Larger fuel injectors & voltage offset vs flow

[Guest]

This is more a heads up than anything. As far as I can recall, the most in depth guidance I've encountered regarding tuning a fuel injector size change across the forums has been limited to setting a base injector pulse width arrived at by calculation or trial and error for an acceptable idle for a start.

Not much is said about injector offset vs. pump or battery voltage. In my case running 8F with 60 lb/hr injectors about 12 psi above their rating, I believe I have discovered the cause of my persistent rich idle or at least part of the cause.

I also setup a history table in Tunerpro5 to follow pump voltage and have recorded voltage variation in excess of 1 volt and nearly as much as 2.

After looking at the offset table in my tune which for the most part shows values for the stock 23ish lb/hr injectors, I got to searching until I found the spec sheet for my DEKA injectors, noted the differences and realized that this over sight contributes to the difficulty in establishing a stable idle after such a change.

Whenever an accessory is turned on causing a voltage change it might affect the fuel delivery. When the engine/alternator warms up the voltage changes also.

I'll be making one important change for a more stable fuel pump voltage to go along with intended value changes, bypassing the stock pump wiring in favor of a "Hot wire" arrangement with direct battery feed to the relay and then to the pump for better current and voltage delivery.

At the moment I can hear my fuel pump oscillate with the turn signal on. I'll be looking at an alternator replacement also as it seems to drop off on occasion.

I found this link on the forum addressing EGR and fuel delivery that includes the formula for calculating the base pulse constant;

http://60degreev6.com/forum/showthread.php/49299-Setting-the-injector-constant?highlight=Fuel+injector+tuning

The spec sheets are for the 60 lb/hr injectors I use. Here is the link also;

http://www.siemensdeka.com/index.php?main_page=product_info&cPath=1&products_id=12

[Joseph Upson]

I put the theory to the test this morning and sure enough there was an immediate effect on idle AFR after adjusting the pump voltage vs injector offset. I also adjusted the base pulse constant (BPC) down one point.

I adjusted the idle VE table but the response to the changes were poor until an excessive value change was made and even with that it did not reflect as well in the AFR reading but moreso in the idle quality.

I stumbled across another table while confirming that certain injector related tables; lbs/hr, gal/hr, single inject, etc were not effective and therefore inactive in my tune; fuel injector offset vs base pulse width.

When I lowered the value corresponding to my idle rpm using the cell follower, the engine idle started swinging giving a rich/lean reading on the AFR display eventhough the chip is set for openloop operation and no correction should be made though I suspect engine stall prevention logic must have been the cause (When I adjusted the voltage table to low the engine just stalled out). The surge related rich/lean operation was characteristic of what is experienced and complained about by many when running closedloop operation after injector size is increased, while cruising along in closed loop you can feel the rich/lean crosses as well as see them on the AFR gauge.

The problem is figuring out which table to change to correct the problem which no one seems to have done definitively but looking at the values in my table and given the response to my temporary change it appears the BPW offset table is the possible solution.

When you consider the code mask many of us are running was calibrated for injectors that are as much as 20 yrs old and some of us are using newer more efficient injectors, chances are pretty good these tables are even more significant to a stable tune than first thought and should probably be the first tables adjusted before any serious VE correction is made except where it's essential.

[SappySE107]

This is common in OBD2 and aftermarket computers. Doesn't work the same at all for OBD1 unfortunately.

[unchained01]

That to me seems to be a valid theory , as you have explained . There are a few guys on other forums experiencing so of the same after injector changes or after brining the care out after storage or weather getting colder and using accessories more IE: heater / defroster Separating the fuel pump circuit and installing a larger output Alt and upgrading you wiring and grounds . Should make a larger difference in voltage drop and more consistant pulsewidth over a greater range od temps and load. One other thing that not one talks about which may help is shielding all of the injector wiring , making sure there are separte twisted pairs and not allowed to touch any metal at all (i know its extreme but we are talking milivolts here) to keep any other signal out all together . Great write up Joe ,Although i know nothing about OBD1 tuning others here maybe able help alot more.

[Joseph Upson]

This is common in OBD2 and aftermarket computers. Doesn't work the same at all for OBD1 unfortunately.

Agreed, but every little bit helps. The trip home today was much more like what I hoped to see in the datalog, the average AFR was 14.xxx all over the history table instead of 13-16 and that's in openloop so the fuel map is much closer than what it has been or what I was able to achieve consistently before the changes made this morning. There seems to be a point where the VE table just doesn't have any effect or seems that way with changes beyond a certain value level.

