I read up on this at one point and it has to do with the reversion of exhaust gas due to the firing order and valve events. VERY summarized explanation there..

 

Yeah. there's a bit more to it than just order of fire and pairing. When you look at the actual uses of divided housings, they are mainly used for larger turbo applications in which the engines they are working with don't have the exhaust energy to really "spin" that turbine wheel effectively unless its in conjunction with using larger displacement (Tractors, big V8s, Wenkel Rotary engines, etc). This is why you typically see high PEAK power numbers from these, is because most of the larger 75mm+ diameter exhaust wheels rotate at a lower speed but can make big power. For 2.5 litres and under, that unfortunately means you have to really rev that engine in order to get the same exhaust pulse energy as the larger displacement engines. (This is the easy part to understand).

What's not so easy is the actual twin scroll turbine housing that use a divided throughout the entire volute of the exhaust housing. This is where firing order and exhaust pulse energy with that firing order come into play. Especially for the fact that most twin scroll turbines you use are used with smaller turbine wheels, under 65mm diameter (in the aftermarket) and easily under 50mm diameter in factory models. Because engineers understand that the effect of this division is based upon smaller displacement engines that are using smaller exhaust wheels with lower exhaust energy speeds where "spool" actually counts.

But there is a law of diminishing returns with this effect. As long as the firing order is more consistent, and the exhaust manifold is paired properly, even if it isn't the perfect firing order, as long as it's consistent, you'll get the desired effect.



I've explained in several forums and such over the years. A divided housing is a necessity to becoming a full twin scroll volute, but not a sufficiency in doing so.

A true twin scroll has a full divider for the exhaust pulses at least 180* within the volute



While a simply large DIVIDED housing used on may high torque-low rpm applications like tractor/trucks that most are using on these cars do not. Divided housings only look at the initial throat entry area and converge about an inch into the volute.



In which case, the benefits are negated in some sizings.




I"ll re-title this as a Discussion topic , because this is interesting to talk about. I had some other material on this forum already this year, perhaps it just needs to be revived and merged.

 

I DID say it was very summarized. Lol

 

 

Evo turbos are divided all the way still the inner scroll is different. Its a huge discusion really.
Short story: Saves the closing cylinder from cross cylinder contamination.
The more boost in combination with short runners the more you need it.
Imo the response was a side effect.