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Follow On From The Mafia's Comment

MrStabby

By MrStabby,
in Engines & Forced Induction

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MrStabby Grand Master

MrStabby

Posted
Posted

The airflow meters are simply joined together so they read the same.

The GTR simply measures both, adds the values together and divides by 2.

I didn't want to pollute the other thread about using RB26 ecu on RB20 - Mafia are you saying that all injectors are trimmed the same based on averged airflow - ie the motor is not divided into two lots of three cylinders, they're just averaged across all 6? Makes sense because if you divided control you could be on multiple points of the fuel map.

What about o2 closed loop? If that wasn't separated what does the ECU do if one o2 reads rich and the other lean?

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GTSBoy Grand Master

GTSBoy

Posted
Posted

Yeah, well your point about possibly being on two parts of the map simultaneously tells the story. There is only 1 TP value calculated by the ECU, so it is worked out by summing both meters together. More to the point, the total airflow just joins back together after the turbos anyway - so there is no way to pretend that the two AFMs and the two turbos belong to half the motor each.

So, because of that, after the intercooler, the engine may as well be any other single turbo RB from the point of view of the ECU - apart from having 2 O2 sensors. But then that's not unusual anyway - most V6 or V8 engines end up with two oxy sensors and the ECU just does whatever it does to decide which of them it's trusting (unless the ECU is tricky enough to do trimming per bank, which I'm sure a lot of newer ones do).

I really don't know why Nissan bothered with 2 AFMs on the 26....it just gives two points of failure.

rob82 Mentor

rob82

Posted
Posted

Yeah, well your point about possibly being on two parts of the map simultaneously tells the story. There is only 1 TP value calculated by the ECU, so it is worked out by summing both meters together. More to the point, the total airflow just joins back together after the turbos anyway - so there is no way to pretend that the two AFMs and the two turbos belong to half the motor each.

So, because of that, after the intercooler, the engine may as well be any other single turbo RB from the point of view of the ECU - apart from having 2 O2 sensors. But then that's not unusual anyway - most V6 or V8 engines end up with two oxy sensors and the ECU just does whatever it does to decide which of them it's trusting (unless the ECU is tricky enough to do trimming per bank, which I'm sure a lot of newer ones do).

I really don't know why Nissan bothered with 2 AFMs on the 26....it just gives two points of failure.

The gtr from r32 will trim independently between the front three and rear three cylinders. And it will apply learnt LTFT to fueling outside of closed loop. Something a lot of so called tuners miss!

STATUS Grand Master

STATUS

Posted
Posted

The gtr from r32 will trim independently between the front three and rear three cylinders. And it will apply learnt LTFT to fueling outside of closed loop. Something a lot of so called tuners miss!

yup hence why both AFM and O2 wires need to be common.

MrStabby Grand Master

MrStabby

Posted
  • Author
Posted

The gtr from r32 will trim independently between the front three and rear three cylinders. And it will apply learnt LTFT to fueling outside of closed loop. Something a lot of so called tuners miss!

Are you talking just o2 feedback or the whole shebang? What does LTFT stand for? XXfuel trim?

rob82 Mentor

rob82

Posted
Posted

Are you talking just o2 feedback or the whole shebang? What does LTFT stand for? XXfuel trim?

I'm only going to comment on what I've seen and that is that there is varying injector puslewidths between the front three and rear three cylinders. I beleive this only to be based on information learned from the oxygen sensors not independant fueling based on differing MAF signals.

LTFT - long term fuel trims. If STFT (short term fuel trims) are positive or negative in a particular cell for long enough the ECU will update that cell with a LTFT of equal value and zero the STFT.

JonnoHR31 Grand Master

JonnoHR31

Posted
Posted

it would only use the o2 sensor and only under light load. the way the gtr's are set up you cant use the afm's to adjust cylinders. still pretty advanced fueling for an ecu that was designed in 88-89

rob82 Mentor

rob82

Posted
Posted

it would only use the o2 sensor and only under light load. the way the gtr's are set up you cant use the afm's to adjust cylinders. still pretty advanced fueling for an ecu that was designed in 88-89

No - The O2 sensors only work at light loads but what they learn in closed loop they DO apply to open load fuelling. Hence if you have negative LTFT numbers in closed loop in the highest load cell then you have a negative fuelling adder in open loop.

MrStabby Grand Master

MrStabby

Posted
  • Author
Posted

More to the point, the total airflow just joins back together after the turbos anyway - so there is no way to pretend that the two AFMs and the two turbos belong to half the motor each.

Facepalm - of course. I must remember to engage my brain.

MrStabby Grand Master

MrStabby

Posted
  • Author
Posted

No - The O2 sensors only work at light loads but what they learn in closed loop they DO apply to open load fuelling. Hence if you have negative LTFT numbers in closed loop in the highest load cell then you have a negative fuelling adder in open loop.

Interesting. I would guess many tuners dont bother to do much with the closed loop cells, because the closed loop will fix them. But given that behaviour, taking the trouble to set the closed loop cells open loop and tuning them to about 14.7 will be important to make sure the LTFT doesn't bugger up your tune!

I'm pretty surprised the old ECUs have LTFT - its impressive for that vintage.

Thanks for the info :cheers:

STATUS Grand Master

STATUS

Posted
Posted

Interesting. I would guess many tuners dont bother to do much with the closed loop cells, because the closed loop will fix them. But given that behaviour, taking the trouble to set the closed loop cells open loop and tuning them to about 14.7 will be important to make sure the LTFT doesn't bugger up your tune!

I'm pretty surprised the old ECUs have LTFT - its impressive for that vintage.

Thanks for the info :cheers:

As with any tune though the cruise areas are setup first to make sure latency is correct etc then load in increments next

then back to cruise idle for cleanup.

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