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Hi Joe - I'll send this by email... but thought it might be of interest to others...
I recently installed your OBD+ unit and am trying to chase down some fuel injection issues. In the event I end up getting new injectors, I'm curious about whether the OBD+ can support injectors with a slightly lower impendence than the stock 4.5? I'm having trouble finding 4.5 ohm injectors for less than $400 each. There appear to be 2.5 ohm injectors that work if a resistor is placed in-line to increase the resistance. My understanding is that _not_ doing this runs the risk of overheating your DME.
Would it be possible to safely run 2.5ohm Lucas injectors with the OBD+ (without changing the resistance of the injector)? I really don't want to risk melting my new, very nice OBD+. But (new) injector options for turbos seem to be very limited and having the ability to do this would be great.
Hi Joe - I'll send this by email... but thought it might be of interest to others...
I recently installed your OBD+ unit and am trying to chase down some fuel injection issues. In the event I end up getting new injectors, I'm curious about whether the OBD+ can support injectors with a slightly lower impendence than the stock 4.5? I'm having trouble finding 4.5 ohm injectors for less than $400 each. There appear to be 2.5 ohm injectors that work if a resistor is placed in-line to increase the resistance. My understanding is that _not_ doing this runs the risk of overheating your DME.
Would it be possible to safely run 2.5ohm Lucas injectors with the OBD+ (without changing the resistance of the injector)? I really don't want to risk melting my new, very nice OBD+. But (new) injector options for turbos seem to be very limited and having the ability to do this would be great.
Thanks for all of your support.
This is a subject I have been meaning to address for some time. There are misconceptions out there regarding fuel injector compatibility for the 944. This is especially true with the concept of “impedance matching” injectors. For example, the factory DME for the Turbo and NA have the exact same injector driver and are calibrated identically despite different injector impedance. The reason for this is that injector impedance doesn’t necessarily matter. What is key is pushing the correct amount of current through the injector for it to operate correctly. In the case of stock 944 NA and Turbo injectors, they just happen to require the same drive current.
When considering an aftermarket low-Z type injector, the question is what current guarantees it will open and the minimum current required to keep it open (i.e. Peak and Hold currents). The impedance is related to this in terms of calculating the power required to drive the injector. But it is not necessary to know because the DME injector driver is completely current controlled. It will drive the injectors at maximum until the programed “peek” current is reached. After that point, it will switch to a lower “hold” current (actually a modulated hold current). The “Peak” and “Hold” strategy is used to reduce the injector on/off time due to the time it takes to charge then discharge its inductance.
Rather than using Low-Z injectors, I suggest using High-Z “saturation” type injectors such as the Siemens Deka. This type of injector just needs battery voltage applied to it and will shut off relatively immediately when that battery voltage is removed. This is compatible with the factory DME and SPORT DME (OBD+ DME included) because it will “saturate” the injector with battery voltage while attempting to reach the “Peak” current. However, because the impedance is much higher, it will never reach that peak current and essentially act like a saturation driver.
If you are looking for a stock injector replacement, the main issue with High-Z injectors is finding one that matches the flow rate of the factory injector. Or at least achieve the same flow rate by installing an adjustable fuel pressure regulator and/or changing the FQS setting to increase/decrease fuel by a few percent.
If you are looking to install an aftermarket Low-Z injector, you will need to determine the peak and hold current it requires. The factory DME and SPORT DME are programed to 1.25 amps per injector. I have never bothered to determine the hold current. Mainly because it is not a calibration value for the S400 injector chip in the way peak injection current is. If the peak and hold values are not known, you can send the injectors into me and most likely I will be able to determine reasonable values. If these values are different than the factory, I can recalibrate the SPORT DME injector driver to the new value.
If you do run after market injectors with a program intended for factory injectors, be sure your NBO2 sensor is installed and working properly. This is so the DME can automatically trim the fuel mixture and help compensate for injector flow and lag differences.
Also, the SPORT DME has a modern and very robust transistor that is specifically designed to drive inductive loads. It also has a huge amount of heat sinking. It can drive virtually any injector in existence regardless of impedance.
