Fuel System Upgrade - Fueling The Fire - Modified Magazine

It’s no secret that to make horsepower, you need the right mixture of air and fuel. In Part 1 we installed a Turbonetics GT-K 700 turbocharger and a Full-Race twin-scroll manifold, which will provide all the compressed intake charge we need to reach our horsepower target of a comfy 400 flaming Doritos at the rear wheels. But a well-conceived engine build capable of delivering power across a broad rpm range also needs that air to travel a smooth and unrestricted path into the combustion chambers, where witchcraft happens as those mysterious spinning triangles convert the compressed air charge along with the added fuel and spark into what we hope will be a competitive horsepower number.

With the exhaust side of the motor taken care of, we continued our quest for efficiency and power on the intake side of the engine. The OE lower intake manifold (LIM) is plagued with unequal runners, power-robbing restrictive bends, and inner diameter casting. After exploring various forums and speaking with fellow rotorheads, we heard the same name popping up: Xcessive Manufacturing and its LIM.

Xcessive Manufacturing has been around since 2002, specializing in engineering, fabrication, and CAD design using composites and other materials. If you can think it up, Xcessive can make it a reality. After a pleasant chat with Rich Kobliha of Xcessive Manufacturing, we learned that the rear rotors on 13B engines like ours experience very lean conditions at high boost that can lead to catastrophic failure. To address this, Rich improved intake flow by designing the manifold with equal-length direct runners that allow direct flow into the intake ports. This also ensures equal fuel and intake charge to the front and rear rotors. The Xcessive manifold is cast from 356 aluminum and also features extra injector bosses that allow you to use the company’s single rail fuel system or a conventional dual fuel rail system. We busted out the Dremel tool to port-match it to our street ported intake, just to ensure the intake air charge is as smooth and undisturbed as possible.

With the intake side sorted out, we shifted our focus to the now inadequate fuel system. For those of you unfamiliar with rotary fuel systems, they use primary and secondary fuel rails and injectors. The primary injectors are active during idle and light cruising activities, usually 3,800 rpm and below. Secondary injectors come to life at higher rpm and boost pressures.

We turned to CJ Motorsports for its full Stage 3 fuel system. CJ Motorsports has been producing high-quality matched fuel systems and products since 2002. We spoke to Charles Konarski about the company’s focus on the fuel system: “We’ve focused on fuel systems products because, at that time, it was overlooked, and people were simply throwing things together or trying to tune around fuel issues.” The RX-7 fuel system has gone through different stages of development to complement changes in injector technology and hardware such as the Xcessive LIM.

We chose the Stage 3 fuel system for our fuel delivery needs, which includes matched primary and secondary fuel rails that are beautifully CNC machined from billet aluminum. The supplied fuel lines are braided stainless, Teflon lined, and finished with AN fittings, while the injector bosses included in the kit are pressed into injector bungs in the LIM to provide stable seating for the injectors.

An innovative feature that is only available on CJ Motorsports fuel rails is the incorporation of a fuel pulsation damper (FPD), a technology designed to prevent fuel pressure from fluctuating. These fluctuations are caused by oscillating pressure waves created by the opening and closing of fuel injectors and are at a much higher frequency than can be monitored with a fuel pressure gauge. This can cause inconsistent fuel pressure and fuel metering, with obvious consequences to the motor, especially for forced-induction applications where a lean condition can get very expensive. And as you rotorheads out there know, apex seals are especially sensitive to detonation from running lean.

We are aware of horror stories of FPDs in rotaries causing engine fires and burning everything to a crisp. However, the usual culprits are old, high-mileage, poorly maintained FPDs where the inner diaphragm has ruptured and caused a fuel leak. Nowadays, improvements in design and diaphragm materials result in more durable FPDs, so we’re completely confident in CJ’s design and the more consistent fuel pressure and metering that comes with it. But for you paranoid, old-school types CJ Motorsports also has FPD-less systems.

The final important pieces to our fuel system are, of course, the injectors. Without the right injectors tuning can become a real nightmare, a lesson we learned in the past by using tired old injectors that weren’t able to supply the quality or quantity of fuel needed. For our 13B build, we opted for Injector Dynamics (ID) injectors, one of the most respected aftermarket fuel injector suppliers in the business today. Injector Dynamics is a joint partnership between Yaw Power Products and T1 Race Development and has been synonymous with producing top-notch injectors for your high-horsepower needs.

The big brain behind ID’s high-quality fuel delivery is Paul Yaw, who developed dynamic characterization along with Motec USA, a method he uses to test all the company’s injectors in real-world conditions and with various fuel types. The results of these tests are what they call “dynamically matched” injector sets. They are matched based on their dynamic flow rate across the pulse-width range to each other. Another advantage to the ID injectors is that they provide you with compensation data for injector dead time. Injector dead time, in simple terms, is the time delay between the ECU signal and when the injector receives the signal to open. This data makes setting up and tuning the ECU easier and results in more consistent air/fuel ratios (AFRs), especially when atmospheric pressure and voltages vary.

For our application, we decided to go with ID850cc primary injectors and ID2,000cc secondary injectors. The ID850s have a nominal flow rate of 885cc/min at 43.5 psi and can still maintain a smooth idle. The ID2,000s have a nominal flow rate of 2,225cc/min at the same psi. Obviously these flow rates will depend on tuning variables that we will cover at the tuning stage. For comparison, the stock RX-7 fuel system used 550cc primaries and 850cc secondaries and restrictive, smaller-diameter fuel lines and rails. We know we will be demanding a lot from our fuel system, and with the monster ID2,000cc, we can rest easy that we will have no shortage of go-juice when our right foot demands it.

With our fuel system now sorted, we are one step closer to having a running, fire-spitting FD. But before we can set the rear bumper on fire, we need to add spark to the party, so in Part 3 you can expect us to bring down the fire from Mount Olympus, or at least cover the ignition coils and wires in some detail. Actions have consequences, so you can also look forward to us addressing the heat byproducts of spinning Doritos and turbine wheels with several different heat management products. Until then, stay cool and don’t run too lean.