What resistance to use with low impedance injectors?

[Zmanco]

At the track this weekend I was having power dropouts when the day got warmer. My theory right now is that the injector drivers were overheating and shutting down for a few seconds.

 

So I have a few questions:

 

1) For those of you using a resistor in series with LOW impedance injectors, what value are you using? I have 3 x 6 ohms in parallel with a measured 2.2 ohm resistance. Perhaps this is too low? I can't remember now what led me to choose this value.

 

2) Can anyone confirm that these FETs behave that way? In other words, does the IRFIZ34N have an over temp protection mechanism? The datasheet doesn't say anything that I could find that says it has this feature.

 

3) Here's a thread I have going on the megasquirt site that so far hasn't yielded anything substantial. Can anyone suggest an alternate theory for why I'm seeing the power dropouts? One problem with my theory is that the power drops completely which would mean that both drivers would need to shut down and then resume simultaneously, something that seems improbable to me.

 

4) Could it be the ignition coil driver overheating instead? I tried reducing dwell to only 2ms and it didn't make any difference. The VB921 datasheet doesn't say anything about overtemp protection either.

 

This is a really hard one in that I can't reproduce it on the street. The nearest track is 80 miles each way with limited availability and costs $90 per half day. I really need to work this out "on the bench".

Edited June 7, 2010 by Zmanco

[Zmanco]

I forgot that I'm using the BIP373 coil driver which does shut down when it overheats.

 

Questions:

 

1. For those of you using a high current coil (I'm using an MSD Blaster) what are you using for max dwell? I was at 3.2 ms, and took a run at 2.0 which didn't seem to make a difference, but wonder if maybe I need to go even shorter?

 

2. I felt the coil when I came off the track and it was warm like everything else in the engine compartment, but not so hot I couldn't keep my hand on it. Shouldn't the coil be hotter than that if the dwell was long enough to cause the driver to shut down? My instincts say "yes".

 

I feel like I'm fumbling in the dark here, so if anyone has some other thoughts I'd love to hear them.

Edited June 8, 2010 by Zmanco

[Tennesseejed]

Zmanco, you're not getting a lot of responses here so I'll nibble.

 

I'd look at your dropping resistor values. I read your post over on msextra.com and still am unclear about what size resistors your using. 3 Ohm or 6 Ohm? 3 Ohm would be low.

 

I assume you wired your injectors as alternate banks on MS. I.e., three injectors each bank.

 

If you haven't lately, check out the info on www.megamanual.com. Specifically the "Injector Resistor" section under "Injectors and Fuel Supply" which is about halfway down the page.

 

Low resistor values result in higher current. I = V/R so current is inversely proportional to resistance. I.e., as resistance increases, current decreases, and vice-versa. Using low value resistor means that when the MS grounds the injector circuit to open the injector, high current flows through the circuit.

 

If you have PWM on your version of MS, I'd recommend trying a 60% setting the next time you're are at the track. I would more or less simulate doubling your injector resistors without having to muck around with wiring.

[goodoldjam]

I've only driven my setup a few times. I was convinced my ignition driver was overheating. Actually it turned out to be bad grounds, car would heat up and then the weirdness would occur and the car would cut in and out. Any of the issues i have had so far have been power related. Now if i have a issue I check all the important connections first, before I start blaming the ecu. I would recommend going through and cleaning your connections first.

Edited June 16, 2010 by goodoldjam

[Zmanco]

I'm running the equivalent of 6 ohms per injector. After reading up on the injector driver I no longer thinks it's part of it. It dissipates less than 1 W at a full 14A!

 

I'm pretty anal about wiring as I hate intermittent issues like this. But the symptoms on this one are temperature dependent. It only happened on the track where the engine is obviously turning higher revs and sees greater load, it only happened on the later warmer runs, it only happened after several laps, and it stopped as soon as I began driving in a normal fashion, such as on the cool down lap.

 

At this point I think it's caused by the coil driver overheating due to the combination of:

1) Blaster II coil with lower than stock resistance

2) I put a little bit more thermal paste on than I think I should have

3) the transistor next to it on the heat sink controls the IAC and I recently changed a setting on MS that basically leaves that valve open all the time. That means that transistor is adding heat to the heat sink right next to the coil driver.

4) there is no ventilation in the MS enclosure which is mounted under the passenger seat which further reduces any airflow over the case.

 

Rather than chase this further, I've decided to swap to EDIS. I knew I was eventually going to go that route, so now is as good of a time as any Derek's kit is already on its way and I'm off to the JY on Saturday.

[Brad-ManQ45]

I may be off base here, but electrically speaking, when you are talking about using low impedance injectors, the resistors should NOT be in parallel, but in series - in parallel, you are lowering resistance even more - in series you are increasing resistance.

 

That said - to take maximum advantage of low impedance injectors (you didn't mention what version of MS software you are running), racingmini_mtl has injector driver boards that let you run low impedance injectors and get maximum resolution from them - fine idle and better tip-in response.

 

I bought two sets of two - one for the L28ET project and one for a V8 project.

 

Please let us know how the EDIS works out...

[Zmanco]

I may be off base here, but electrically speaking, when you are talking about using low impedance injectors, the resistors should NOT be in parallel, but in series - in parallel, you are lowering resistance even more - in series you are increasing resistance.

Just to clear things up, I have two sets of three injectors. Each set of injectors is wired in parallel. If 6 ohms is the proper resistance for each injector by itself, then when all three are wired in parallel, the proper resistance is 2 ohms. Unfortunately I was not able to find 2 ohm resistors with enough power dissipation, hence I created one by wiring three 6 ohm resistors in parallel. Hope that helps.

 

That said - to take maximum advantage of low impedance injectors (you didn't mention what version of MS software you are running), racingmini_mtl has injector driver boards that let you run low impedance injectors and get maximum resolution from them - fine idle and better tip-in response.

Indeed, there are better approaches. I've been watching the development of MS3 and will probably at some point just swap to it. But for now, it's getting hard just from a packaging point of view, to add any more boards/modules as I'm running out of space on the mounting board under the passenger seat. I didn't see how far this project was going to go, so didn't make the best use of space. For example, I just ordered a KnockSense knock sensor module which I need to install as well. Plus, right now at least, I don't have any problems with idle quality even with these 440cc injectors. In other words, at least right now, it ain't broken

 

Please let us know how the EDIS works out...

I ran out of time last weekend and won't be back home until this weekend. If things go really well I'll get it up and running on Saturday morning and have some time to tune and shake it down before a track day on Sunday. That would let me confirm that the intermittent problem is indeed solved. Unfortunately I am not able to recreate it on the street.

[jacob80]

I'm also having an issue with the car leaning out at idle once its warmed up. Perhaps our issues are very similar if not the same. I too am running low impedance injectors but am using the built in fly back board in the ECU. I'll be watching this thread closely. Good luck!

[bradyzq]

A typical high impedance injector will be 12-16 Ohms impedance. You can use that range for your calcs.