Low Vs High-Impedance Injectors - A Thesis

[asphaltgambler]

Well, now being educated (thoroughly) I could write a college thesis on low VS high-impedance fuel injectors. OR the difference between early Motronic, GM, Jag through late 1980's VS later fuel management.

The quandary being the earlier low-impedance injectors are scarce, if you can find them at all - it's a small $$ fortune. Some people suggest that you can swap the later high-impedance, more readily available injectors, install them in your earlier, 911, 924, 944, BMW, Jag. Don't believe that.

Can discuss here, if anyone had tried this, or is thinking about trying it............

[76FJ55]

Not quite sure were you are headed with this thread. I opened it expecting to read the thesis on fuel injectors.

Are you asking if people have direct experience with installing high impedance in our cars designed for low impedance injectors and their success or failures in doing so?

[FA-18C]

Asphalt - would welcome to get your input. There are numerous threads about alternatives to stock (now unobtainium), but even some of the alternatives are getting scarce. I am sending my old ones for a complete clean and check, and installed Lucas. Some have had major issues with these, I am on the fence here.

The pulpit is yours

[Peter M]

Oh! Click baiting on Pelican Forum now!

Ok, I'll bite!

Ashy, can you tell me why you think the 0 280 155 931's I put in my '88 Carrera didn't make it run perfectly and the dyno run that showed ideal AFR's was erroneous? Was I imagining the benefits of improved atomisation?

Yes, I realise there are differences in dead time, flow rates and a reduction in current draw with the substitution of a modern injector but surely the proof of the pudding is in the eating?

[asphaltgambler]

Ok, sorry for the delay in responding - been a busy week. First generation Low-impedance / high current injectors that were used on Euro, GM and many other applications in the 80's were the result of design and thinking of that time. Which is; large coil, snap the pintle open quickly with 4+ amps of current "peak" then lower and briefly modulate the current. Then off so as not to overheat the injector coil. Otherwise known as 'peak and hold', The modulation of the electrical drivers on the ECM board to this was programmed into the 'Chip' or brain of the ECM.

Later engineering tests revealed that while a good idea on paper, a better, lower cost way to do the same work was to use High-impedance / low current (1-2 amp) fuel injectors and simply program the "chip" apply full current open to close. The injector coil size could be greatly reduced, the wiring to the injectors could be much smaller gauge, lastly the injector size is about 1/2 the diameter of the older generation units - and it all works just fine.

Here's the issue when cross-breeding from personal experience. I used the later style High-impedance injectors in my '85 3.4 Hot-Rod with the plastic 964 intake setup because I was told early on that they would work - well they sort-of work, but not really. The problem lays with how the early Motronic ECM peak and hold firmware programing is contrary to the later injectors design. Starting with the 964 (Porsche) they made the 'switch'. Those injectors look similar but you can see the coil size; circumference is smaller side-by-side. Modern injectors are even smaller, more efficient - the size of a magic marker.

My setup: '85 Targa, fresh 3.4, 10:1, Super cup cams with modified lobe separation for low-end torque, plastic 964 intake with vario-ram flap, programmable MAF conversion, live wide-band O2.

In short the later injectors just would not flow enough fuel primarily because the early ECM was pulsing it when they are not designed to operate that way. Also when hot or heat soak the engine just would not fire (provide enough fuel) to start and run correctly.

Also, with my programmable MAF unit, I can add or subtract fuel (through the map interface via my laptop) in tiny or larger increment across a very wide range of conditions. Then write that file to the MAF, cycle the key and start the engine. With-in 60-80 seconds have the wide-band online and see live what changes throughout the RPM I've made.

No matter how much fuel I added to the MAF, I couldn't get a good fuel curve from idle to WOT. In doubt as to whether the issue still was with the possible mismatch between the early Motronic chip programing and the later high-impedance injectors, i sent them off to be cleaned, tested, flowed by a reputable shop. I get them back, install them and still have essentially the same issue. I search the interwebs, get different answers from different reputable people.

At this point, I'm at a crossroads. Note, I am a very experienced tuner, engine guy with decades of real world trial and error. I decide I have gone as far and eliminated everything but the later injectors. I found replacements (Lucas for Jag's) are still available. Unfortunately, while there seems to be more than a few Bosch part # equivalents in flow rate, some 80's GM products to name some, sadly no one stocks them.

The Lucas part is same spec, flow rate, correct Low-impedance / peak and hold injector. So anyone here with an early (pre-89-90) can use these. You-ll have to do a little digging as our host prolly would not want me to post. However, you can PM me for the source if needed.

I purchased the correct ones for my application and all problems went away immediately. As a matter of fact I had to remove a bunch of fuel from my fuel map file to get it to run @14:1 average until WOT. All issues out the window.

