12v VR6 air/fuel mixture?

[KI11Z]

Hiya, I have a question dub doods!

How does the stock ecu calculate the air/fuel mixture to compensate for the varied runner lengths? Is it a pre-programmed thing?

So when I switch to a stand alone, am I going to have to run pyros on each extractor for each cylinder to monitor the air/fuel? I can't see another way for the computer to adjust the mixture for each cylinder?

Many thanks in advance

. . . . Alex

[KI11Z]

not one member on here knows

[Peter Jones]

What makes you think the computer is adjusting the mixtures on a per cylinder basis?

To do that would require the equivalent of 4 injection systems.

Some very modern cars are getting close to this with newer injection technologies but your VR6 is most likely just applying the same timing to all the injectors.

I'm not specifically familiar with the VR6 system but a lot of systems around that era just fired all the injectors at the same time with the same signal.

Bosch publish some really good training manuls on EFI, see if you can grab one off e-bay, they help a lot in understanding this stuff.

Pete

[KI11Z]

Well its something my engine builder brought up with me; Seeing that the distance from throttle to the intake valves 2,4,6 is shorter than the distance to the intake valves 1,3,5, 2,4,6 should be getting more air? And running all 6 cylinders from the one intake, you couldnt really assume all cylinders are getting the same amount of air could you? So would vw have pre-programmed to allow for the differences?

Also, how can you get a stand alone to monitor the air/fuel properly? as mentioned above, do i need to have pyros installed on each extractor for each cylinder to monitor the air/fuel mixtures?

Thanks for replying Peter

[MattyT]

You'd be start hitting up ppl on vortex for hints I reckon. Not many people locally have gone a stand alone ECU on a vr6.

[KI11Z]

i've just done that, made a thread on the tex so hopefully something will happen over there

[Peter Jones]

I'm no expert by a long shot but no, you shouldn't need need more than one.

You might be better off finding one of the EFI forums to ask this question, there's a few forums associated with each of the different brands of ECUs.

Mostly sensor readings are just sort of averaged out accross the bank of cylinders.

It's a CPU cycle thing. Especially in the ealy days the CPUs weren't fast enough to do too much at once so the sensors might only be read every couple of revolutions of the motor.

Here's some maths for example.

@ 6000 rpm the crank is rotating 100 times a second.

In a 4 cylinder 4 stroke motor it's firing twice per revolution or 200 times per second. That makes the time between firings .005 seconds.

In that 5ms the CPU in the ECU has tread all the sensors and use those readings to calculate the opening duration of the injectors. (Plus a bunch of other stuff). You need a resonably quick CPU to all of that.

Back in 1994 ECUs were running at about 16MHz which is for the sake of simplicity lets say 16 million cpu cycles per second.

But we don't have a whole second to play with - we have 5 ms. so instead of 16 million CPU cycles we've got 80,000 clock cycles of CPU time between firings. That might sound like a lot but remember these thing are only processing 8 BITS of data at a time. (10101010) this means that to even read the letter "A" off this page wich is comprised of 2 bytes, it's going to take 2 clock cycles at least to read it off it's input pins.

Put several thousand lines of code in there, read a few sensors etc, and you simply run out of time to do the maths between firings.

Add to all of that the fact that the sensors don't probably react to changes within .005 seconds and that there's not much point checking them more that once every few seconds anyway.

So it's a timing thing, generally speaking you don't do the injector timing calculations for each revolution let alone each cylinder individually.

There's a natural hysteresis in the system too. Engines can't go from 0 rpm to 6000 rpm in one revolution (I had one go from 8500rpm to 0 rpm in one revolution once - that was terminal experience for that motor.). Like wise they can't go from cold to hot in one revolution either. In terms of engine revolutions, changes in motors operating parameters happen fairly slowly. So if your ECU sets the injector timing it may not need to bother changing it for a number of engine revolutions.

In your standalone system you might be able to adjust the injector duration by a fixed offset per cylinder to compensate for runner length issues but it's unlikely to be able to be adjusted dynamically by a sensor feedback loop.

Just set all the offsets at 0 get the overall AFR right and then only play with the duration offsets based on spark plug colouration if you absolutley have to.

Please note that the above is only my opinion and hazy recollection you'd be well advised to seek expert help on this topic.

Pete

[KI11Z]

wow, hectic post pete cheers!

i assume its not so the computer can adjust every cycle because it shouldnt fluctuate that much i'd assume, it'd be moreso the initial set up and knowing how much air each cylinder is getting? either way i guess installing pyros is the only real option, just more cost thats all

[Peter Jones]

You might get an idea by measuring the dwell times of each of your current injectors at a constant RPM. You could map out the injector timing across a number of rev ranges to try and base line the map.

Pete

[G60JAY]

This might help with some technical info!



http://www.geocities.com/MotorCity/1108/faq.vr6.html