The two often ignored tables made a tremendous difference. I'm not spending much time trying to perfect it now because I have a small exhaust leak ahead of the narrow band O2 sensor. My wideband is about 10" away from the turbo and that's what I used to make adjustments.

At some point I'll sit down and attempt to enter the spec sheet values and proper base constant value after I install the adjustable fuel pressure regulator and reduce fuel pressure to the spec rating. My alternator is definately part of the problem, the ride home was in warmer weather and during a hot idle log battery voltage dropped to about 12.3 with the coolant fan, park lights and heater on, otherwise it hoovers just over 13 volts.

[SappySE107]

I would not lower the pressure to 39psi on a turbo car and I would not consider changing voltage tables until you have a real stable voltage.

[Joseph Upson]

I would not lower the pressure to 39psi on a turbo car and I would not consider changing voltage tables until you have a real stable voltage.

You wouldn't lower the pressure to spec for the injectors even with a 1:1 boost sensing fuel pressure regulator? My preference is higher static fuel pressure than the current ~50 psi for good atomization but I'm not sure the relatively higher pressure is not contributing to idle AFR inconsistency. I don't favor the idea but I want a proper idle among other things and it seems the best approach is being maticulus with the numbers.

The stock injectors for the 3900 are about 28 lb/hr and the DEKA 60s about 62 lb/hr at 42 psi so I'm already above that with current static pressure readings and uncertainty as to how well the 730 ecm is managing them at low pulse width.

My current base pulse width at idle is about .119

I understand the concern about holding off on injector offset changes until the voltage is more stable but I don't expect it to cause much of a problem since my adjustments have been restricted to the low end reading where idle sits to clean up the AFR and eliminate the rich fuel smell.

Is there something I've forgotten to factor in?

[Driver_10]

Im going to use 2 "Peak-n-hold" driver cards and some 70lb Low-z injectors. They can be used on any ecm that controls the injector by switching negative.

http://www.jbperf.com/p&h_board/v2_0.html

It should also help clean up the RPM signal since injector coil "flyback" voltage is kept out of the Megasquirt altogether. Im hoping (more like praying) that I can get way with 14-15 afr's while cruising.

[Joseph Upson]

Im going to use 2 "Peak-n-hold" driver cards and some 70lb Low-z injectors. They can be used on any ecm that controls the injector by switching negative.

http://www.jbperf.com/p&h_board/v2_0.html

It should also help clean up the RPM signal since injector coil "flyback" voltage is kept out of the Megasquirt altogether. Im hoping (more like praying) that I can get way with 14-15 afr's while cruising.

It's much wiser to use no more fuel injector than what's needed especially in a daily driver. The side effects/ trouble with tuning higher capacity injectors is not worth it until all the necessary parameters needing adjustment can be identified. I didn't have an idle problem until I installed the 60s I noticed from looking at video of the car with slightly larger injectors installed.

[Driver_10]

It's much wiser to use no more fuel injector than what's needed especially in a daily driver. The side effects/ trouble with tuning higher capacity injectors is not worth it until all the necessary parameters needing adjustment can be identified. I didn't have an idle problem until I installed the 60s I noticed from looking at video of the car with slightly larger injectors installed.

My turbo flows a little over 72lbs of air/ min. A set of 70lb injectors will support a little over 630-650hp. I need the extra fuel volume, so smaller fuel injectors aren't a possibility. Low-Z injectors allow for faster opening and closing rates than a standard saturated injector. (about .5ms faster)

[Joseph Upson]

My turbo flows a little over 72lbs of air/ min. A set of 70lb injectors will support a little over 630-650hp. I need the extra fuel volume, so smaller fuel injectors aren't a possibility. Low-Z injectors allow for faster opening and closing rates than a standard saturated injector. (about .5ms faster)

If you have a use for it no problem but my turbo is capable of about the same (T67) and my injectors at maximum allowed pressure flow ~85 lb/hr of fuel but I know I'll likely never be anywhere near the limits of either. Not sure the low impedance injectors being able to open and close faster would sufficiently address the potential problem of flowing too much fuel at idle I've been having. In your case the flow is even more and I suspect you have a smaller displacement motor if it's a 60 degree mill. I think the problem is more a calibration and voltage stability problem on my end though. I'm also planning to drop down one turbine housing size for a dedicated street performer which is mostly 0-60.

