Phenolic Spacer Development for VQ35s

[nismology]

Originally Posted by 2slow

Which signals do the S/VAFC modify?

MAF sensor voltage.

[2slow]

Ahhh. Then I was wrong; I keep getting hung up on vehicles not using fuel trims for open loop operation. Whatever metric is used to define load (MAP, MAF, or a calculated/correlated value) is calibrated beyond normal operating parameters (VE). Eventually this ability will run out as peak MAF/MAP voltage or injector duty cycle is achieved.

[nismology]

To my understanding it's a simple formula to determine load as far as these ECU's are concerned: (X) MAF sensor voltage + (Y) Engine speed = (Z) Injector pulse width. The only way the a/f would change is if the target open loop a/f ratio at that particular load cell is different; otherwise it should remain the same.

[throttlehappy46]

Originally Posted by 2slow

Many vehicles don't use fuel trim feedback to modify base fuel delivery tables, or increase fuel delivery even if more air is measured. Based on Aaron's posts, it seems the Maxima's ecu falls into this category. As such, the fuel trims (or calibration method) will keep constant closed loop AFR, but will not accommodate air flow increases at/near WOT. The AFR changes are likely not significant as calibrations generally run rich at/near WOT to cool and protect the catalyst(s).

Nearly all OBDII vehicles require short and long term fuel trims(for emissions compliance testing) which is why often an 'ECU reset' will make the car feel quicker for a few days until fuel trims kill power again by adding too much fuel or retarding timing etc to take into account engine wear, poor O2 sensor output etc.

Nissan ECU's do not carry an 'AFR target' but a base injection pulse width for a certain cell value in open loop. Hence why using a Z32 air flow meter requires a complete retune as it is not trying to maintaina certain AFRs but a base injection pulse for a given amount of air.

Once a change is made to the induction or exhaust system, the volumetric efficiency of the engine will change allowing the engine to run more ignition advance(choked up exhaust promotes more exhaust valve heat and hence more detonation prone etc so reduced timing as an example) but by offering a colder and denser air, the knock retard map is likely to allow more timing with the benefit being that a stronger induction can be generated on the next intake stroke due to a thorough combustion from more timing and denser oxygen air on the just emptying exhaust stroke leaves more vacuum inside the bore, which increases volumetric efficiency

Narrow band O2 sensors are just not accurate enough for open loop AFR control

[Aaron92SE]

Originally Posted by 2slow

I assume you are talking about AFR at WOT (open loop)

Yes, I was only talking about open loop AFR.

[clashez1983]

were could find out how to drop a 3.5 in a 1995 maxima

[2slow]

Originally Posted by throttlehappy46

Nearly all OBDII vehicles require short and long term fuel trims(for emissions compliance testing) which is why often an 'ECU reset' will make the car feel quicker for a few days until fuel trims kill power again by adding too much fuel or retarding timing etc to take into account engine wear, poor O2 sensor output etc.

How do fuel trims affect ignition timing?

Originally Posted by throttlehappy46

Nissan ECU's do not carry an 'AFR target' but a base injection pulse width for a certain cell value in open loop. Hence why using a Z32 air flow meter requires a complete retune as it is not trying to maintaina certain AFRs but a base injection pulse for a given amount of air.

The ECU should carry a base map of fixed injection durations and closed loop AFR targets. The AFR targets can be set with either a software target and a 'wideband' sensor or by a 'narrowband' sensor which activates around a fxed AFR. Without performing more research I do not know how the 5.5 Maxima handles this operation. Without an AFR target, the system would have no feedback error and would be unable to make corrections (via fuel trims). The base map acts as the default settings in the event of oxygen sensor failures and open loop operation.

When changing to another MAF (Z32, etc.) a new calibration is necessary as the new MAF's correlation of voltage to air flow is different. Consequently, this correlation must be 'taught' to the ECU as the original base map references MAF voltage and will no longer provide the correct AFR.

Originally Posted by throttlehappy46

Once a change is made to the induction or exhaust system, the volumetric efficiency of the engine will change allowing the engine to run more ignition advance(choked up exhaust promotes more exhaust valve heat and hence more detonation prone etc so reduced timing as an example) but by offering a colder and denser air, the knock retard map is likely to allow more timing with the benefit being that a stronger induction can be generated on the next intake stroke due to a thorough combustion from more timing and denser oxygen air on the just emptying exhaust stroke leaves more vacuum inside the bore, which increases volumetric efficiency

Yes and no. Increasing VE (air flow) allows more fuel be burned and more chemical energy converted to motive power. Supplying a cooler intake air charge will increase an engine's knock limit (decrease octane requirement) and allow increasing ignition timing towards MBT.

Originally Posted by throttlehappy46

Narrow band O2 sensors are just not accurate enough for open loop AFR control

Yup, that is why most vehicles use 'wideband' primary oxygen sensors.

[2slow]

Originally Posted by nismology

To my understanding it's a simple formula to determine load as far as these ECU's are concerned: (X) MAF sensor voltage + (Y) Engine speed = (Z) Injector pulse width. The only way the a/f would change is if the target open loop a/f ratio at that particular load cell is different; otherwise it should remain the same.

That is correct, usually it is in tabular form with X & Y as axis and Z as the table values. The target closed loop values are almost always a lambda of 1, where other operation modes (WOT, catalyst protection, etc..) use open loop fuelling.