A major downfall of other Manufacturers sequential twin turbo systems is the transition from single to double turbo’s in operation.
This is due to the second larger turbo not spinning fast enough when its called on, resulting in the engine “coughing” momentarily.
Mazda Motors Corporation solved this problem on the Rotary twin turbo by spooling the second larger turbo to a pre-boost speed of around 100,000 rpm with exhaust gas circulation.
When the time comes for the second turbo to cut in, a by-pass valve is shut to “surge” the compressor, spinning it to over 140,000 rpm.
This then assures a smooth coupling with the smaller primary turbo. Once at the required speed, it receives a full share of exhaust flow to add its boost with the primary turbo.
The Mazda Rotary RX7 twin turbo chargers are fitted to a “dynamic pressure” cast iron exhaust manifold shaped to minimise the gap between the exhaust ports and the intake of the turbos, improving boost by as much as 35 per cent.
The 13B-REW was fitted standard with an air-to-air intercooler mounted above the radiator with separate ducting.
The Mazda RX7 ECU controlled fuel injection system uses air density measurement instead of the common air flow metering to allow a smoother air flow and more precise fuel management.
Coupled with the precise shape of the plenum chamber and inlet tract length, the added benefit of the increased power and throttle response is the additional improvement in fuel consumption for the city/highway cycle.
Two injectors per rotor look after the varied engine loads. The primary injector takes fuel from the side of the injector body instead of the top. The resulting reduction in fuel travel through the injector body gives a quicker and more precise engine re-start after high speed/load applications. The secondary injector is mounted upstream of the inlet tract operating during mid and high rpm. Change over to twin injector operation takes place around 2,750 rpm or lower if the load commands it.
Ignition timing of the four platinum tipped spark plugs (two per rotor) is controlled via the ECU which will automatically retard timing if detonation is detected. Light weight high energy coils are utilised to take advantage of the rotary’s unique combustion characteristics.
Ensuring the emissions are lower than required, the use of a two stage catalyst system were the main three way converter gives the lowest flow resistance available. A single muffler is used for noise reduction and the whole system weighs considerably less compared to the series V RX-7.
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