Largest injectors that can be driven by PCM?

Im curious as to the largest size injectors that can be driven by the PCM before having to go with FAST or something similiar. I know the PCM only reads one sort of injector, but Im not sure if it is low impedance or high impedance. Can I go without FAST or the like if I use a large injector (72# or higher) with the same impedance as the stock.. I dont remember if the stock is low or high, but I seem to remember that the larger injectors arent available in the one the PCM reads(sorry I cant remember which it is). Thanks for any info.

High Impedence is what the stock computer can control. It can control up to 50# highZ injectores.

But there again when you go to large highZ injectors they start getting sloppy and slow to open.

This is where LowZ injectors come into play. You can go as big as you need and they are not sloppy like large highZ injectors. Normally you will need a aftermarket computer to run these. But Jeff 'LJ' Stevens has developed a lowZ driver box so that you can use these injectors with a stock computer. Check out http://www.acceleronics.com for more info. I just recieved this box and will be trying it on my new setup.

The stock PCM cannot be used to drive low impedance injectors, they draw too much current and it will overheat. Most large injectors are low impedance, though large high impedance injectors do exist. In any case, the largest that the PCM can use seems to be 50lbs (high impedance). I am not sure what happens with larger injectors, just that it "doesn't work".

If you need to use low impedance injectors (ie larger the 50lbs/h) you will need something like LJ's supplemental injector driver (allows the stock PCM to drive large, low impedance injectors) or an aftermarket ECM like the FAST. Another option for a blower car is the Carroll Superfueler. I am using one of these to drive three low impedance 92lb injectors. The SF has three injector drivers and is controlled by boost.

Hope this helps. And if someone out there knows why the stock PCM can't drive large high impedance injectors I'd like to know the explanation.

Rich Krause

Originally posted by rskrause
And if someone out there knows why the stock PCM can't drive large high impedance injectors I'd like to know the explanation.

Rich Krause

Rich, this is taken from LJ's site:

Assuming your car came with high-impedance injectors from the factory, your engine control computer most likely turns them on and off with a simple electronic switch. It's not much more complicated than touching the two injector terminals to your plus and minus battery terminals whenever you need fuel to flow. Touch for open, remove for close. As long as you don't hold the injector open for a long time, the injector will not burn out.
But you can't do that with low-impedance injectors, also referred to as peak and hold injectors, or you will burn them out! These injectors need to have their current regulated (or modulated, or carefully-controlled) by the computer. A simple on/off switch will not work. The second problem is that the engine computer's injector outputs probably cannot handle the electric current demand of low-impedance injectors, which is roughly 4 times bigger than high-impedance currents. This much larger current flow will probably burn out the computer, if the injector doesn't burn out first!

Rich and Ken,

Thanks very much for the info, you both have answered my questions time and again, I appreciate it :bow: What would be more driveable, the FAST setup, the Superfueler, or the acceleronics device? My car is a 3 of day of the week driver during the fall/winter/spring, and almost daily in the summer, and I want a setup that will let it be pretty driveable, which is a lot to ask with large injectors, but in your opinion, what would be the best? Thanks a lot guys, both of you have answered lot of my questions for my build up, Ive got everything mapped out except fueling and once I get this sorted out, time for the fun stuff (ordering, installing, machine shop, etc ;) ) Thanks again!

No Prob Chris,
Really they all have their good and bad points. Its just want you like/want/need.

Best choice is aftermarket computer with large lowZ injectors, but that is the most costly. The superfueler is good because you can use the stock computer but im not sure if it can handle the needs everyone needs, i think IIRC that Rich is on the high end of duty cycle even with the superfueler. LJ's box with large lowZ injectors seems like a good solution because you can run whatever size injs you want and dont have to pay the high price of a aftmrt computer, but it is a new product so some may be skeptical...

I may try LJs box then...something for me to do some research on and think about I guess. Thanks for the info!

Originally posted by kmook
No Prob Chris,
Really they all have their good and bad points. Its just want you like/want/need.

Best choice is aftermarket computer with large lowZ injectors, but that is the most costly. The superfueler is good because you can use the stock computer but im not sure if it can handle the needs everyone needs, i think IIRC that Rich is on the high end of duty cycle even with the superfueler. LJ's box with large lowZ injectors seems like a good solution because you can run whatever size injs you want and dont have to pay the high price of a aftmrt computer, but it is a new product so some may be skeptical...

Ditto on the "No Prob" :)

Ken makes a good point and if I were doing my car again now I would be tempted to try LJ's box. The thing is, I do like the concept of tuning fuel for boost, which the low-Z box does not accomodate and the SF does.

The complete aftermaket ECU's have many advantages besides allowing use of low impedance injectors. But this comes at a price measured not just in $$$. Among the problems are that you must give up the MAF meter and that while tuning for WOT/max power is a piece of cake, tuning for drivability is far from easy. If someone tells you different they are either very lucky, very talented, or don't know what the heck they are talking about! Because of the MAF the stock PCM usually handles fairly radical combos without a major problem. An add-on like LJ's box or the SF to handle boost doesn't effect operation at other than WOT and the stock PCM can merrily do its' thing. Getting very large injectors to idle can be a problem with either LJ's box or an aftermarket ECU just due the operational characteristics of the injectors - the smallest controllable pulse width is ~1.3ms and if you motor needs less fuel than this supplies at idle, it will not idle well. But I have seen up to 72lb injectors idle fine on a stroker so for most people this is not an issue anyway.