[Driver_10]

If you have a use for it no problem but my turbo is capable of about the same (T67) and my injectors at maximum allowed pressure flow ~85 lb/hr of fuel but I know I'll likely never be anywhere near the limits of either. Not sure the low impedance injectors being able to open and close faster would sufficiently address the potential problem of flowing too much fuel at idle I've been having. In your case the flow is even more and I suspect you have a smaller displacement motor if it's a 60 degree mill. I think the problem is more a calibration and voltage stability problem on my end though. I'm also planning to drop down one turbine housing size for a dedicated street performer which is mostly 0-60.

I imagine that you must be spiking the pressure up and maxing the PWM to get that kind of flow.

Problem one... Increasing fuel pressure will cause slower closing times
Problem two... overheated coil windings and potential damage to injectors

The peak-n-hold style injectors have larger windings and lower resistance as a result. Because of this, they handle greater amounts of amperage. They use high amperage to pull the needle away from the opening and then use a lesser current to hold the injector open. (hence the term "peak-n-hold")

The problem with them is that they can burn the transistors out of an average ECU due to the high amperage. They can be used with inline resistors, but that defeats the purpose of using low-z injectors all-together.

[Joseph Upson]

I imagine that you must be spiking the pressure up and maxing the PWM to get that kind of flow.

Problem one... Increasing fuel pressure will cause slower closing times
Problem two... overheated coil windings and potential damage to injectors

The peak-n-hold style injectors have larger windings and lower resistance as a result. Because of this, they handle greater amounts of amperage. They use high amperage to pull the needle away from the opening and then use a lesser current to hold the injector open. (hence the term "peak-n-hold")

The problem with them is that they can burn the transistors out of an average ECU due to the high amperage. They can be used with inline resistors, but that defeats the purpose of using low-z injectors all-together.

I know all about the differences between the injectors but I'm not referring to operating outside of the equipments capability, higher fuel pressure is an acceptable option within the injectors limits and a given with a boost sensing pressure regulator.
85 psi flow rate was listed in the ad for my injectors I don't run pressure anywhere near that.

The offset table would handle delayed openning times related to higher fuel pressure as delayed openning occurs with low voltage as well so there's one more reason to tune that table for your injectors whenever operating outside the specified range.

I'm starting to lean away from the "more than you need camp" in favor of the "make up your mind about what you want" and use just enough to achieve it rule for maximum efficiency.

My turbo is a little oversized for what I need along with the injectors and for a street performer that means slower 0-60 times. There's no practical setting on the street to take advantage of a high end motor for me. My choices were for my stroked motor where these parts are better suited.

[Driver_10]

I know all about the differences between the injectors but I'm not referring to operating outside of the equipments capability, higher fuel pressure is an acceptable option within the injectors limits and a given with a boost sensing pressure regulator.
85 psi flow rate was listed in the ad for my injectors I don't run pressure anywhere near that.

The offset table would handle delayed openning times related to higher fuel pressure as delayed openning occurs with low voltage as well so there's one more reason to tune that table for your injectors whenever operating outside the specified range.

I'm starting to lean away from the "more than you need camp" in favor of the "make up your mind about what you want" and use just enough to achieve it rule for maximum efficiency.

My turbo is a little oversized for what I need along with the injectors and for a street performer that means slower 0-60 times. There's no practical setting on the street to take advantage of a high end motor for me. My choices were for my stroked motor where these parts are better suited.

Im sorry, I wasnt trying to be patronizing or anything. I had the impression that you honestly didn't know about low-z injectors. I guess I misinterpreted what you wrote. I keep forgetting the crowd that Im speaking to at times.
My bad...

[Joseph Upson]

No problem, I learned the difference about saturated and peak-hold injectors years ago when much of this stuff was still a mystery to the open market in regards to interchangeability. I was hoping Sappy would reply in regards to my question about why I shouldn't run lower fuel pressure given my scenario just in case a circumstance I'm not aware of is present and a potential sticking point.

It wouldn't be a bad idea to take the time to crunch the numbers and verify that the 60s can handle higher fuel pressure without loosing the ability to run low enough pulse widths for idle. Considering they already flow more than twice as much fuel as the stock 28s at around 16 psi less than what the stock injectors are run at and they are now set to about 50 psi I suspect I'll have no choice but to lower static pressure.