Summary, my experience: Use the correct style (impedance) injector that the ECM chip was designed to drive. Period.

I know there was a Pelicanite here, (still) that went through this a while back, went to the trouble of setting up a solution: selling a reprogrammed early Motronic Chip, along with the later style injectors to those who could not find the correct replacement low-impedance injectors. While that is one way to skin a cat, My recommendation, just obtain the correct impedance injectors for your early car, they're out there, available and reasonable cost for the set.

[Peter M]

Ashy,
Thanks for the response but frankly that isn't worthy of a thesis, maybe a bedtime story!

However I am pleased that you solved your tune issue with the change of injectors

All I can report is a positive experience with the adoption of Bosch 0 280 155 931 in my case.

I wish you well.

Regards
Peter

[asphaltgambler]

Peter- please elaborate. What do have, and what was going on?

[mysocal911]

Originally Posted by asphaltgambler

Ok, sorry for the delay in responding - been a busy week. First generation Low-impedance / high current injectors that were used on Euro, GM and many other applications in the 80's were the result of design and thinking of that time. Which is; large coil, snap the pintle open quickly with 4+ amps of current "peak" then lower and briefly modulate the current. Then off so as not to overheat the injector coil. Otherwise known as 'peak and hold', The modulation of the electrical drivers on the ECM board to this was programmed into the 'Chip' or brain of the ECM.

Later engineering tests revealed that while a good idea on paper, a better, lower cost way to do the same work was to use High-impedance / low current (1-2 amp) fuel injectors and simply program the "chip" apply full current open to close. The injector coil size could be greatly reduced, the wiring to the injectors could be much smaller gauge, lastly the injector size is about 1/2 the diameter of the older generation units - and it all works just fine.

Here's the issue when cross-breeding from personal experience. I used the later style High-impedance injectors in my '85 3.4 Hot-Rod with the plastic 964 intake setup because I was told early on that they would work - well they sort-of work, but not really. The problem lays with how the early Motronic ECM peak and hold firmware programing is contrary to the later injectors design. Starting with the 964 (Porsche) they made the 'switch'. Those injectors look similar but you can see the coil size; circumference is smaller side-by-side. Modern injectors are even smaller, more efficient - the size of a magic marker.

My setup: '85 Targa, fresh 3.4, 10:1, Super cup cams with modified lobe separation for low-end torque, plastic 964 intake with vario-ram flap, programmable MAF conversion, live wide-band O2.

In short the later injectors just would not flow enough fuel primarily because the early ECM was pulsing it when they are not designed to operate that way. Also when hot or heat soak the engine just would not fire (provide enough fuel) to start and run correctly.

Also, with my programmable MAF unit, I can add or subtract fuel (through the map interface via my laptop) in tiny or larger increment across a very wide range of conditions. Then write that file to the MAF, cycle the key and start the engine. With-in 60-80 seconds have the wide-band online and see live what changes throughout the RPM I've made.

No matter how much fuel I added to the MAF, I couldn't get a good fuel curve from idle to WOT. In doubt as to whether the issue still was with the possible mismatch between the early Motronic chip programing and the later high-impedance injectors, i sent them off to be cleaned, tested, flowed by a reputable shop. I get them back, install them and still have essentially the same issue. I search the interwebs, get different answers from different reputable people.

At this point, I'm at a crossroads. Note, I am a very experienced tuner, engine guy with decades of real world trial and error. I decide I have gone as far and eliminated everything but the later injectors. I found replacements (Lucas for Jag's) are still available. Unfortunately, while there seems to be more than a few Bosch part # equivalents in flow rate, some 80's GM products to name some, sadly no one stocks them.

The Lucas part is same spec, flow rate, correct Low-impedance / peak and hold injector. So anyone here with an early (pre-89-90) can use these. You-ll have to do a little digging as our host prolly would not want me to post. However, you can PM me for the source if needed.

I purchased the correct ones for my application and all problems went away immediately. As a matter of fact I had to remove a bunch of fuel from my fuel map file to get it to run @14:1 average until WOT. All issues out the window.

Summary, my experience: Use the correct style (impedance) injector that the ECM chip was designed to drive. Period.

I know there was a Pelicanite here, (still) that went through this a while back, went to the trouble of setting up a solution: selling a reprogrammed early Motronic Chip, along with the later style injectors to those who could not find the correct replacement low-impedance injectors. While that is one way to skin a cat, My recommendation, just obtain the correct impedance injectors for your early car, they're out there, available and reasonable cost for the set.

