8 Injector Fuel System

If you have a turbo kit on your second generation "non-turbo" Talon or Eclipse, fuel is a big concern for you. Stage I and II kits utilize a high gain fuel pressure regulator (FPR) that increases fuel pressure in relation to boost pressure. For various reasons, you may have considered (or are using) larger fuel injectors. This generally requires the use of an Apex'i S-AFC in addition to the high gain FPR. This is also not a perfect solution. The S-AFC tricks the ECU into running the larger injectors. The "trick" is that the S-AFC interupts the intake manifold pressure signal to the ECU and modifies it. The ECU goes about its business using the modified pressure signal. Manifold pressure is used to calculate fuel needs. It is also used in some other calculations. By modifying the manifold pressure signal, you not only modify the expected fuel needs, but also things like ignition timing. Poor timing can lead to knock, detonation, lowered efficiency, and other undesireable things. The S-AFC has other limitations. It can only modify the pressure signal by +/- 50%. This places an upper limit on your injector size. There are also some issues with modifying the output of a sensor that maxes out at around 1 psi of boost. The stock pressure sensor varies its output linearly from 0 to 4.9 volts. Once you hit about 1 psi of boost, the sensor output stays at 4.9 volts no matter how much pressure is in the manifold. This just leads to some innaccuracies in fueling. Many people use the S-AFC with good results. It is a decent option, but I believe there is a better choice for similar money. That choice is the extra injector controller (EIC).

Background

Please read up on how the MFI system works before continuing on.

You might also want to read up on choosing injectors. That also covers how injectors work.

Example setup

The setup shown uses a modified stock lower intake manifold to house 4 extra injectors. The injectors are fed by a 1997 stock fuel rail as are the injectors in the stock location (fuel rails have the test port on the end). The fuel rails are mounted in series and the injectors are 252 cc/min Accel injectors meant for a Ford. An Aeromotive 1:1 FPR is used to match fuel pressure to intake manifold pressure. The extra injectors are controlled by an 034 EIC. See this link for information about this EIC. It is a universal PC programmable EIC and easily adapted to our car. It reads manifold pressure with its own sensor and reads RPM from the tachometer signal. You can create 3D fuel maps and alter many variables to suit your needs.

The stock ECU continues to control the 4 injectors in the stock mounting position. These stock injectors provide all of the engine's fuel needs from full vacuum to 0 psi of boost. Between these operating points, stock driveability and efficiency are retained. From 0 psi to your maximum boost, fuel needs are provided by the combined efforts of the stock injectors and the extra injectors. The stock ECU still controls the stock injectors. The EIC controls how much fuel the extra injectors supply. The EIC lets you provide the exact amount of fuel that you desire when boosting.

Since the EIC can provide however much fuel you want, you don't need a high gain FPR. All you need is a 1:1 FPR. This regulator raises fuel pressure 1 psi for every 1 psi increase in manifold pressure. Since fuel flow from an injector is based on manifold pressure in addition to fuel pressure, this ensures that the EIC will always have the same amount of fuel to work with. In this case, the fuel rails are mounted in series, with the FPR in the return line. Fuel pressure is essentially the same in both rails. You do want to choose a high performance FPR that will hold its settings. The Aeromotive 13109 is a very nice unit with less than 1 psi variation from cold to hot (cheap ones can vary 15 psi).

The setup shown uses 252 cc/min Accel injectors meant for a Ford. These injectors are the same dimensions and similar impedances (10-15 ohm) as the stock units. They are a drop in replacement if you account for the increased flow potential (stock is 235 cc/min). You just have to lower the fuel pressure some to account for that. I am using these injectors because they are of a high quality and affordable. You can often find fuel injectors meant for domestic V8's for much less than what "import" injectors cost. You generally have to buy them in sets of 8. In this case, that is not a bad thing. You can also let the stock ECU control 4 stock injectors and have the EIC control 4 big injectors. It all depends on your fuel needs. In this case, the ability to dump 504 cc/min fuel per cylinder was plenty.

Building the manifold

Once you have chosen injectors, you need to pick a mounting location in the intake manifold. It is best to buy a spare manifold to work on. You can mount the injector on the bottom of the manifold, near the head. This is a good location, except that it can interfere with the alternator. Mounting them here is ideal, but you have to measure alot and do some test fitting to get it to work. A far easier solution is to mount them near the bend on the underside of the manifold. This keeps the injectors away from the alternator and other items. Wherever you put the injectors; you must do a visual inspection of the car to make sure that everything will fit. Since these secondary injectors are only used when alot of air is moving through the intake, their location is not all that important. The velocity of the air will carry the fuel right into the engine. If these injectors were expected to run at idle and low loads, location would be important for driveability.
You need something for the injector O-rings to seal against. This requires a very smooth cylindrical surface. The inside of drawn aluminum tubing is appropriate. You can buy special injector bosses (mounts), but tubing is often cheaper. You want the injector O-ring to fit tightly into the inside of the pipe.