As far as the limits of the SF, you can easily get low-Z injectors up to 160lbs/h, so I have a ways to go with it yet!

Rich Krause

would it be possible to tune for the larger injectors to idle better with the SF or LJ's box via LT1 edit? i havent received my copy yet so im not sure whats programmable and what isnt doable. Thanks for the help guys!

Originally posted by ChrisJ325
would it be possible to tune for the larger injectors to idle better with the SF or LJ's box via LT1 edit? i havent received my copy yet so im not sure whats programmable and what isnt doable. Thanks for the help guys!


Im pretty sure you dont do any programming for the large injectors with the SF. Correct me if im wrong.

Originally posted by kmook
Im pretty sure you dont do any programming for the large injectors with the SF. Correct me if im wrong.

The SF would normally be set up to activate only under boost. Mine, for example, is set up to turn on at ~4lbs of boost. So it does not control the supplemental (large) injectors under any conditions other than WOT. The issue with large injectors and idle has little to do with what circuit is driving them The issue is simply that they cannot be controlled below ~1.3msec duty cycle. The time they take to open and close approachs this interval and they can't deliver a predictable amount of fuel at shorter duty cycles.

Rich Krause

below ~1.3msec duty cycle.

You mean "pulse width". :)

Oops, Fred, you are right (as usual). I meant pulse width.

Rich Krause

What I don't quite understand is what the acceleronics box comprises? I have used high and low impedance injectors on hondas, and the only accomodation you must make is an injector resistor when running peak-hold injectors (low impedance). Why is this not possible with the $EE ecu?

Originally posted by brain
What I don't quite understand is what the acceleronics box comprises? I have used high and low impedance injectors on hondas, and the only accomodation you must make is an injector resistor when running peak-hold injectors (low impedance). Why is this not possible with the $EE ecu?

I'm sure there is more to it other wise i spent way to much money ;) But im sure that it is more intricate and that there was some decent R&D and testing that had to be done to make sure it works with our ecu.

I spoke with LJ via email and he might pop on here tonight to answer any questions and correct us on anything we might have wrong so far.

Ken, not trying to say you wasted money, but I did read some of what he had on his site. I would like a further explanation if he would, as it seems almost like overkill. Some hondas have saturated injectors stock, pretty much all of them from 92 and up, except for the ludes. In 88 - 91 they ran peak/hold, and just had an injector resistor. We just changed over a stock Integra with saturated 240 injectors to peak/hold 550s. No problems with the ecu or injectors. Unless the ecu has some sort of voltage drop circuitry for the peak/hold styles, which I don't see why it would, as it was designed with saturated in mind, it won't last according to LJ. Hmm.. just trying to understand this some more. Please post any info you find out! I thought about trying some big injectors in my car with a resistor to drop the voltage down, like a honda would. I have a set of 83s laying around from a mustang that is in the shop right now.

The $EE PCM simply grounds one side of the injector to open it. The other side of the injector is connected to battery voltage via the ignition switch (12V supplied to injectors only when the ignition is on). High impedance injectors can handle full voltage across them without burning out the wire coil inside, and the drive transistor in the PCM can handle the ~1 amp of current needed to open the injector, and hold it injector open.

What if you try to drive low impedance injectors with the PCM without changing anything? When the PCM grounds the low-Z injector to open it, the current through the injector wire coil and the PCM drive transistor be be somewhere in the neighborhood of 3 to 6 amps. This much current, if it stays on for more than a few milliseconds, will overheat the injector wire coil and the PCM drive transistor, causing one or both of these components to fry (sooner or later). You might get away with it for idling and cruising, where the pulsewidths are just a few milliseconds long, but you'll let the smoke out at WOT when the pulsewidths go into the upper teens and the duty cycle gets to 80%. Electronic stuff does not work after you let the smoke out.

What about just putting a 10 ohm resistor in series with the low impedance injector, which would limit the current to 1 amp, just like the stock high-Z injectors? Low-Z injectors are *designed* to be yanked open with about 4 amps of current. The pintle (or disk, or what-have-you) gets heavier as the rated flow gets bigger, so the wire coil inside the injector has to yank harder to open it quickly. More current = harder yanking! A big, low-Z injector might not even open, or would open sluggishly and erratically at best, if it only saw 1 amp.

So low-Z injectors are peak and hold. The peak current *needs* to be on the order of 4 amps to yank those suckers open. the hold current *needs* to be around 1 amp to prevent the injector coil wiring from overheating and failing.

This is exactly what the Acceleronics Impedance Converter does. It intecepts the 8 injector signals from the PCM and converts each one to peak and hold.

My $.02 on this is; if you need to get that last 10hp out of your engine, you *should* spend 10 times the money (as in thousands instead of hundreds) for a wide band FAST system. If you're budget minded, the Converter at $375 is a very effective solution. I've had no trouble tuning my blown car to run on 75's, and I expect no trouble when I drop in the 92's to support the new ys-trim.

My schedule doesn't allow me to hang out on all the web boards that are talking about the box (I've sold many to Stangers), so please email me directly if you have more questions.


LJ
http://para.noid.org/~lj
www.acceleronics.com (http://www.acceleronics.com)

Thanks very much for the info LJ!