Toyota Supra mkiv
550cc Injector Upgrade

550cc Injector Upgrade

The Japanese mkiv Supra injectors are 430cc per minute injectors and at 80% duty cycle, they will run at about 55hp per injector or 330hp for all six injectors. The 80% duty cycle is the accepted industry standard because with injectors being solenoids, we want to avoid overheating the injectors as this will cause uneven or irregular response from the injectors. At 100% duty they will peak at 68hp each or 410hp for all six injectors. Also at high revs and high boost the injectors may not be able to deliver the required amount of fuel in the time available, even when running at 100% duty cycle. See the RC Fuel Injection article and the September 2000 Performance Car Issue #45 for the source of this information.

Some mkiv owners are running their Japanese mkiv's TT's with more flywheel hp than this (equates to about 315 rwhp on Aussie/NZ dynos), so these calculations are obviously conservative. However, there will come a point where the std mkiv injectors cannot provide enough fuel to keep the fuel ratio sufficiently rich (about 11.5 - 12:1) for the high boost required to produce these hp figures on standard turbos.

From the hp figures some of the group members have achieved and their fueling under those conditions, I would say any more then around 330-340rwhp will very likely result in lean conditions under high boost. A very old saying "better safe than sorry" is the best attitude to adopt, as the mkiv motors are not indestructable. I know of at least one mkiv in Aussie that destroyed itself at 22psi boost, and I have destroyed several spark plugs when I had runaway boost to 25+psi.

Note: Please read all the following instructions carefully and completely before starting. This information is provided on the basis that it is as complete and accurate as possible, but remember, you and you alone are responsible for any modifications that you perform on your vehicles. If you do not have the necessary knowledge to perform these modifications yourself, then pay for an experienced automotive engineer to do them for you.

This information has kindly been provided courtesy of Lindsay in Christchurch, New Zealand who has already modified his Japanese mkiv TT to use the larger 550cc Export injectors. His purpose was to help eliminate the possibilty of detonation occuring under high boost conditions and to avoid running near 100% duty cycle on the std 430cc injectors.

Electrical differences:
  • Std Japanese Import injectors - 430cc High Impedance
  • Std USA/UK Export injectors - 550cc Low Impedance
  • The low impedance type injectors require an EFI or Solenoid resistor to run on 12 volts.

Japanese injector wiring diagram showing the wires for injectors 2-6 coming from the harness at the rear of the car and the wire for injector 1 coming from the harness where it goes behind and under the cam cover.

After adding the EFI Resistor (as per the USA mkiv's) the injector wires now all come from the harness at the rear of the car as the old wire to the no. 1 injector has now been rerouted to the EFI Resistor.

Tools required:
  • Soldering iron.
  • Wire cutters/stripper.
  • screwdriver.
  • misc. sockets, spanners, etc.
Materials required:
  • Second hand (or new) 550cc UK/USA mkiv Supra injectors.
  • EFI 6 ohm Resistor (as per USA/UK mkiv's) with 1 inlet and 6 outlet wires.
  • Plug from another car to match EFI resistor plug (or buy the USA/UK mkiv one).
  • 2m of 7 wire (1 x 2.0mm, 6 x 1.25mm) auto electrical cable.
  • Roll of black insulating tape.
  • Solder.
  • CRC can.
  • Optionally, the USA fuel rail, which is the same as the Japanese one.
  • Optionally, the USA spec inlet cam which allows for about 5% more airflow at the top end.

  • Before starting disconnect and remove the battery.
  • Remove the top half of the inlet manifold with throttle body to access the wiring and injectors.
  • Open up the injector harness where it joins the main engine harness, just above no.6 inlet manifold port.
  • For your wiring extension to the EFI resistor, use 7 core automotive cable (6 x 1.25mm and 1 x 2.0mm wires).
  • NB: Splice, solder and heatshrink connections in a staggered pattern, to avoid bunching of the joined wires.
  • Seperate the 5 black with orange strip wires (one goes to each injector) for no.2 to no.6 injectors and cut them at different intervals to avoid bunching.
  • Note: The ignition feed black/orange wire to no.1 injector is in the harness that comes in over the top of the cam box, behind the cam wheels.
  • Cut and insulate no.1 ignition supply side as this wire will be taped into at the harness near the firewall.
  • Join the 6 x 1.25mm wires to each black/orange wire leading to the injector plugs (solder and heat shrink).
  • Join together the 5 black/orange wires emerging from the main engine harness together with the 7th wire (2mm) out of the 7 core cable.
  • The EFI resistor should have a common line in and 6 wires out, each with a 6 ohm resistance. The EFI resistor Lindsay used is the white box (with red 19X5 written on it) in the picture below. The yellow plug is the connection to the 6 wires leading to the injectors.

