Dyno tuning

[HNK]

What is the difference in reading AFR from the tail pipe as opposed the rear O2 sensor bung?

[EC_Tune]

Can be positive or negative differential. I've seen both.

[HNK]

Can be positive or negative differential. I've seen both.

So if lets say i get a ratio of 12.4 from the tail pipe, that could mean leaner or richer from the collector? I remember reading somewhere that tail pipe reading would read leaner by about .3-.5.

thnx

[WS6FirebirdTA00]

Like Doug said, it could swing either way. I have seen dead on numbers, I have seen lean numbers, I have seen richer number. It all depends on the WB setups being used and if there are any leaks.

[brianko]

So if lets say i get a ratio of 12.4 from the tail pipe, that could mean leaner or richer from the collector? I remember reading somewhere that tail pipe reading would read leaner by about .3-.5.

That's about right, at least with my setup. Also, remember that the A/F curve needs to be shifted slightly to the left, since there is a small change delay with tailpipe numbers.

I don't put a lot of weight in tailpipe numbers that are in states of transition (for instance, when PE kicks in). Once the A/F values settle down, you get the data you really need.

Best of all worlds is to read A/F at the collector. But that's not always possible.

[EaglePerformance]

Most dyno runs are supposed to be done at the gear in the tranny that comes closest to 1:1, the thought being that you will be at maximum load gear wise and speed, (aerodynamic). A good loaded dyno will simulate this. Part of the problem using a intertia dyno like a DJ 248 is that the force needed to spin the roller, (it is a fixed mass, HP is figured from an acceleration formula), which is about 2600 lbs, is that unless your car is exactly that weight, the load you are tuning for will be different. So it is far better to be a bit rich in the lower gears than too lean in the top gears. A 4,000lb car tuned to the max on an inertia dyno will be lean and over advanced in the real world. Sharp dyno operators, (with a LOT of experience), have figured this out and make allowances, and will finish tune on the street/strip. Lower rpm, higher vacuum loads can be simulated on a loaded dyno, so drivability, fuel mapping is able to be made right.