Fuel Injection / Motronic Guru's!
 

Hey folks,
The Motronic is a bit of a mystery to me. I'm not actually interested in using it, but I'm curious how it works as a starting point to help fine tune my MegaSquirt (on a 924 2.0L). There's quite a bit of discussion on the MS forum about the number of injector squirts per combustion cycle, and when to time them (even without going sequential).

Just a couple simple questions (for now) -

1) are pairs of injectors fired together with Motronic? i.e. 1&4, 2&3, etc.?

2) if so, which injectors are tied together? I'm guessing 1&4 and 2&3. The idea on MS is to pair injectors with the firing order -

1 3 4 2 firing order
1 2 1 2 injector pairing

Cheers!
nick

as far as i know there is 2 drivers and they are paired as you guessed but it doesn't realy matter as they are not sequential until about 1993.
well it only matters when your matching the resistance of the injector drivers changing injectors .
don't worry Trans axle will be here soon our German connection he will set you strait

One driver, they all fire together. There are two wires coming from the DME pin, this is to reduce the resistance which reduces the voltage drop. I have looked at the DME board and the schematics and confirmed this. This has been discussed to death, do a search.

lol, well with the search and within this thread, I'm seeing some folks say they fire all at once, and some say they fire in pairs. If either is the case, are they firing only once per 720 degrees of rotation? That would be surprising, and give very unequal fueling...

https://rennlist.com/forums/showthre...atch+injectors

https://rennlist.com/forums/showthre...atch+injectors

Cheers for the links Brian, although I'm not sure what they're getting at? The "racerx" link seems to be very confused as to what batch and sequential firing actually is. Sequential shoots fuel only (more or less) when the intake valve is open - for each cylinder. Batch fires all injectors at once. If they're all fired once per cycle (720 degrees) then one cylinder is getting fuel when the intake valve is open, one cylinder is getting a shot on the compression stroke, another cylinder on the power stroke, and the other on the exhaust stroke.

It sounds like the consensus is that Motronic fires all the injectors at once. Next question is - is it firing the injectors four times per cycle? That would at least ensure equal fuelling. Neither of those threads (or any others I searched for) mention this. The second thread is just for a Motronic display?

Most engines with 1-3-4-2 firing order seem to pair injectors and fire them twice per cycle. So that each cylinder gets a squirt on the intake stroke and the power stroke (what the racerx link incorrectly describes as sequential). This ensures equal fueling at least and is what I would expect from Motronic.

Motronic for the 951/944 cars fires in batch mode, all four injectors firing together once per revolution. This gives you two firings per cycle. Each injector fires onto the back side of each cylinder's intake valve. All the required fuel for one cycle will be present for each cylinder on the intake stroke when the valve opens.

I noticed a common misconception in the original posters thoughts –
At full load in either sequential or batch firing the majority of the fuel will hit the closed intake valve – this is actually a good thing since the quite hot intake valve will aid in the vaporization of the fuel. When you see that the fuel injectors are running at or over 80% duty cycle (typical for full load) the 80% represents 80% of the 720 degree cycle (or 80% of two successive 360 degree rotations for a batch fired). Either way – a lot of time squirting on a closed valve.
When you look at the spray pattern of a fuel injector you will also understand that the injector never fires the fuel directly into the cylinder.
If I recall the Motronic unit correctly there are two drivers but they are tied together to run as one batch fired system.
Summary- basically the timing of the injector firing is irrelevant except for a full sequential car that is dead cold – only then will you see a difference. The plus of running full sequential is that you can get a larger injector to idle at the correct mixture since.

Two pins on the DME going to two injectors each, but only one driver sinking current from all 4 injectors. OEM sequential systems use high impedance injectors with a low side current driver for each injector.

That explains it, I have seen cars with 'pairs' of injectors not working - figure it was a common driver, but it must be my old friend the wiring harness...!

Maybe I'm missing something, but if you have 2 squirts per cycle for all cylinders, then you're definitely not firing the injectors on the back of the intake valves for all cylinders. (thanks for the correction on firing time, you're definitely right Chris!)

Here's a pretty little chart :)
Cylinder number is at the stop, the time in the rotation of the squirt is on the left, and I=intake, E=exhaust, P=power, C=compression strokes -

___#1___#2___#3___#4
____I____C____E____P
s___C___P_____I____E
____P___E____C____I
s___E___I_____P____C

If you squirt all injectors at once twice per cycle, then #1 and #4 pistons are getting a squirt at the start of the compression and exhaust strokes, but #2 and #3 get their squirts at the beginning of the intake and power strokes. Above ~2500rpm or so, it's not going to make a difference because the timing is short. But under that, it absolutely makes a difference - at least with large injectors... I'm very surprised Motronic runs this way?

In the real world there is not much difference in response due to injection timing. The fuel will stay in suspension until the valve opens. When the system is cold it will enrichen the mixture to make up for the slow vaporization and the amount of fuel that will wet the intake walls.
So don’t get too worried about it. If you are programming your new system I would program it to complete the injection cycle long before TDC (but that can get tricky if you need to run over 80% duty cycle on the injectors).

hmm, well certainly thanks for the info. It's definitely a surprise. I've changed my squirts from basically the same all-at-once twice per cycle mode as the stock 944, to alternating the two banks so each cylinder is getting a squirt twice per cycle, but at the same points in the cycle.

i.e. 1&4 get their squirts on compression and exhaust, and so do 3&4.

I, and quite a number of other turbo-4cyl folks, are seeing quite dramatic improvements in idle and off-idle tuning and drivability. Maybe this is only really apparent with larger injectors (440cc on a 2.0L)?

Thanks again! :)

Is it possible to make the inj's fire in sequential or semi-sequential order with software?

well some programmable efi boxes haves outputs than can be used for either feul of direct fire ignightion and these ones you can manipulate the injection just like ignightion timeing . but even with all this magic most feel its not worth the time . and in answear to your question i would say no due to lack of hard ware. As far as i know the DME has no sequencing it syncrinizes the distubutor at start up then forgets . Trans axle must be on holiday.
and as for Idle quality it is so you can fire a big injector once per cycle every second rpm .