The 8051 processor can be programmed, e.g. masked ROM/EPROM, to provide any digital input to the actual injector final-stage circuitry,
but it's the Interdesign analog driver IC that's designed only for low impedance injectors. The low impedance type of injector used
by Bosch began when the transition from D-Jectronic (Map) to L-Jectronic (AFM) type of ECM occurred in the mid to late '70s,
e.g. VW Vanagon & 914. No attempt to re-program (Mickey Mouse) the digital logic of the 8051 for high impedance injectors
is possible without a total analog re-design of the output injector circuitry of the 911 3.2 DME ECM, and thereby achieve the
maximum performance provided by the original low impedance injectors.

[mysocal911]

Now you could totally eliminate the key low impedance I.C. (Interdesign) injector driver and use the original Darlingtion NPN, now driving six 12 ohm high impedance injectors,
(0 280 155 931 - 6 amps max in parallel), driven by a high current pre-driver NPN (2N4401), This pre-driver is restively connected to one of the 8051 outputs (pins 1-8).
Now the injector 8051 assembly source code would be very similar to the 964 fuel processor code, i.e. in functionality without sequential injection. Although, the 964 DME ECM
uses a dedicator processor for the injectors, i.e. a sequential injection, a single 8051 could drive a batch injector mode like the present 911 3.2 system with ignition timing.

[Peter M]

Originally Posted by asphaltgambler

Peter- please elaborate. What do have, and what was going on?

Mr Gambler,
My '88 Carrera was suffering from a leaky injector. Like you I was loath to spend a huge sum on NOS replacements and couldn't see the value in reconditioning the existing as they were 240,000km old. (I actually reconditioned the injectors in my previous 3.2 but it had much lower mileage.)

Consequently I investigated alternatives and became aware that a well regarded Porsche workshop was successfully using modern style 4 hole Bosch injectors in 3.2's. However they did not disclose the model number so I still had work to do

I subsequently purchased and installed a set of 0 280 155 931 injectors. Being aware of the different performance characteristics I tested the AFR's on a dyno to satisfy myself that they were safe. After two dyno pulls, I recall the dyno operation describing the AFR's as "perfect". I found the driveability and idle quality to be equal or better than my previous 3.2 so was pretty happy all round.

The car I installed these injectors on was the typical 3.2 with Wong chip and M&K 1in 1out muffler. I drove it in this configuration and with these injectors for a couple of years before worn valve guides put me at the top of a slippery slope that resulted in the engine being modified further (3.4/993SS cams/twin plug/headers/2in 1 out muffler/twin knock sensors/twin 02 sensors/full sequential on CoP ignition and injection/AFM delete to MAP/Motec 130)

Consequently when I saw the title of your thread it peaked my interest due to my own direct and positive experience.

Regards

[mysocal911]

Originally Posted by Peter M

Mr Gambler,
My '88 Carrera was suffering from a leaky injector. Like you I was loath to spend a huge sum on NOS replacements and couldn't see the value in reconditioning the existing as they were 240,000km old. (I actually reconditioned the injectors in my previous 3.2 but it had much lower mileage.)

Consequently I investigated alternatives and became aware that a well regarded Porsche workshop was successfully using modern style 4 hole Bosch injectors in 3.2's. However they did not disclose the model number so I still had work to do

I subsequently purchased and installed a set of 0 280 155 931 injectors. Being aware of the different performance characteristics I tested the AFR's on a dyno to satisfy myself that they were safe. After two dyno pulls, I recall the dyno operation describing the AFR's as "perfect". I found the driveability and idle quality to be equal or better than my previous 3.2 so was pretty happy all round.

The car I installed these injectors on was the typical 3.2 with Wong chip and M&K 1in 1out muffler. I drove it in this configuration and with these injectors for a couple of years before worn valve guides put me at the top of a slippery slope that resulted in the engine being modified further (3.4/993SS cams/twin plug/headers/2in 1 out muffler/twin knock sensors/twin 02 sensors/full sequential on CoP ignition and injection/AFM delete to MAP/Motec 130)

Consequently when I saw the title of your thread it peaked my interest due to my own direct and positive experience.

Regards

Please provide a scope trace of your injector signal (pins 14/15) of the 911 3.2 DME ECM using the 0 280 155 931 injectors at 2K RPM, ideally under partial load on a dyno.
Ideally, one would do the same test with the low impedance OEM injectors, and testing both using a stock DME ECM chip. Who knows what injector pulse widths are used
or modified on the non-stock chip? Many Porsche techs indicate an excessively rich running engine at all RPMs and loads with high impedance injectors, resulting in failed
emission tests.

[asphaltgambler]

Great responses guys, so the key limitation is the driver module on early Motronic when trying to use later high-impedance injectors, not the chip as I understand your explanation.

