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Rich AFR on the dyno

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Ok here it goes, where I come from, a dyno is very limited. Basically there is only 1 4wd dyno. The dyno dynamics. I had my car tuned on the road using opensource. Wideband AFR shows a steady 11.1:1 on the road consistently during wot . Later when I took it to the dyno to see how much power it makes.

So on the dyno, 1st pull: peak boost shows AFR is 11, but has a sudden drop to 10.4 after 5500rpm. 2nd pull: AFR is shows 10.4 still. Back on the road, afr is back to a consistent 11.1:1 during wot.

I have very limited on dyno operation so my question is, what may have caused the afr to become suddenly rich on the dyno? I can only assume that the ECU went into failsafe mode hence causing the rich condition. Could it be caused due to the airflow? I have dyno graph attached.

Attached Files

The first concern would be what are you using for AFR measurement on the road vs dyno? Are they both reading the same?

If the dyno is being used correctly and the ECT, IAT and boost are consistent with what you are seeing on the road then there is no reason why the AFR should be different. There is also a large power difference between the two pulls. What else changed? Is the boost level consistent?

How good is the fresh air supply on the dyno?

If the exhaust isn't vented properly, you can turn a dyno room into a giant Exhaust Gas Recirculation system, and your measured O2 will go down (i.e., the AFR reads rich even though it isn't really). I have seen cars tuned to perfection on the dyno, but when run the next day on the track start off way too lean (like 10-15%).

The fact that it got worse the longer it ran, but changed immediately on the street support that theory.

As Andre said, boost pressure is first parameter to check.

It can change from road to dyno condition due to rolling resistance and gear ratio used, and consequently affect AFR.

IAT can also be affected by dyno ventilation.

As the other mentioned. Do logs on the street and on the dyno an compare them, to find any difference. If the final calculated injection time. boost pressure and barometric pressure is exactlly the same, there is probably some exhaust fumes in the air.It's usuall to see a tick more boost on the dyno than on the streets in lower gears, because you put a lot of load to the engine on the dyno. The load boost difference depends hardly from the duration of the run, how hot the exhaust system is from previous runs and what the cloosed loop boost control (if it is setted up) does.

I made the same experience with rich AFR's on other companys dyno (before i had my own). As longer as i was on the dyno as richer was the engine running. On top of the, the engine makes less power, because the oxigen content in the air is lower and it isn't well for your health.

Many (if not the most) dyno rooms has much to less airflow and a too low air exchange rate. That was the reason why I planned my dynoroom for a long time and have now one of the highest air speed in Europe. The Dynofan press a laminar air flow whit up to 175km/ airspeed through the coolers in the engine bay. Consquently I have the luxury problem, that I have to pay attention to not running the engine to cold instead as usually to hot. :-)

Sorry for the offtopic, but what dyno fan do you have that does 175km/h? How many m3/h does it do at what pressure? Link to the fan?