Fine Tuning: Inj. Offset v BATT

[XfireZ51]

Like many other ZR-1 owners, I swapped out my injectors some time ago to the Accel 21#. Pretty much an accepted practice for owners. When I began modifying my 84 Xfire,I found out it wasn't good enough to throw parts on it. Once you did that, the hard part came with need to tune it and then fine tune it. Most modifications will require at least a datalog to see if anything material has changed in the performance of the motor. Some parameters may require a tweak or a refresh. With the swap of injectors,they appear to work well enough without any further changes, but like anything having to do with tuning, it can always be better. Sort of the curse of tuning. You do it because you can. So it is with installing a set of Accel 21# injectors. Two things to consider when doing this swap.

1. The Accels are rated 21#/hr but at 39psi. Our fuel rail runs 43psi. The additional pressure makes these injectors effectively 23#. They work well because the calibration uses 22.78#

2. The Accels do not have the same injector opening time that the stock calibration calls for. This is where the Inj. Offset comes in.

Since emissions and performance requires a consistent delivery of fuel, the calibration must account for variation in the electrical power and the
affect on the fuel injector operation. As voltage is reduced the ECM compensates for the increased injector opening time in order to deliver the same amount of fuel. As voltage increases, the offset decreases. The electrical behavior of the injector varies by manufacturer and at times by the injector size. There is also a variation on the basis of the pulsewidth duration itself, but its not as critical.
So, the LT-5 calibration comes with a set of Inj. Offset values matched to the charateristics of the Bosch Multecs used. The Accel injector offset is different from that of the Bosch. Some manufacturers make the Inj. Offset available to tuners and publish them. Bosch does, Accel says they don't know what I am talking about. This is important since the more accurate the information is, the closer to optimum you get w control of fuel. I finally came across a source for the information on Accel injectors although the table of values I have are for 55# injectors, not 21. I was told that Accel injectors have the same Offset between the 15-75# injectors. Looking at the values, it appears that the Accels are a "slower" opening injector so more offset needs to be used to match the "effective" pulsewidth of the Bosch. Increasing the Offset is a method of increasing the fuel delivered and reducing injector duty cycle. It affects the idle portion of the VE table the most since the opening time of the injector is a much larger percentage of the overall calculated pulsewidth. But it can still help reduce VE% at WOT where it could be capped at 99% w a high
(> 85%) injector duty cycle. It can help avoid the need for installing "larger" injectors which would require additional calibration changes.
At (23#/hr x 16 injectors) / BSFC of .45 = horsepower supported at
100% DC x 80%DC = 654hp.
I have played around w the Offset and found that modifying it helps to steady the AFR on the WB. You get less variation in the idle although I haven't made the change called for in the table of values I just got.
When the weather gets better, we'll see.
Maybe the guys at RC or FIC could help w confirming the Offset for certain injectors we use on the ZRs.

[vilant]

Thanks for the info. I just changed mine to the Delphi inj. from FIC. I have noticed she idles high at first but settles down after it's warmed up. Related? Don't know, but it would good to know what differences, if any, there are between my Deplhi's and the originals. I just bought a bunch of software that allows me tune the car. Haven't learned enough yet to take that step, but it's nice to know if I need to change something, I can.

[XfireZ51]

The high idle is part of the IAC map for warmup. Want the motor to warmup asap for emissions purposes.

[scottfab]

Makes one wonder about the offset variance due to temp of the injectors too. Not every injector vendor will have the same thermal effect. I'm thinking more of pintle vs rotory injectors but even the variance of one pintle design vs another would be interesting. A very hot inj vs a cold one.
Then there is the variance from inj to inj in the same batch. I know the stock injectors were way off from each other. The RC ones are rotary and matched.

Something else comes to mind, not inj related.
Some have relocated the air temperature sensor. Without tuning
for that move there maybe even more optimization that could be done.

[XfireZ51]

Scott,

Funny you should mention the MAT/IAT relocate. Something else I have been toying with looking at in terms of making sure the ECM is accurately interpreting the data its getting. The MAT v Compensation table assumes the temps the MAT will see are much higher than an IAT will report to the ECM. Air temp at the radiator will vary much less than at the airhorn. Then there's the issue of the MAT sensor heat soak.