  • Connect via a plug (you may have to search for a matching plug at a wreckers), the ignition feed from the main engine harness (2.0mm wire) to the common line in.
  • Connect individually the other six wires (1.25mm) via a plug out, leading to the injectors plug. This effectively connects the 6 ohm resistor series with each injector.
  • Mount resistor on the left hand side (Japanese mkiv passenger side) of the front wheel arche.
  • Reform wiring harness, tape up and secure.
  • Before installing the second hand 550cc injectors, have them cleaned and flow tested first at 40psi. Make sure they all deliver exactly the same amount of fuel. Eg. Mine were flow tested at 40psi, and delivered 92cc each for 10 seconds. To get 60 seconds of flow, multiply by 6 (6 x 92cc = 552cc). This is a very accurate flow for injectors rated at 550cc per minute. These second hand 550cc injectors came from Powerhouse Racing in the USA.
  • Check all "O" rings and replace if necessary. Wrap a cloth around the replacement 550cc injector to cover the filter gauze.
  • File off the 2 side locater ribs (they don't match the Japanese injector plug) to allow electrical injector plug to fit. Leave a little rib to allow for a tight fit.
  • Remove old 430cc injectors. Note position of sealing rubbers.
  • Fit new 550cc injectors; spray some CRC on "O" rings first and they go in real easy.
  • Connect and secure wiring harness plugs to each injector.
  • Now you need to check there are no fuel leaks. On a mkiv you need to turn the key to CRANK to get fuel pressure. The fuel pump starts at 12v while cranking at then immediately reverts back to 9v once the engine fires. Leave the soleniod resistor unplugged (so the engine won't fire) and temporarily reinstate battery power to crank the motor over for a few seconds but at least 3 times. Chances are the fuel rail will be empty and the return line drained back to the tank. Saftey Tip: always have a reliable fire extingusher handy when working on high pressure fuel systems!!!!
  • TIP: Drill and Tap a 1/8" hole in the back of the top half of the inlet manifold before reinstalling it. It's a good place to hook a boost gauge, etc.
  • Reassemble motor, top up coolant, etc.
  • Start up motor. It will sound rich at first, which is a slower warmup, and slower idle speed, etc.
  • Don't despair! Warm it up and drive it normally. Over the next 30-50km (or less) the ECU via the oxygen sensor will relearn to accomodate the larger injectors. The mkiv ECU has enough "intelligence" to be able to do this.
  • Note: Each time you disconnect the battery or pull the ECU fuse, the ECU will reset itself back to its base settings again, and run rich on startup. Just drive it normally and it will relearn again.

Lindsay changed over to the 550cc injectors on his mkiv, in conjunction with changing the inlet cam for an Export spec one. He also removed the viscous fan and replaced it with a 16" electric fan.

See the dyno sheet below that show the hp changes as a result of these modifications. The dotted line (341.9rwhp) is the before and the solid line (357.4rwhp) is the after, an increase of 15.5 hp (4.5%). As you can see all the extra power is after 4500rpm, and is mainly due to the export inlet cam. The bigger injectors don't give any more power (probably a little less on Lindsay's as it's running too rich at the moment) and the 16" fan only gives a saving when the fan is actually running. Peak torque has improved a little more with an increase from 308.3ft/lb to 328.4ft/lb (6.5%).

The oxygen sensor information is only used by the ECU in learn mode, which runs up to around 5000rpm, so changing to the 550cc injectors will make it run a bit rich at the top end, even after the ECU has done its learning. The export inlet cam allows for about 5% more airflow at the top end, so helps compensate for the extra fueling to a certain degree. Here's the calc. 550 / 430 = 1.28 / 1.05 = 1.22. So it's still running around 22% rich. The ECU may be smart enough to not run quite this rich, but based on Lindsay's CO readings, it still runs too rich at high revs.

How can I fix this? You can alter the MAP sensor signal by use of variable potentiometers to trick the ECU into supplying less fuel at the top end. You could make this yourself (see Autospeed DIY Injectors article) or buy an item such as the flowmate fuel controller that allows you to adjust the MAP sensor signal, before it reaches the ECU. This should then allow the original fuel economy to be largely maintained. Most fuel cut devices also alter the MAP sensor signal to eliminate the fuel cut that occurs at around 15-16psi on the mkiv Supra. You cannot run them both at the same time as they both usually plug into the ECU at the same place.

NB: Blowing-up an engine (especially a turbo one) developing a heap of power is as easy as one power run with lean mixtures. Mixtures should be monitored by an air/fuel ratio meter. Doing this with the car on a dyno gives the best control over what is happening. Start off rich and then lean things out - don't go the other way.

When adjusting the MAP sensor signal to the ECU, first disconnect the oxygen sensor, otherwise the ECU will be trying to learn while you are altering the MAP sensor signals to the ECU. Concentrate on getting the high loads right first (5000-7000rpm), rather than the light loads. However, when the mixtures are set perfectly for high load conditions, then they may not be right at light loads because of non-linearity in the engine's fuel needs when compared with its unmodified state. If the light loads look a bit too rich (or lean for that matter), don't worry; when the oxygen sensor is re-connected they'll lean (or richen) as the ECU learns its new mixtures.

What about the fuel cut that occurs at 15-16psi? If you reduced the MAP sensor signal by say 15%, then just using the flowmate (or the homemade device) alone, would increase fuel cut from 15-16psi boost (30-31psi absolute) to around 20-21psi boost (35-36psi absolute) which is higher then the max boost you would want to run anyway.

Any questions or comments, then please email Lindsay

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Last modified on Tuesday, July 19th, 2000