it means the turbo doesnt have a common entry point from all 4 cylinders, it takes an input from 1 cylinder and another input from the other 3. If you look at some pictures of the CT26 you will see how it splits.
Actually if you look at the manifold, 1 and 4 go to the one side and 2 and 3 go to the other side.
The whole idea behind twin entry is isolating the runners and the natural vacuum/pull/push effect caused by the valve actuation. The idea is cylinders 1 and 4 "
pulse" together and 2 and 3 "pulse' together. (on a 4 cylinder 1 and 4 and 2 and 3 fire at the same time). When one side is pulsing, it creates a force that could potentially cause turbulence in the exhaust ports so they isolate it by putting each side to its own entry in the turbo. Its supposed to make spoolup and response better.
If all 4 cylinders dump into one large collector volume, you lose a bit of kinetic energy.
So the "divided scroll" or "twin entry" idea is kind of like this - take the turbine housing, divide it in half, port 1 & 4 into one half, and 2 & 3 into the other half, then the transition from "off boost" to "boost threshold" should be aided by the maintenance of kinetic energy contained in the syncopated gas flow between the pulse-timed cylinder pairs. It all sounds good in theory, but if you make a convoluted manifold that sacrifices gas flow for pulse-tuning you could very easily give away more than you gain. Within the confines of a MR2 engine bay, it might be difficult to properly execute a decent header for a divided scroll turbo. Pictures of the old turbocharged F1 cars with 4-cyl motors clearly show the application of the divided-scroll theory applied to reality, so I'm guessing there is some merit to it. A prominent bord member who is in the business of making headers claimed the divided-scroll method does not give you any measurable benefit, although I think the benefit is more in transitional presure increase vs. RPM, not in peak horsepower numbers in an almost static mode.
Either way, it's a matter of opinion/choice. Do you choose to make a very complicated header, and spend extra money on an expensive turbine housing, or just go with the flow (no pun intended) and run a regular turbo?!? We are talking about changes in boost threshold of maybe 300-500rpm at best, and admittedly, most of this technology and approach is from the era of turbos BEFORE the advent of twin ball bearings, and ceramic ball beraings and things of that nature, so which is best?
standard old turbo?
single ceramic bearing Turbonetics turbo?
twin ball bearing Garrett turbo?
twin ball bearing Garrett turbo with a divided-scroll?
"Traditionally, passenger car turbochargers have had only one volute through which exhaust gases leaving the engine cylinders can enter the turbine housing (where they are used to create torque). Unfortunately, this single entry design can sometimes lead to combustion inefficiencies, since some cylinders are expelling burnt gases while others are inhaling fresh air for combustion. What happens is that the hot, dense exhaust gases from the first set of cylinders are drawn into the second set of cylinders, causing the combustion occurring in those second cyf the fuel it's consuming.
A twin-scroll, or twin-entry, housing allows each set of cylinders to release its exhaust gases into a dedicated volute. That way, it doesn't interfere with the other set of cylinders, which is taking in fresh air at that same time."