Fuel System Tech

The fuel system is oftentimes the most misunderstood part of modifying an RX-7. As we know, not having enough fuel to go with the air you're pumping in the motor can lead to an increase in combustion temperatures, which causes detonation. On this page, I will explore different avenues of modifying the fuel system, and the pros and cons of each.

First off, how is the fuel system handled on a stock car? On 2nd generation RX-7's, the fuel delivery is handled by a fuel injection system, based on the Bosch L-Jetronic fuel injection system. There are 4 fuel injectors that actually deliver fuel into the motor - 2 are primary injectors, and 2 are secondary. Primaries fire below 3800 RPM, and the secondaries are brought on, along with the primaries, at 3800 RPM to redline.

The stock fuel injectors are top-feed Nippondenso injectors - 460cc in the non-turbo models, and 550cc in the turbo models. The size of the injector denotes how much fuel the injector can flow in a given amount of time at a given fuel pressure.

Fuel pressure is the second part of the equation. There is an in-tank electric fuel pump which feeds fuel to the injectors. The extra, unused fuel is recirculated back to the gas tank. To make sure there's enough fuel pressure behind the injectors, there is a fuel pressure regulator in the system. It's located after the fuel injectors in the fuel path - it "pinches" the fuel return to bring pressure up. This is similar to pinching a garden hose - the water coming out of the hose gets to be less, but pressure in the hose increases (you can see the hose bulge up). The stock fuel pressure regulator (FPR from here on) has a vacuum reference - vacuum and boost can raise and lower the base fuel pressure, so the fuel pressure matches the engine's demand.

The fuel injection system itself has many sensors that the ECU looks at to determine the appropriate amount of fuel to inject. These are:

Airflow meter - this is attached to the stock airbox. Air entering the engine is measured by volume, either by a flapper door that the air pushes against (86-88), or a sliding cone the air pushes against (89-91). This lets the ECU know how much air is entering the engine.

Water temp sensor - this is on the back of the water pump housing. A stone-cold engine doesn't atomize fuel well, so the ECU injects more fuel when the car is cold to make up for this. As the engine warms, the ECU slowly takes fuel out of the mixture.

Air temp sensor - this is on the throttle body inlet (turbo) or on the upper intake manifold (non-turbo). This reads the temperature of the incoming air. Colder air is denser, and requires more fuel, since there's more available oxygen in a given air volume for combustion.

Throttle position sensor (TPS) - this is located on the throttle body. The TPS for the 2nd generation cars is pretty basic - it tells the ECU when the throttle is closed, and when the throttle is wide-open. The wide-open range of the TPS is actually about 1/4 of the full throttle travel. The '89-91 TPS's have an extra throttle sensor that's full-range. This is used for the ECU-controlled oil metering pump, and not used in fuel injection calculations, but the full-range TPS can be used with aftermarket computers.

RPM signal - this comes from the crank angle sensor (CAS), which is in the front driver's side corner of the engine. It knows exactly the position the engine is in for the ECU to orchestrate fuel and ignition events.

Pressure sensor - this is on the passenger side shock tower with a vacuum hose running to it. This can sense how much vacuum or boost is in the motor, but no one is totally sure how much the ECU uses this input.

So, let's say you start modifying your car. Most modifications concentrate on changing the volumetric efficiency of the motor - getting more air in, and getting the exhaust out. An engine is simply an air pump - cram more air and fuel in the engine, you get a bigger bang, and make more power. Making the engine breathe easier makes it more efficient, resulting in more power.

The stock system can deal with a few modifications, and still be able to accurately supply the air/fuel mixture to the car. When you get past a certain point, you operate beyond the boundaries of the stock ECU, and you are not able to maintain a safe air/fuel ratio.

So, what are different ways of maintaining a safe air/fuel ratio?

First off, you can work around the parameters of the stock system to deliver more fuel. This includes intercepting and modifying the inputs to the ECU to fool it, changing the output devices the ECU controls, or adding more fuel on top of what the ECU is providing.

First up is fooling the ECU by modifying inputs. People have tried using the stock water temp sensor to richen the mixture. This works to some extent, but doesn't give a lot of headway for more power, is impossible to tune for smooth power, and just isn't very elegant. The airflow meter signal can also be modified -this has become very popular in the aftermarket recently. These devices really aren't helpful for *adding* more fuel, as the stock ECU will use the injectors for all they're worth. But, you can install larger injectors, usually in the primaries, and use the device to tune the injectors in for a better amount of power. The Apex'i Super-AFC is a good example of this type of device.

