Max Low Impedance Injectors on PWM?

[thildemar]

Hi Everyone,

I am looking into options for using Megasquirt with a Jaguar 5.3L V12. Initially I am thinking of firing each bank of 6 cylinders alternating (same as factory). I believe the injectors are 2.5 ohm. Will the v3 or newer boards handle two sets of 6 2.5 ohm injectors in parallel doing PWM or do I need resistors inline with each?

Thanks!

[racerron]

Injector outputs should handle the extra injectors. I think you have to do a current limiting.

[billr]

Hmm... I'm not so sure. Those mainboard drivers seem to be rated for 14A, but the six low-z (2.5 ohm) injectors in parallel will draw over 30A peak. The two DB37 pins for each driver may also be over-loaded.

[DaveEFI]

4 low impedance injectors in parallel would also in theory exceed that 14 amps by quite a margin. And loads of V8s with MS around.
The V3 board driver has current limiting as part of that sophisticated circuit. But you couldn't drive low impedance injectors to the full current from 12v anyway without burning them out.

[Edit]

Just to add, the total resistance of injector plus resistor is given as 7-10 ohms in the Lucas fault finding manual. Which would suggest each injector operates on something under two amps of drive current.

[prof315]

You can run 12 low Z injectors on a V3.0 mainboard using PWM current limiting. Been there done that.

[billr]

I misstated my opinion, as well as making a mistake on the "peak" value.

"Peak" for low-Z injectors is generally 4A, not what 12V divided by injector Z is. So, for four on a V-8 that would be 16A, close enough to 14A that I wouldn't expect any real problem. However six in parallel is a 50% increase, 24A would be needed (not the 30+ that I originally indicated).

If the MS driver self-limits to about 14A (as I can believe it does), or you limit using TS settings, I can also believe you can use six injectors in parallel. However, they won't be operating in true "P&H" mode. That's my main point, control of those injectors isn't as precise as you think it is; and that still leaves 12A to be handled by each of those DB37 pins that many folks feel are rather dainty.

[thildemar]

I seem to be hearing "kinda." Or that it may technically work, but probably will run into some issue. Fair statement?

So next question is would adding a resistor inline with each injector be a better solution? If so, with or without PWM?

I am also going to look into finding a set of newer injectors - it would probably be much easier just to deal with high z directly if I can find some that fit.

[prof315]

Hi Zs that fit are indeed a better option, but I repeat. I have run a 5.3 Jag V12 with Low Z injectors using the 2 V3.0/V3.57 mainboard injector drivers without damaging anything.

[DaveEFI]

The V12 Jag is quite similar to my Rover V8 which had the factory injection with original Lucas/Bosch low Z injectors. And if the injectors are of the same physical design, finding Hi-Z ones which are a straight fit may not be that easy. On the Rover, the shape of the inlet manifold fitting is different - as is the fuel rail connection.

[Marek]

The loZ Bosch injectors used between 1992-4 are physically the same size as the HiZ injectors used on the Denso v12 in 1995-6 so will fit between the square 6litre fuel rail and the manifolds in the same way.

You can adequately run 6 loZ injectors per mainboard channel. Just look in the archives or on the Jaguar site where you also asked this question.

If you want to work out the average current that the DB37 pins will have to take then make a spreadsheet with rpm versus pulsewidth and assume a worst case 80% duty cycle, with 1mS opening time pulling 4amps and the remaining open time pulling 1amp per injector and average it out over the 2pins per channel you can allocate to the task on the DB37. Logic says the maximum average current will work out 3-4amps per pin.

In the real world, the duty cycle will be much lower as this is a v12 and will be mostly running at very low rpm compared to its redline unless you specifically want to run at 7000rpm all day and night.

kind regards
Marek

[DaveEFI]

Was it really still using Lo-Z injectors by the 90s? Lucas changed to Hi-Z on the RV8 when the hotwire system arrived in the late 80s.