Project [Rotary] FD RX-7: Part 4 – Turbo System

Project [Rotary] FD RX-7: Part 4 – Turbo System

by M-P Spierer

Designing and installing the turbo setup is one of the most enjoyable aspects of any build. There are many decisions to make when planning out a turbo setup and each decision significantly affects subsequent decisions. It is a balancing act of desired goals and undeniable trade-offs. Many questions need to be answered like, am I looking to maximize mid-range response or top-end power? And, how much response will I be giving up if I choose one turbine housing A/R over another? These are questions that have different answers for every build. For this RX-7 we are trying to maximize response around a specific power limit without breaking the bank.
The beginning of the turbo selection process for this project started over a year ago when power goals were first being determined. A key factor in this process was defining a power limit that would still allow for good reliability track session after track session. With proper cooling (charge air, coolant, and oil), good tuning, and an auxiliary injection system running 100% water (to be featured and installed in a future article), a power limit of 450 wheel horsepower was chosen. Now, I say power limit because this is the maximum horsepower that we will be tuning for to keep the heat generation and engine stress levels in check, but not necessarily the power level that the car will operate at. In actuality, the true target wheel horsepower for normal operation is 425 (an even 500bhp at the engine if we assume 15% drivetrain losses). Either way, we are asking the 13B-REW to produce roughly two times as much power as it did when it came out of the factory in Hiroshima, Japan over 20 years ago.
Once the power goal was established, turbo selection was next. If you remember back to the first article of this project, the car originally used a ball-bearing Garrett T04R (often referred to as a T04Z). This was a great turbo for its time and honestly would have suited the power goal just fine. However, turbo technology has come a long way since the T04 series of turbos were popular and there were good reasons change to the new turbo…the Borg Warner S362 FMW.
The Borg Warner S300SX FMW (S362) is technically in the AirWerks family of Borg Warner turbochargers, but unlike most models in the AirWerks lineup it uses a billet compressor wheel similar to what you might find in an EFR turbo. There are not very many billet compressor wheel, journal bearing turbos out there, but Borg Warner has filled that void with several FMW Airwerks turbos ranging from 57mm to 82mm. I like to think that these turbos are for the people who want an affordable and extremely robust turbo with some of the cutting edge turbo technology found in an EFR series turbo.

Forged aluminum is inherently stronger than cast aluminum, so the benefit of a billet (forged) compressor wheel is that it can be made lighter than its cast counterpart without sacrificing strength. This is the same basic principle that applies to wheels as well. Now if we compare numbers between the old T04R and the new S362 FMW it is plain to see one of the benefits of changing turbos. 

The Borg Warner billet compressor wheel has a 61.4mm/83mm (inducer/exducer) and is capable of flowing ~74 lb/min of air. The T04R's larger 66.7mm/84mm cast compressor wheel could flow ~75 lb/min of air. Basically, with the Borg Warner you have a turbo with a smaller and lighter compressor wheel that has similar airflow capability. The smaller and lighter compressor wheel has a lower moment of inertia and therefore should accelerate (spool/respond) faster than the larger and heavier compressor wheel.

Shot of the turbine side of the S300SX FMW. The turbine housing features a fully divided T4 inlet and 0.91 A/R. It has also been modified by Full-Race to have a 3″ V-band discharge instead of the Borg Warner full marmon discharge and is ceramic coated. All the quality check paint marks shown in this photo leads one to believe that quality control is very important to Borg Warner.
The crown jewel of this turbo has to be the billet compressor wheel or “Forged Milled Wheel” (FMW) as Borg Warner calls it. This compressor wheel features Borg Warner's “Extended Tip Technology” and borrows much of its design directly from the new EFR series of turbos.
For those who are compressor map literate, here you go. I expect the turbo to operate with a maximum pressure ratio between 2.25 and 2.5. This should bring me right across the widest part of the 76% efficiency island and the entire map in general. The peak flow at this pressure ratio is ~66 lb/min which should be good for a realistic 450-475whp in a rotary powered RX-7.

For the exhaust manifold we didn't need to look past Full-Race. Their manifolds are used on some of the fastest cars around in every discipline imaginable. Their reputation coupled with the fact that they offered an off the shelf manifold with all the features required for this build sealed the deal. It was important to have a true twin-scroll manifold for the RX-7 because the exhaust pulses of a two rotor engine allow for full utilization of twin scroll technology and the resulting benefits.

Full-Race also installed 1/8th NPT bungs in each runner so that we can monitor exhaust gas temperatures or pre-turbo exhaust backpressure while tuning. Monitoring the exhaust backpressure and reading dyno plots will allow us to determine if the 0.91 A/R turbine housing was the correct choice or if the 1.00 A/R option would have been better.

Full-Race has an incredible reputation for quality and performance. The manifold used in this build is twin scroll with a T4 turbo flange, dual 44mm wastegate flanges, an EGT sensor bung on each runner, and has been ceramic coated.

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