Small article on Autoweek

[Feeshta]

Yeah, the FD RX7 and MkIV Supra's both used sequential twin turbos, whereas the 300ZX and 3000GT's used parallel twin turbos. Which made sense when you look in the engine bay. 300ZX/3000GT = V6's, exhauste on both sides of the block. RX7/Supra Rotary and I6, exhaust ports all on one side. It made logical sense, IMO anyway.

"The twin turbos operated in sequential mode, not parallel. Initially, all of the exhaust is routed to the first turbine for reduced lag. This resulted in boost and enhanced torque as early as 1800 rpm, where it already produced 300 lb·ft (410 N·m) of torque. At 3500 rpm, some of the exhaust is routed to the second turbine for a "pre-boost" mode, although none of the compressor output is used by the engine at this point. At 4000 rpm, the second turbo's output is used to augment the first turbo's output. Compared to the parallel mode, sequential mode turbos provide quicker low RPM response and increased high RPM boost."

Yes, but unfortunately they are notoriously problematic to maintain. Very complex. There are not very many old Supras still around with the original system still in place and functioning correctly, and that was the most reliable version ever built.

In theory you could do the same thing more simply with a twin-scroll turbo, as the plumbing would be much less complex, but I don't think anyone sees it as really needed these days. There are twin scrolls out there that function basically liked two differently sized turbos, as the scrolls are asymmetrical. You could simply direct most of the flow to the smaller scroll at low RPM and the larger one at higher RPM. You would lose the advantage of properly times exhaust pulses though, unless of course you went with 2 of them.

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[Trackaholic]

There's also that twin-charging thing VW tried for a while with supercharging for low-rpm boost and turbos on the high end.

I think they still do that on some of their smaller engines used in Europe.

Volvo is also doing that on one of their upcoming four cylinder engines. They are using a supercharger on the low end to build instant boost, and then they can use a larger turbo on the high end, for greater power. Their turbo-only motor will likely use a smaller turbo.

-T