When you pick your rough injector location, you need to mark where the injectors will actually penetrate the manifold. Push the injectors into the fuel rail you have chosen. A 1997+ 2GNT rail works well, but you can use alot of different rails. Line up the tips of the injectors on the manifold where you want them to go. Mark each location with a permanent marker. You will likely need a machine shop to drill the holes for the injector bosses. You want the hole drilled at about a 20 degree angle from the horizontal. You want the fuel to spray in nearly parallel with the direction of air flow. You really need a drill press (or better) for this, so a machine shop is often the best choice.
When the holes are drilled, you will need to prep the parts for the epoxy. Both the manifold and bosses should be roughed for the epoxy to stick to. Then clean the pieces with soap and water to clean off any grease. Scrub hard to get it all off. Make sure you wash all of the soap off. You can opt to do a final cleaning with alcohol if you want. Now slip the bosses onto the injectors (they should be mounted on the fuel rail too). Then maneuver the boss/injector/rail combination to slip the bosses into the manifold. The weight of the rail and injectors will hold everything into the holes in the manifold. You can then mix up a large batch of epoxy. Mix it thoroughly and start globbing it onto the joint between the bosses and manifold. Work it all the way around the boss, like you are welding the two together. Don't be stingy; extra epoxy will just make the joint stronger.

Wait the proper amount of time for the epoxy to harden completely (often 24 hours). Then you can concentrate on supporting the fuel rail. For the stock rail, the mounting method above works well. Its just 1x1/8" aluminum bar bolted to the manifold with long bolts running through it to the rail mounts. A nut and lockwasher is used to secure the fuel rail to the end of the long bolts. You can also get proper fuel rail mounts welded to the manifold. Whatever you do, you must have a secure mount for the rail. If the fuel rail can move, it can put some force on the injector bosses and break the epoxy.

If done right, the long bolt method will securely hold the fuel rail. Above, you can see the assembled manifold. You can use the test port on the primary rail as an outlet. The outlet of the primary rail leads into the test port of the secondary rail. You must use a tire valve tool to remove the shrader valve from the test port on each rail. You can choose to run the fuel rails in parallel. Put a tee in the feed line from the fuel pump. Send that to the inlet on both rails. Then run a line from the output of each rail to a dual inlet FPR (the Aeromotive 13109 FPR has dual inlets). Its all up to you. It is easier to run the rails in series.

Wiring the controller

Each EIC has different capabilities and wiring needs. This page will focus on the 034 EIC. If you want to maintain a colored wiring scheme you will want to buy some extra wire. Use something fairly heavy for the EIC power, EIC ground, injector power, and injector drivers. 14 gage will work for that. You can use 24-20 gage wire for the tach signal. To connect the injectors, you will need a spare injector wiring harness. Cut off the end of the harness that would lead back to the ECU. You should have 5 wires. The 12 V power for the injectors runs though one wire. The other four wires are injector drivers for each injector. You want to connect the driver for 1 and 3 together. Connect the driver for 2 and 4 together too. You will only be running 3 wires from the injectors to the EIC. There is a good chassis ground for the EIC on the firewall behind the intake manifold. You should use a relay to supply 12V power from the battery only when the stock ECU is powered. You can just tap into the stock ECU wiring harness to get the switching power for the relay. This will keep the unit from powering the injectors when you shut the car off (and have your software set to supply fuel at or below atmospheric pressure). Add a 5.6-5.7 kilo-ohm resistor in the tach signal wire running to the EIC. This will ensure that the stock tach will still function properly. For my car and many others, it is the white wire in the stock ECU wiring harness. Be careful when routing all wires. Keep them away from the EGR pipe as it can melt them. Solder all connections if possible. Use shrink tubing on all exposed connections. Follow the wiring diagram below.

Here is the engine bay with everything installed. You can see the 034 EIC on the upper left sitting on top of the cruise control box.

The cruise box has been lowered a couple inches so I could mount the controller on top. The FPR is on the upper right where the relays were. The relays were taken off of their bracket and laid down next to the stut tower.

Setting up the software

The 034 controller is PC programmable. There are several parameters that must be input before you can start tuning. I suggest that you set the "fuel cut below" to atmospheric pressure. I have found that this gives the best results. You'll have to power up the EIC with the car off to read the actual atmospheric pressure. Depending on elevation, it could easily vary from 95 to 105 kpa. You can play with that parameter some to improve the transition from 4 to 8 injectors. Adjusting the "MAP cut rate" can help if you have hesitation while spooling or letting off gas. Values of 20 to 30 seem to work well. "Injector inhibit timer" can be useful if you set the "fuel cut below" to a value less than atmospheric pressure. That would keep the secondary injectors from firing while you try to start the engine. The picture below shows some common settings.

