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.