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Sequential Turbocharging

 - First used by Porsche and Abarth.
 - Small turbo used for low to mid boost.
 - Big turbo used for mid to high boost.
 - Small to big cutover in the mid range.
 - Japanese went nuts in the 1990s and released some serious sequentials.

Fiat/Lancia tuning arm Abarth were the first manufacturer to try sequential turbocharging. It gained favour for a few years but was eventually abandoned. Toyota/Mazda/Nissan and later Subaru all tried it in their production sports models. Toyota was most successful with their Supra MkVI.
In 1987 Abarth launched ‘modular turbocharging’ later named sequential turbocharging, in grp S rally engines. The 4cyl 1759cc engines made 600bhp at 8000rpm with 400ftlb of torque. (Some argue that Porsche had a similar system in the 959 a year earlier).

The idea of sequential turbos is that a small turbo provides boost at low speed then a bigger turbo takes over to maintain a high boost level. The small turbo, while providing good throttle response, severely limits power and wastes fuel. Running two turbos full time, results in sluggish performance up to 3000rpm.

The sequential setup was supposed to address these problems. At low speed all exhaust flow is directed to the smaller turbo which has its wastegate fully closed. As exhaust gases increase, the small turbo compressor flow begins to peak. At this point a normal turbos wastegate would open, but in a sequential setup the EMU instructs the second turbos exit valve to open. This action permits flow through he big turbo and allows it to spool up. However, with the inlet strangler valve closed, the compressor vent valve open, it doesn’t supply the engine with anyboost. All flow from the large compressor goes round in a closed loop. Further increases in engine rpm and exhaust flow signals the EMU to fully open the big turbos exhaust blocker valve and simultaneously close the inlets vent valve. The inline strangler valve now opens as boost pressure builds up and the big turbos compressor begins to supply air to the engine. As flow from this compressor continues to rise, boost pressure will again peak. The EMU detects this and pulses the big wastegate to avoid an over boost situation. A fuel cut will be instigated if the wastegate fails to control boost.