Thanks, I try my best to keep it simple, and that image really does help understanding the system greatly.
Also can watch this video, the explanation on the video is superb.
VGT and Twin scroll is nothing new, both pretty much have the same goal which is to expand the turbo's efficiency range. But still out of reach of the mainstream production cars yet due to most probably the high cost for a VGT turbo designed to withstand the heat of a petrol engine, and also the complicated control strategies required to operate the VGT mechanism. If you can't wait, you can get a Porsche
Also, the more moving part, the more chance of problems. I've seen the VGT vanes of a VGT turbo get stuck before so that's another thing to ponder as well.
Twin scroll turbos works by dividing the exhaust pulses, introducing the scavenging effect in a way and reduces interference between the pulses thus improving the engine as well as the turbocharger's efficiency. Since twin scroll turbos work by dividing exhaust pulses, the manifold design is of utmost importance and for a common 4 cyl, cyl 1 and 4 will be paired to 1 scroll while cyl 2 and 3 will be paired to the other scroll. If you install a single external wastegate on the system by combining both scrolls, you pretty much negate the benefits of a twin scroll as mixing between the cylinder pulses can then occur since you had basically combined the scrolls. A proper wastegate system on a twin scroll setup would require two wastegates. On a stock Evo 4+ turbo for example, there is actually 2 wastegate flappers operated by a common actuator so the gas really does stay divided all the way until after the turbo.
For more reading material on twin scroll turbos, can read a few links here
http://www.bmwblog.com/2010/01/02/twin-scroll-turbo-system-design-explained/
http://dsportmag.com/the-tech/twin-scroll-vs-single-scroll-turbo-test-the-great-divide/
https://www.rx7club.com/single-turb...s-single-scroll-vs-twin-scroll-turbos-903210/
so really, if possible, get a twin scroll turbo
No it is really completely different. Twin charging as in 2 identical sized turbos, main purpose is to reduce turbo lag by reducing inertial mass, but still suffers from choking up top. but even that seems to be phasing out now with more efficient turbochargers being introduced. Those people with RB and JZ engines which came with twin turbos are converting to single turbos and getting very good gains.
Compound charge on the other hand, as the video I attached describe, is really like running 2 seperate turbos, one for the low end and one for up top, vastly increasing your powerband. It's slightly different than staged turbo on certain production car where there are actually flaps or valves blocking/rerouting the gas flow while compound charge really is just suck air through this turbo and push air through the other turbo