Another point of contention is the fuel cut - on the turbos, the factory ECU will cut fuel to the rear rotor at around 8.5 psi. This is felt as a violent jerking of the whole car. The fuel cut is there to warn the driver that there's something wrong with the turbo, and to take it to the dealer. Well, when you want to go past that amount of boost, there are a few ways of doing it. First is a fuel cut defender, or FCD. This caps off the amount of boost the ECU sees, so it never sees enough boost for fuel cut. The other way is a reprogrammed ECU that has the fuel cut removed.

An FCD is not a license to run as much boost as you want - you always have to have your fuel and air temperatures right, or the motor WILL blow. A car can be safely made to run quite a bit of power with an FCD, as long as the fuelling and air temps are taken care of.

Another FCD alternative, and a way to increase fuel flow, is the HKS FCON (Fuel CONtroller). The unit is relatively expensive ($1000), but can be found used for much less. The FCON comes with an ECU brain box, a wiring harness that plugs in between the stock ECU and the wiring harness, an FCD, and a boost sensor. The FCON has a chip in it with a new fuel map that has been optimized for an upgraded car. The FCD keeps the ECU happy and away from fuel cut, while the FCON's own boost sensor knows exactly how much boost the car is running, and meters the fuel from its own maps accordingly. The device works extremely well, and can be tuned somewhat with HKS's Graphic Control Computer (GCC). This lets you tune the fuel maps at various RPM points for better power. There are also 2 chips for the FCON - a stock turbo chip, and an upgraded turbo chip for a full turbo setup. The downside to the FCON is the chip is tuned around a stock car's fuel system - stock fuel injectors and stock fuel pressure. Changing these will cause the FCON to run extremely rich, which could possibly be tuned in with the GCC.

Another way to add fuel to the stock ECU is with additional injectors. These injectors are installed in an intercooler pipe or in the upper intake manifold, and have a separate controller to program them. The best additional injector controllers use both RPM and boost to setup the fuel injectors. You set a start RPM and a start boost pressure, then set a rate of gain for the pulsewidth after that point. Tuned properly, the stock ECU and fuel injectors handle most of the driving around work, but when you get on it, the additional injectors come on where the factory injectors run out of steam, adding more fuel to the mixture. AIC's can be a bit tricky to tune, but they work incredibly well.

Changing the car's outputs can also richen fuel mixture up. It's very common to install larger secondary fuel injectors - either 680cc injectors from the GSL-SE RX-7, or new 720cc injectors. Since the secondaries only fire above 3800 RPM, this gives extra fuel where it's needed. These can be further tuned in for optimum power with a airflow meter tuning device, like the Apex'i Super-AFC. This will provide some extra fuel, but the difference in the amount of fuel isnt' that much - it's a little extra headway for more power.

Another popular alternative is upgrading the fuel pressure regulator. There are a number of rising-rate fuel pressure regulators, or boost-dependant fuel pressure regulators (BDFPR). A BDFPR has numerous adjustments that lets you increase the fuel pressure with boost - a vacuum line to the BDFPR tells it how much boost you're running, and adjustments on the unit lets you set where it starts increasing fuel pressure, and in what ratio. A stock fuel pressure regulator adds in some extra fuel pressure wtih boost, but a BDFPR will let you add quite a bit more fuel pressure with boost. This can work, but is tricky to tune properly, and doesn't give a lot of headway for making a lot of power. Also, you really need a fuel pressure gauge in the dash to tune it well - about 70-80psi of fuel pressure is the max you want to run.

A very important upgrade to the system is the fuel pump itself. Running higher fuel pressures or extra injectors can use up all the volume of the stock fuel pump - it simply can't flow enough to maintain fuel pressure. There are a number of fuel pumps that bolt right in and replace the stock pump, giving extra fuel flow. Some upgraded fuel pumps can overcome the stock fuel pressure regulator, causing higher than stock fuel pressure. This sometimes is a good side effect, but this result can be tuned out with an aftermarket fuel pressure regulator.

At a point, the stock ECU just can't cut it any more - you're trying to make lots of power, and you've got a number of add-ons and workarounds to the problem. At this point, it might be wise to consider an aftermarket fuel injection system. This completely replaces the stock ECU, wiring harness, and sensors with brand-new ones. This gives the ultimate flexibility. Aftermarket fuel injection systems all allow for programming - you have complete control over the fuel injector maps, how the ECU responds to sensor output, etc. With a setup like this, there is virtually no limit to adding fuel to the car.

These units do have a few drawbacks, though. Installing the system is quite a bit of work - a knowledge of car electronic systems and fuel injection systems is a must. There's a lot of wiring to run and hook up, and attention to detail is necessary. Tuning the system is also quite a bit of work - getting the car to start every time, idle smoothly, make good, safe power, and remain driveable on the street can be a challenge. The power to tune the fuel system can also be the power to improperly tune the car - it's not hard to make a mistake that could cost your motor! But, the added flexibilty and control these units have can make it worthwhile.

- Dale