[mysocal911]

Originally Posted by asphaltgambler

Great responses guys, so the key limitation is the driver module on early Motronic when trying to use later high-impedance injectors, not the chip as I understand your explanation.

Actually, the DME ECM 8051 fuel algorithm source code, based on AFM inputs, must be modified when using high impedance injectors too.
Just changing fuel map values based on idle, intermediate AFM inputs, and full throttle results in less than optimal AFRs, thereby having
marginal performance at less than full throttle and problematic AFRs for emission testing for a 911 3.2 engine.

[asphaltgambler]

^^Possibly this is what a Pelicanite here was doing: selling a 'modified' Chip with HI injectors as a kit. I do not know if anyone here purchased those in the past or proved out his concept. But it looks like from your explanation that there is more involved than just chip fuel algorithm changes to make the fuel map work correctly through-out the RPM range.

[mysocal911]

Originally Posted by asphaltgambler

^^Possibly this is what a Pelicanite here was doing: selling a 'modified' Chip with HI injectors as a kit. I do not know if anyone here purchased those in the past or proved out his concept. But it looks like from your explanation that there is more involved than just chip fuel algorithm changes to make the fuel map work correctly through-out the RPM range.

Here are typical 911 3.2 EPROM fuel maps which are modified for a "performance chip";



Generally, it's only the Full Throttle map that's modified. When using high impedance injectors, the Part Throttle map is modified in an attempt
to make fuel corrections to optimize AFRs. This approach has significant limitations, though, necessitating major 8051 source code changes,
and not just cell changes to the Part Throttle fuel map for optimal AFRs.

[asphaltgambler]

Interesting - many components to this puzzle I didn't think about or was previously misled

[Peter M]

Originally Posted by mysocal911

Please provide a scope trace of your injector signal (pins 14/15) of the 911 3.2 DME ECM using the 0 280 155 931 injectors at 2K RPM, ideally under partial load on a dyno.
Ideally, one would do the same test with the low impedance OEM injectors, and testing both using a stock DME ECM chip. Who knows what injector pulse widths are used
or modified on the non-stock chip? Many Porsche techs indicate an excessively rich running engine at all RPMs and loads with high impedance injectors, resulting in failed
emission tests.

Dave,
As I said in my earlier post I eventually converted my car to an aftermarket ECU along with many other modifications so am unable to provide a scope trace.

The Wong chip used was the one that came with the car when I bought it and was his generic "premuffler/cat bypass exhaust configuration" mapping which I found in practice with the new high impedance injectors to be very satisfactory. I agree, certain to be far from optimised but good nonetheless and affordably satisfied a need at the time . I also did not have any emission testing requirements to meet.

[mysocal911]

Originally Posted by Peter M

Dave,
As I said in my earlier post I eventually converted my car to an aftermarket ECU along with many other modifications so am unable to provide a scope trace.

The Wong chip used was the one that came with the car when I bough it and was his generic "premuffler/cat bypass exhaust configuration" mapping which I found in practice with the new high impedance injectors to be very satisfactory. I agree, certain to be far from optimised but good nonetheless and affordably satisfied a need at the time . I also did not have any emission testing requirements to meet.

Thanks for your feedback.

[asphaltgambler]

So the "mystery" is now solved understanding the difference to the early / later injectors is much more than just impedance differences. Looking back with advice/ recommendations from credible Porsche guys, most said it would make little to no difference. As I said in my OP, I bought the complete 964 intake with injectors not knowing until later that was the first of the next generation of high-impedance injectors, not batch-fired. Also found out those injectors are rated @190cc the 3.2's are @250cc flow rate.

Mysocal911's input with exactly how the early motronic vs later ecm hardware/ firmware configuration puts the period on the end of the sentence. It's that plus my real world experience that once and for all proves without any doubt the two configurations are seperate architectures and do not work when substituting injectors from the other platform.

In my situation, I have an infinitely programmable MAF unit with an accurate live wide-band and can see live in real time what's going on under different loads/ throttle position. So if ever was there an opportunity to scientifically prove this out, one way or another - I just did.

The crux of all this is the lack of available low-impedance new injectors. There is a guy who has a wepage that has done extensive testing to find similar Bosch injectors with same or very close flow rates. A little googlin' you can find that.

Lastly, this journey IS thesis worthy as no one else has absolutely proved it out, and those who say they've had success with installing high-impedance injectors in early motronic systems have no documented proof that they in fact do work with nothing other than seat of pants dyno. And there in lyes where the my thesis ends and the bedtime story begins.

[Peter M]

LOL! This still isn't a thesis!

I have Mainline dyno runs showing very satisfactory AFR's with my 0 280 155 931 injector equipped 3.2 that I can confirm also drove very well.

But I'm obviously wasting my time here...Cheers!