Why AFRs and power change after first dyno run?
 

A month ago I did a few dyno runs on a stock 2017 BRZ with performance package. Upside is it seems to make all the power as advertised, the downside is it only does it for the first run, and everything that follows is significantly richer and down on power; see attached picture displaying the first run and the run that followed a few seconds later. Does anybody know why this is?

I left out the other dyno runs out but they very closely followed the "rich run."

Mileage: ~3000miles
Fuel: ~93AKI Sunoco Unleaded (91+100)
Weather: 91F 29.5in-Hg 22% Humidity
Engine modifications: STI radiator cap, Redline 0w-20

It's dumping fuel to cool things on the 2nd pull.

That's about a 4% difference, which is a normal margin or error for dynos.

I don't get the graph. Do you get more power with an 12.79 AFR and less with 10.66 AFR?

Yep, generally, lean = more power. But too lean = kaboom.

Ok with the 2017 ECU their a lot of extra stuff going on with fueling than the older ecu calibrations.

The old logic you had just closed loop and open loop fueling.

the 2017 you have closed loop, open loop and wide open throttle fuel maps.

Also when in open loop (hard acceleration and full throttle) their are now 3 maps the ecu can choose OL fuel map A or B or the WOT fuel map.

It appears it runs the WOT map at full throttle untill the exhaust/cat temps get to a certian level then it default back to one of the much richer open loop fuel maps.

Hence the first runs were probably using the WOT fuel map at AFR 12.5 then as the exhaust/cat temp threshold cut in it switched to the richer OL fuel maps A or B

This is done likely to preserve the life of the primary cat in header (its supposed to last 10 years or something). Once you go catless header (or you dont care about primary cat lifespan) most tuners will likely equalise these tables for max power all the time and run afr arround 12-12.5 for max power at all times. while keeping safe exhaust gass temps

same deal with the multiple timing maps now available.

Tail pipe AFR sensors generally read on the lean side on car with cats (stock their are two).

I guess this is the main reason why they didn't spec the car with a 215ps output and stayed on 205ps. The +10ps are there, but not always ...

Thanks for the info on multiple maps for full throttle.

At first I figured it was catalyst protection, but then a search didn't bring anything up about this engine having EGT sensors, so how does it decide when and when not to run the 12.5:1 map?

It would be nice for me if I could have the 12.5:1 ratio all the time because I'm already down on acceleration compared to most other cars and the gas mileage is suspiciously poor at the track.

ecu uses the front o2 sensor heater circuit to estimate cat temp

basicly measures resistance heater circuit using current through heater as reference.


fuel consumption will always be terrible on track , i use about 30l/100km on E85 on track in a 2013

We were questioning for a long time why the new cars are not so strong, even though we had many dynos with a +15whp. Some insisted that the gains are only +5whp and not +15whp and they were saying that the main difference is with the shorter FD. Now it is clear what is hapenning. Most of the gains are related with the tune and the ECU changes the AFRs after some time. This has as a result to lose power. Thanks for the finding.

I guess the only option is to have a custom tune. Gas mileage will also improve a bit.

All my questions are answered. Thanks Steve99! :cheers:

That's a lot of E85! I've been getting about 26l/100km 9.1mpg on 91AKI gas in the 2017.