIS.
This is a very incorrect assumption. We are dealing in Absolute pressure, not gauge pressure here. For those of you that may not know or understand the difference, Absolute pressure is Baro + measure pressure, so that means as you change in altitude you move from one line to another as well as side to side on the chart I provided. So if you connect a gauge to the exhaust and watch the exhaust pulse come down a pipe you are going to see a pressure increase each time the pulse travels down the pipe and thats what you need to know, the absolute pressure when the sensor is read.
Each line on the chart I made for you all, is a different altitude and you follow the curve of the line based on absolute exhaust pressure. You would have to measure your system to see what you have at the location of your sensor but it is not uncommon to see +/- 0.5 bar pressure changes on a open exhaust and that will rise as engine speed and volume increase. So based on the total power output you will and do get different readings. So what you read at say 2000 RPM versus what you read at 8000 RPM can and will have completely different correction facts needed to get an accurate readings.
Also remember what I told you all before, and that is the curve I did was for 13.23 AFR ONLY! As the mixture goes richer the curves change and the amount of mixture change gets greater and greater. As the mixture goes leaner the curves get smaller too, so as you get back to a 14.68 mixture there is next to no curve at all and the correction there really doesn't much matter. Now as you go leaner than 14.68 it all starts back the other direction again but I did not think any of you would much care about the Lean burn side of things. The range of error at a fixed altitude is going to be about 0.63 AFR if you use the whole chart at a mixture ratio of 13.23. That gets greater as you run richer mixtures and smaller as you run leaner mixture up to 14.68.
For the record I am not saying do not use one, what I am saying is understand the tool and what it can and cannot do! I've seen to many people burned by a meter that they believed to be correct and did not understand why they burned a piston when the data told them the mixture was fine.