Tuning

You need a datalogger to do this properly. You can't easily predict how much fuel your setup will actually supply, so you need some help. You can get dataloggers from a variety of sources. The one from OBD-2 is a good choice. After starting the car and verifying that everything is working as it should (no fuel or air leaks), you need to start tuning. Assuming that you disconnected the negative battery terminal before installing all of this, your ECU will need to relearn its fuel settings. Hook the datalogger up to the car and start reading long term and short term fuel trim (engine needs to be running).

To start, adjust the fuel pressure to minimize long term fuel trim (you should get it to +/- 2%). The car is driveable now. Carefully cruise around town (below 0 psi boost) reading fuel trim. It helps to have a passenger read the data. You must keep the ECU in closed loop mode for all of this tuning. Fuel trims are meaningless if you are in open loop mode (about 80% throttle or more). You now want to check what the long term trim is at 0 psi boost. You want it to be well below 25%. Lets say 15% or less. If it ever reaches 25%, the short term trim will have to start compensating for the maxed out long term trim. In addition, the fuel map for the stock ECU tops out around 0 psi boost. When you go beyond the range of the stock ECU fuel map, it will stop adjusting long term fuel trim. For example: If you were at -17% long term trim around 0 psi boost, the long term trim will stay at -17% as long as you are above 0 psi boost, NO MATTER WHAT your fuel needs are. If your long term trim is maxed out and locked, the short term trim may have to compensate for this, ruining its ability to react to changing engine conditions. Short term trim will continue to adjust even if you go beyond the range of the stock ECU fuel map. Once you verify that your long term trim is acceptable at 0 psi boost, you can continue.

Tuning for boosted conditions is just as easy, but a little more dangerous. You must continue to drive the car so that the ECU stays in closed loop. Your will rely on the fuel trims to tell you how much fuel to add or subtract. You first want to slowly boost to 2 or 3 psi, while reading fuel trim. Both long and short term trim will likely be positive (if you started with a low number for the injector scaler). A positive fuel trim means that the stock injectors are having to add extra fuel to keep the engine running right. The maximum fuel trim is 25% for each trim factor. If your short term trim is very high, increase the injector scaler to add some more fuel. It is best to tune with the short term trim at a low number. You need "breathing room", so the ECU can make adjustments. If your short term trim is negative, just leave it for now. Running rich will give you some safety. Once you have your short term trim near zero at 2-3 psi, move up to 5-6 psi. At this point, the extra injectors will be providing a large percentage of the necessary fuel. You should adjust the injector scaler to minimize both long and short term fuel trims. Stop when they are +/- 5%. You should now be able to safely boost to 8-9 psi. Do the same thing here as you did for 5-6 psi. You will probably just have to increase the injector scaler a little bit. Keep doing this until you reach your maximum boost pressure. You want to have your short and long term trims at +/- 5% at max boost (still in closed loop mode).

Your EGTs may climb near 1600F while you are boosting under closed loop. This is because the ECU adjusts fuel for maximum efficiency from the catalytic converter. Don't keep the engine running at a high EGT for long...just long enough to get a good reading for the fuel trim. You may want to let the engine cool for a mile after boosting. You will be boosting repeatedly, so you don't want to overload the cooling system. So once your fuel trims are near zero at full boost, you should be safe to go into open loop (full throttle). The stock ECU will inject some extra fuel when it goes into open loop. You should be running rich now and the stock ECU no longer adjusts using fuel trim. If it is too rich for your taste (really low EGTs and high O2 voltage), you can decrease the injector scaler to supply less fuel. Once you have the engine nearly tuned, you will want to do some high load testing. Find an interstate road with a steep uphill grade and boost up it. This places a high load on the engine and you can adjust the fuel to run safely at these loads. When you have it tuned to run safely at high loads, it should do fine at lower loads. Many people like to keep the EGTs around 1400-1500 at max load. EGTs will slowly climb if you boost for a long period. Just keep any eye on the EGT gauge if you decide to boost for a long time.

Thats pretty much it. It takes alot of time to build, install, and tune an extra injector setup, but I believe that it is worth it. Take your time and be careful. There are many different ways to add extra injectors to your car, you don't have to do it exactly like this. Think it over and choose a solution that works for your needs.

Contributed by Corbin
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Cars Modifications Power Turbo Fuel_System 8 Injector Fuel System

Document statistics: Last modified on 2008-12-14 13:00:15 by Corbin