I'm not sure what direction I'm going yet. I'd just get the original Bosch injectors if I could find them. At the end of the day... I'm trying for "as painless as possible." If there's something I can drop in and not have to do anything... that would be preferred (but I'm not willing to go with dropping $1600 on injectors from Porsche).
Alternately... I'd certainly be willing to send one to you. These are what I was looking at:
Dear Joe: Any updates on the MAF unit these days? Just wondering..Almost ready to pull the trigger on my DME overhaul...since you don't have anything else to do (LOL!) I was wondering if a remote base and a small cable harness could be constructed to accept you super whifferdill fan relay and plug into the early car fan relay spot on the early relay board. Perhaps it could be mounted on the inside of the sidewall near the hood release cable handle..Just spitballing here..Might increase your sales base on these units.(hopefully)..
Dear Joe: Any updates on the MAF unit these days? Just wondering..Almost ready to pull the trigger on my DME overhaul...since you don't have anything else to do (LOL!) I was wondering if a remote base and a small cable harness could be constructed to accept you super whifferdill fan relay and plug into the early car fan relay spot on the early relay board. Perhaps it could be mounted on the inside of the sidewall near the hood release cable handle..Just spitballing here..Might increase your sales base on these units.(hopefully)..
I would not recommend using any of my Solid-State Relays for any application other than what they where designed for. I will add a Fan SSR for the early 944/924S to my project backlog and start collecting information on it. I will also see if I can get some enclosure samples in and an original relay to reference. I am already looking into the fuel pump relay for the 924, hopefully I can develop them in parallel. Optimistically, I might have a prototype of each toward the end of next year.
If I understand your post on fuel injectors correctly... the following would work with no other modifications (using an OBD+ DME) other than a 3bar FPR? It's a Lucas unit that flows 30lbs/hr at 43.5psi with a resistance of 12ohms:
Still haven't figured out what is going on with my injectors... but looking for alternatives if I end up needing new ones.
Thanks for your help and support...
Dear Millermatic: Have you changed out the Injector harness? I heard that Lindsey sells one that might be compatible with your car..Hope that this would "sort out" any problems..Might be cheaper than a new bunch of injectors. BTW, I got my car started for the first time. I bet that it hasn't run in about 20 years..anyway, the new Buick injectors all did fine, with no leaks. Exhaust color is normal without any black soot. Haven't pulled the plugs to check yet. Only have about 20 minutes run time so far. You might Google Sorenson Electronics, give them a call and see if they have a cheaper injector that would do well for you. You'll need the Bosch # of course. I do know that AZ no longer directly carries their line of parts, or so I was told. I don't know of any other FLAPS that do, but you can find out what corporation carries their stuff when you call. HTH some.
Dear Millermatic: Have you changed out the Injector harness? I heard that Lindsey sells one that might be compatible with your car..Hope that this would "sort out" any problems..Might be cheaper than a new bunch of injectors. BTW, I got my car started for the first time. I bet that it hasn't run in about 20 years..anyway, the new Buick injectors all did fine, with no leaks. Exhaust color is normal without any black soot. Haven't pulled the plugs to check yet. Only have about 20 minutes run time so far. You might Google Sorenson Electronics, give them a call and see if they have a cheaper injector that would do well for you. You'll need the Bosch # of course. I do know that AZ no longer directly carries their line of parts, or so I was told. I don't know of any other FLAPS that do, but you can find out what corporation carries their stuff when you call. HTH some.
Thanks, Tiger. When I started this project about two years ago I realized that all of the heat shielding on the original wiring was brittle (if it was still there). The battery cables were missing large patches of insulation. It was mess. I now have Lindsey's complete harness and new battery cables.
That's not to say I don't have a wiring problem. You have to splice Lindsey's harness into your existing harness. It's not a total replacement. Bit of a (total) pain. (But I don't think wiring is the issue).
To be fair... the car runs well. It just starts rough. I've got enough time invested in it now that I'm determined to get it right. Per my new OBD+ (thanks, Joe!), I've noticed my ICV isn't in the "green" zone... so that's on my list, too. (It's not far off).
Congrats on getting the car started!
Last edited by Millermatic; 11-17-2021 at 10:32 AM.
10-29-2021, 11:52 PM
