The more power the more fuel required. A simple correlation. Fuel is supplied by the fuel injectors. Simple again. So what do we do if at a given boost level the injectors cannot supply enough fuel to meet the demands of the ECU to produce the power? Get bigger injectors or increase the fuel pressure to the injectors so they squirt more for each open and closing cycle. As background, on my car equipped with an Apexi PowerFC, at 12psi I was seeing fuel injector duty cycles well into the 90% range. This means that they are operating at 90%, or more, of what they are capable of. It is a common thought among the performance crowd that operating above 85% reduces the ability of the injector to meter fuel accurately. In essence rather than opening and closing the injector is basically just operating open all the time and injecting fuel in a steady stream. One important thing to note is that the injector may not be injecting more fuel, just not metering it as it should in the high RPM band. This is the case because as the RPMs increase the "window" available in which the injector has the time to squirt in the fuel gets shorter and shorter the faster the engine spins. It thus will get to a point where it simply cannot inject in enough fuel in the time window allowed. Back to my personal experience, seeing injector duty cycles in the 90+%, some may argue that this could be reduced by more careful tuning. This could be true as my wide band O2 sensor is indicating a rich mixture in the mid 10's at WOT in the higher RPM range. However, in the interest of long term planning, and the desire for some margin for safety I decided installing larger secondary fuel injectors would not be a bad thing.
There has been much debate regarding the best route to take when upgrading the fuel injectors with the following options: 1)Bored out OE injectors (850cc to 1300cc), 2)Bored out OE injectors (850cc to 1200cc), and 3)1600cc Bosch injectors. The debate lays with the argument over reliability, that being the bored out OE Denso injectors are less reliable than off the shelf brand new Bosch injectors. However, the Bosch injectors are not without their drawbacks. They require a new fuel rail, as they are different by size and design. And the Bosch injectors require more fine tuning to make the transition between the primary injector operation and the secondary injector operation, i.e., 550cc to 1600cc is a much bigger jump than 550cc to 1300cc. Some other things to consider also are the cost of each, the Bosch injectors and new fuel rail being considerably more expensive, and also the real need for the very large 1600cc injectors. 1300cc secondary injectors are just adequate enough to feed a 400rwhp motor, the most I woudl ever seek to attain. Given all the above I weighed the pros and cons of each, the fact that I still run the stock turbos on the original motor, and the balance of my checkbook I decided to go with a set of the bored out to 1300cc Denso injectors. I purchased a set of 850cc injectors for cheap to use as my cores and sent them off the SR Motorsports for the work. RC Engineering does the actual work but I decided to go through SR as they were the same price and I had a gift certificate to use. Having a spare set of injectors also means minimal down time for your car as it took about 10 days to get the injectors back from SR.
With the upper intake manifold off to relocate the air intake sensor the next steps were somewhat straight forward. Perhaps the trickiest part is removal of the air control valve in order to gain access to the rear secondary injector. The ACV is held in place by 3 8mm nuts. Two on the top and one absolutely buried underneath. The real problem is that the solonoid rack and vacuum lines make getting a socket on that one 8mm nut underneath very VERY difficult. I ended up using a 1/4" drive ratchet with a tiny extension and socket. With the ACV moved to the side (one electrical connector must be unplugged) access to the top of the secondary fuel rail is pretty easy.
The injectors are simply held in by a retainer cap. Disconnect the injector from the electrical harness and remove the retainer cap. The injector can be pulled out gently with a pair of pliers on the orange portion of the top cover. Mine came out pretty easily with twisting pulling motion.
With the factory injectors out check the socket for debris or anything that might inhibit a good seat of the new injectors. It is imperative the injectors seat perfectly or else a fuel leak is a certainty. With a side feed injector there are two O-rings. The upper prevents fuel from leaking out into the engine bay and the lower prevents fuel from bypassing the injector and dumping directly into the intake. Without a perfectly seated injector both will occur resulting in fuel dumping onto the floor under the car and a car that won't run. Don't ask me how I know! So when installing the new injectors use a little bit of bearing grease or engine oil on the O-rings (the injectors will come with new O-rings - never re-use the old ones) and push them all the way in by hand. Do not use the retainer caps to press them as you tighten the screws. There would be a good chance you will pinch an O-ring. Don't ask me how I know. See a trend here? With the injectors in, simply reassemble in reverse order: retainer caps on, injector plugged in, ACV reconnected, etc. It is adviseable to pressure test the fuel rail per the FSM to ensure you have no leaks before completing the reassembly. And a trick to getting that lower 8mm nut back on the ACV is to put a small piece of paper in the socket and wedge the nut in the end of the socket so it won't fall out until it's on the stud and you pull the socket off the nut.
So there you have it. Not much to the install really. Most of the effort is
just getting to the point where the UIM has been removed. The next step is to make
any tuning adjustments on your ECU to accommodate the larger secondary injectors.
With the Apexi Power FC and Datalogit this is fairly elementary.
Using other peoples experience with the best settings to assure a smoother injector transition from primaries to secondaries I used the following inputs:
Lag: 0.16 (SR setpoints at 0.04 but I've seen #'s as high as 0.20)
Time: 0.98 (from 1.5 for the stock 850's)
In addition to, of course, changing injector size to from 850 to 1300.
To prove that all worked as intended a few pulls to redline at 12psi while datalogging indicated a peak fuel injector duty cycle of only 79%. Exactly where I'd hoped it would be. And with a smooth injector transition.
As a sidebar to my work installing the new injectors, this fairly straightforward 4 hour job turned into a 15 hour nightmare. After installing one injector and not getting it seated properly due to a pinched O-ring, I opened up the car again to see what was amiss. In the process of removing the ACV, and being somewhat annoyed that I'd screwed up the job somehow, in my haste I neglected to cover the lower intake runner ports, and as if God had willed it himself I dropped one of the 8mm ACV nuts straight down the intake runner! Click here to see what transpired next.
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