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Old 11-15-2008, 11:30 PM   #1
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Default Need Advice with Low-Z RC550cc Injectors

Hey guys,

I have a set of RC550 cc injectors that are low impedance (3.5 ohm, IIRC). Anyways, I've got two options now...

1) I could run to RadioShack and buy these resistors:
10 Ohm 10W 5% Wirewound Resistor (2-Pack) - RadioShack.com
and solder in series to make them hi-Z

or

2) Keep them low-Z and do grounding upgrades to the MSPNP9093 that I have.

I'd prefer to do 1, b/c it seems like much less wire routing, but someone on the forum (maybe y8s?) says to use low-Z. Is low-Z better due to the quicker response time? If I converted the injectors to hi-Z, could I get similar performance using the hires code?

What do you guys think? Thanks!
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Old 11-15-2008, 11:42 PM   #2
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A mspnp will never control low ohm injectors. You gotta be MS2 to do that. So only option for you if you keep the MSPNP is to wire in resistors. FWIW, I run low ohm 550cc RC's but I'm MS2E. I should have wired in resistors a LONG time ago. The benefits aren't **** really.
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Old 11-16-2008, 01:05 AM   #3
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Pat, MS1 can run high or low Z injectors, and MSPNP can run low Z injectors with minimal work and standard firmware. HOWEVER, the MS1 Extra HiRez firmware does not support low Z injectors.

I would add resistors and run HiRez. Another option could be to build a peak and hold board.
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Old 11-16-2008, 01:10 AM   #4
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Ben, how so? You mean you can add the flyback circuit to the MS1 and then setup the fuel injectors PWM for peak and hold with MS1 code? I thought this was a MS2 only thing.

Anyways, to the OP I still suggest wiring in resistors even if you can setup MS1 or MSPPN to drive them. I run low ohm injectors and it has been causing noise in the electrical system like crazy. Today I finally lifted the middle pins of the TPI125's and ran them to the +12V on the fuel injectors to complete the loop for the PWM circuit and keep them from adding noise to the MS itself. Helped with noise, but didn't fix it. If I wasn't this far into it, I'd back up and wire in resistors. I just hate to accept defeat this far into it.
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Old 11-16-2008, 01:19 AM   #5
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Pat,
I've never explored it. I assume that some hardware would be necessary.
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Old 11-16-2008, 08:45 AM   #6
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The MS1 and MS2 use the exact same hardware for fuel injector drive, and they can both do PWM drive of Lo-Z injectors. On page 4 of the 3.0 the schematic, note the circuits consisting of Q9/Q12, Q10/Q13, and associated parts. Those are the PWM flyback clamps.

The difference, as Ben pointed out, is the Hi-Rez variant of the MS1Extra code cannot do this, as the CPU resources which were (in the original builds) allocated to doing the PWM timing are instead hijacked and put to work doing the main timing.

However, there is a third solution, beyond merely running resistors or using the non-HR code. JBPerf, the same company that brought us the JimStim, sells a Peak&Hold Injector Driver Board for Megasquirt that is compatible with all MSs. One cool thing is that this board does the P&H work in dedicated hardware, using a pair of LM1949 Injector Drive Controllers. So you configure the MS as though it were driving hi-Z injectors, and let the board do the work. Another nice feature is that you get dedicated injector grounds just like a "real" ECU.
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Old 11-16-2008, 10:14 AM   #7
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Thanks for the input, guys. I think I'll just wire up some resistors in series. My only concern is: will the wire-wound power resistors cause any inductive noise in the injector signal? Will the high underhood temps overheat the resistors?
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Old 11-16-2008, 10:22 AM   #8
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No, and no.

I'd recommend resistors in the 20 watt range if you can find them. They will be slightly more temperature-stable, regardless of where they are mounted.
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