So a friend was asking me what the difference between a single and a twin scroll turbo was.
I dont know so I figure I would bring it up here and get other peoples pros and cons on this topic
he has an rb25 and wants to make 5-600 hp but wants as little lag as possible
let me know so I can let him know what the difference and maybe what you suggest for turbo
Thanx, Brad
The path that leads through the turbine housing is called a “volute”. a twin-scroll turbine housing is basically two smaller volutes (when compare to single scroll), side by side. the idea is that the velocity of the exhaust gases being segregated to both the smaller and the larger diameter of the turbine wheel allows for A) higher flow on the volute that surrounds the smaller diameter of the turbine housing (exducer), and B) more velocity, thus more pressure build up on the larger part of the turbine wheel (inducer). So essentially, it’s makes the turbine housing of a large turbo system seem as though it’s smaller due to increased velocity, but it is infact comparable in flow to a larger turbine housing. Just as an example (not verifiable through flow calculations), a 0.89 a/r turbine housing will act like a 0.76 a/r turbine housing in low RPM but flow like a 1.0 a/r turbine housing at high rpm. The numbers are wildly separated in this example, just to illustrate the function of the dual volute system, but in reality the difference between theoretical and actual a/r sizes is much tighter.
Mazda tested dual volute design in the early 80’s with technical “white papers” that you can find online, and they reported a 15% increase in torque/hp per same RPM as a single volute turbo. That doesn’t mean it made more power, it just made the same power sooner, while flowing at the same or higher volume at the top end.
You can find twin-scroll turbos in OEM applications like the RX7 TII and HX series equipped Pickup trucks (Cummins Diesel, etc). What you will find, though, is that most Twin Volute designs are not made or optimized for passenger cars unless fitted with parts like external wastegates. In order to have a properly controllable twin scroll turbo, you need one with TWO volute swing-valves (flapper doors, wastegate doors, etc.) The RX7 has two swing valves on its twin scroll turbos.
Wheras the HX series turbos have only one swing valve, meaning you it dumps the pressure from one side of the turbine housing, but not the other… so boost control on a car that runs on the low end of the scale is very twitchy, or sometimes not easily controllable. i.e - unpredictable:
The reason it’s ok for diesel applications is because they run at around 20-40 psi under normal to heavy operation, so the swing valve is not a wastegate made to expel any and all pressure in the forced air pumping system for turbo diesel applications, it is just a very rudimentary boost controller.
For this reason, people with the HX series turbos actually make the hole on the swingvalve larger. The stock HX swingvalve is 18mm in diam. Though it can be sized up to about 30mm. This allows you to bleed almost all pressure out of the single volute and makes controlling boost much easier, especially at high RPM. Thus making HX turbos suitable for race applications if you actually wanted to use one and have it last long and not overboost/blow up your motor.
If you want to make 500-600hp, you should be able to find a race-application twin-scroll turbo that has NO swing-valves, but you will need to use a manifold that has two wastegate flanges, and the manifold must be divided in pairs of 3 cylinders in the case of an RB25.
So you’ll be spending more money to have that option on your turbo (in most cases), and you’ll be spending more money on a manifold, and buying two wastegates, and plumbing two wastegates back into the exhaust, which requires downpipe modification or additional dump pipes to be made and routed.
15% is pretty good, considering all that… but you won’t make more power.
Well nissan went so far as to mave their RBxxT and VG30ET manifolds divided to improve response, so it shows they believe in it. Of course there are advantages to going twin-turbo too, and where price supports it, they go that way…