[Tyler Townsley]

Originally Posted by XfireZ51

Scott,

Funny you should mention the MAT/IAT relocate. Something else I have been toying with looking at in terms of making sure the ECM is accurately interpreting the data its getting. The MAT v Compensation table assumes the temps the MAT will see are much higher than an IAT will report to the ECM. Air temp at the radiator will vary much less than at the airhorn. Then there's the issue of the MAT sensor heat soak.

It should be noted here you are talking about closed loop situations and open loop situations as if they are the same, they are not.
Most of the above is talking about drivability issues but apply in open loop, once the engine goes closed loop (154 deg) the closed loop adjustments essentially render them moot. IE if the O2 sensor sees a rich mixture caused by inaccurate MAT it will adjust fuel to lean the mixture as long as it is closed loop.

The other thing to remember is when you go PE (pedal to the medal) none of the above apply as you go straight to PE tables that ignore any input from the various sensors.


Tyler

[scottfab]

Originally Posted by XfireZ51

Scott,

Funny you should mention the MAT/IAT relocate. Something else I have been toying with looking at in terms of making sure the ECM is accurately interpreting the data its getting. The MAT v Compensation table assumes the temps the MAT will see are much higher than an IAT will report to the ECM. Air temp at the radiator will vary much less than at the airhorn. Then there's the issue of the MAT sensor heat soak.

Yes the issue of heat soak has always concerned me and is certainly a good reason to relocate the sensor and yet I have resisted doing it.
There is solid empirical evidence to the heat soak issue on a hot day after a run to the store then getting back in to find performance is muted. It's a "bummer" kind of feeling. What I'd like to do is leave the sensor there but insulate it via the treads without offsetting it out too much. It's on my list. I have good bench mark data recorded with my diacom so a comparison can be made.

In any event it's fun to look at and understand all the minute details of operation and tuning.

[XfireZ51]

Scott,

I wrote this on another board dealing w the issue of re-mapping the MAT table when it has been relocated and used as an IAT instead.

I appreciate that you keep bringing this topic up because it does interest me particularly since I moved by MAT sensor from the airhorn to a spot behind the air filter but ahead of the airbridge(which BTW needs to be stiffened or it will collapse at WOT). Now the table in the $D0A mask is a
MAT Temp C v Compensation. The temps in the table go to 151 C which makes sense since the airhorn is attached to the motor. However, by moving the MAT to an IAT location, my thinking is the IAT will never "see" that 151 C temp since incoming air probably won't go above say 51-55C. And at speed, the IAT should be measuring close to ambient temp air. IOW, much of the action the IAT will see is at the lower part of the table. Let's say I drive the car from spring to fall, so temps may range from 5C to 45 or so.
Having said that, if I had a corresponding MAT installed, while the IAT is seeing a fairly stable temp for air, the MAT temps could continue to rise from heat soak and additonal friction on intake. So I tried to correlate what the current table for MAT has as a compensation and move those values further down the table to the IAT measurements corresponding to the MAT measurements. Let's say the motor is operating nominally at a coolant temp of 80-84C. However, I am driving in 10C weather. Since the ECM is seeing
10C temps, I felt I needed to have the 80C MAT compensation values in the
10C cell of the table at least as a beginning point.
BTW, feel free to tell me I am totally out to lunch on this, but it helps to write it out and check the logic of what I am doing.

[scottfab]

Originally Posted by XfireZ51

Scott,

I wrote this on another board dealing w the issue of re-mapping the MAT table when it has been relocated and used as an IAT instead.

...snip....

IAT = Incoming Air Temp
but you could use terms like
PAT = Plastic Air Temp
or
BTF = Bellows Air Temp
kidding...

I once measured the temp on the intake manifold at 181F (83C) on a heat soaked situation. (I was actually just testing the IR temp gun I had just purchased) so 150C in the table seems high. I could be remembering this wrong though. I suppose that temperature could go higher if the coolant routed to the TB has been diverted but my coolant still goes through it. In this situation it would seem the coolant keeps the manifold from getting wildly hot?
I know coolant was routed through the TB to keep ice from forming in the manifold but seems to work the other way too.

What about the danger of using IAT when the air is bound to be heated by the manifold on the way in? Actual temp is probably higher? Obviously there is not much danger or we'd be hearing of engine damage from those who have move it from the MAT location but.... in as far as tuning goes and being accurate??? dunno
Maybe the error is so minor that it's in the noise floor.
That's the thing about insomnia. It allows you to think of such things