Twin Entry Turbo

[Bibbs]

Multi-cylinder engines have ranges in which there is some overlap in the exhaust valve opening timing of one cylinder and its sequent cylinder. This can cause interference to the exhaust gas flow where the exhaust gases from the two cylinders join and results in exhaust energy loss in the turbocharged engine. Also, cylinders which have just finished the exhaust process are affected by the high back pressure of the exhaust initial process, so that high pressure gas remains behind more easily. This  hinders smooth intake flow to the engine.

This exhaust gas interference is eliminated in the 3S-GTE engine by providing

two exhaust ports, for cylinders No. 1 and No. 4, and for cylinders No. 2 and

No. 3, together with two scrolls inside the turbine housing. This design increases the engine's low-speed performance and acceleration response.

(Taken from Turbo magazine, May 1990)

[Bibbs]

more details from Terry Heick @ www.freshalloy.com

In describing the benfits of the twin-scroll design utilized in the Evo

VIIIs 16G, Mitsubishi used the following diagram, and description.

"With the twin scroll turbine housing design, the exhaust gases flow through

the exhaust manifold into a pair of passages, instead of a single larger

volume passage.This helps the cast iron turbine housing to smoothly and

efficiently direct the exhaust energy over the turbine wheel, yielding

improvements in the engine's low-to-mid range engine throttle response and

torque production."

[Bibbs]

and again from Adrian Forbes (dunno where he got it from)

The turbine is rarely subjected to constant exhaust pressure. In pulse

turbocharged commercial diesel engines, twin-entry turbines allow exhaust

gas pulsations to be optimized, because a higher turbine pressure ratio is

reached in a shorter time. Thus, through the increasing pressure ratio, the

efficiency rises, improving the all-important time interval when a high,

more efficient mass flow is passing through the turbine. As a result of this

improved exhaust gas energy utilization, the engine's boost pressure

characteristics and, hence, torque behavior is improved, particularly at low

engine speeds.

To prevent the various cylinders from interfering with each other during the

charge exchange cycles, three cylinders are connected into one exhaust gas

manifold. Twin-entry turbines then allow the exhaust gas flow to be fed

separately through the turbine.