Low or High Impedence Injectors

[v12pilot]

I'm installing a MS2 w/3.57 board onto a Jaguar V-12. The engine originally has 12 Nippondenso LOW Impedance injectors.

From what I'm reading the LOW injectors will possibly require resistors and a few other mods to MS2 to keep from overheating components. I've come across HIGH Impedance injectors from a Volvo that appear to be closely matched to my OEM Jaguar injectors. (The information that I have is for a 5.3L V-12 and mine is a 6.0L, according to the Megamanual calculator I should be looking for 19.6 lbs/hr for my 6.0)

I appreciate any and all suggestions/recommendations.

Thanks,

Chad

Comparison Table:
Part Number lbs/hr cc/min grm/min PSI BARS lbs/hr cc/min 80% 95% Feed Imped
0-280-150-779 19.0 199.7 143.6 43.5 3.0 19 199.7 29.2 34.7 Top High
0-280-150-164 17.2 180.8 130 43.5 3.0 17.2 180.8 26.5 31.4 Top Low

[v12pilot]

The 17.2 lbs/hr injector is a 5.3 Jaguar V12
The 19.0 lbs/hr is a Volvo 850

[trakkies]

I'm happily running low impedance injectors on a R V-8 with no resistors - the later MSII boards have flyback current limiting.
Not sure if this applies to a 12 cylinder - the MegaManual has details of what can and can't be done, though.

[v12pilot]

That's the thing that has my attention; By adding 4 more Loz injectors to the circuits will that cause a problem? or will the flyback function protect the MS2 EMS regardless of the number of injectors and their impedance?

At $400.00 for the (pre-assembled) MS2 unit I'm being "real" careful about what I wire to it.

Chad

[Philip Lochner]

Let me say categorically that anyone who says MS2 is not really capable of driving LoZ injectors is ill informed. My Jag V12 XJS has been running LoZ injectors without current limit resistors on the same MS since 06 and is categoric proof of that.

The attached scope trace shows how MS2 handles the LoZ injectors.
Blue trace (Volt)is the control signal of the transistor switching the injector on and off
red trace (Volt) is the voltage over the 0.05Ohm current limit resistor (R37 and R38)
Black trace (Amps) is the current through the injector calculated by dividing the red trace by 0.05 (I = V/R)
Green trace is on the transistor side of the injector so that one can see the flyback clamping. (Volts)

This trace is for one of the LoZ injectors. The black trace is calibrated in Amps and you can see it peaks at 1.7A and then PWM is applied at 35% which then keeps the injector open at 1.3A (during the on time) for the required duration. My car is running at 20% and is still doing fine. The vertical dotted line is where the injector is fully open (1ms). This trace is at 14.45V. At lower voltages the injector takes longer to open but interestingly also requires less current to open. (1.6ms, 0.83A @ 7.6V)

In fact, my MS running the Jag (LoZ) is running cooler than my MS running the Land Rover (HiZ) but that is due to the LR MS having to drive the IAC in "always on" mode - the only accurate mode of driving IAC in closed loop. The IAC driver generates a lot of heat.

BUT: There is a mod that is advised when going with LoZ injectors and that means lifting the legs of two transistors, installing wires from those legs directly back to the 12V feed of the injector harness using some of the spare pins on the board. ( http://www.megamanual.com/ms2/V3trouble.htm par 16)

So, if money is not a problem, get HiZ and be done with it. Installing current resistors are bad news IMO. It increases the opening time of the injector but more than that it increases the DIFFERENCES in opening times between the injectors and this could make it IMPOSSIBLE to get consistent fuelling accross cylinders at small pulsewidths such as you have at idle and low throttle openings (lean cruise) and would force you to run with excessively rich mixtures only because you need to make sure that all cylinders get enough fuel. Been there, done that.

Going with injectors that flow more, there is a potential danger that if your injectors flow too much, you will have difficulty in maintaining a smooth idle and lean mixtures at low manifold pressures due to the PW becoming too small. Now 5% don't sound like much I know and its hard for me to say if that is too much so all I can say is try to get the same flow as the OEM if at all possible. THEY DO FLOW ENOUGH.

Now here comes the "dealclincher": It is IMPERATIVE that you start with injectors that all flow the same amount of fuel otherwise you would end up chasing an inconsistend AFR, so, I advise strongly that you have those used injectors cleaned and tested. If this will cost close to or as much as new ones then....

[Philip Lochner]

As further "evidence" of how well MS2/v3 can cope with LoZ injectors I attach a scope trace of one of the two injector outputs of the MS2/v3 on my car.

In other words, this is for 6 LoZ injectors (2.3Ohm 4.4mH) on ONE injector channel. This trace was taken as I stomped on the gas to produce an exceptionally long injector pulse due to AE being added.

From this one can see:

1) It looks like I've got my PWM threshold (1.25ms) set somewhat too low (but close enough it seems) because one can not (clearly) see the injector opening "dip";
2) peak current is about 7.5A and if you consider that MS can handle 14A per injector circuit, one could easily run a V16 Merlin with MS on LoZ injectors if need be;
3) My PWM duty is set such that the "on-current" settles at 2A (0.3A per injector) and is certainly enough to hold them open (by virtue of engine not stumbling during such AE).