There's always a limit somewhere, obviously. With turbos it's a careful balance of having a turbo big enough to hit your target power vs response. Bigger turbos make more power but spool slower due to the higher inertia of the turbine wheels and lower gas velocity through the bigger housing. Twin turbo set ups were used to overcome this, either by using 2 equal-sized, but relatively small turbos for fast response and enough flow, or a small turbo for response and a larger turbo for top end boost.GG. wrote: ↑Wed May 22, 2019 10:08 amOk - but presumably boost limited because of the overall size of the single turbo despite the fact that the exhaust gasses are delivered in two streams to minimise lag - i.e. less tuning potential? I would also imagine swapping to a bigger turbo would not be easy unless compatible with the twin scroll setup?NotoriousREV wrote: ↑Tue May 21, 2019 8:15 pmThat’s not how Twin-scroll turbos work. It’s a single impeller but with the exhaust gasses split in two and delivered separately to it at different angles, with the blades angled to make efficient use of the flow from each nozzle. One nozzle works best for response, the other for high boost. Overall you end up with a turbo with great response and great top end flow.
Sounds like a clever solution to minimise cost whilst not succumbing to the lag typical of a single turbo. I expect fiddling with the exhaust plumbing upstream from the turbo is cheaper than fitting two which was the main driver behind doing that.
And of course, back to my overall point, ditching Toyota tech in favour of using a BMW setup.
The twin scrolls work by pushing half the gas flow at high velocity at the smaller diameter part of the impeller to get it to spin up quickly, and then using the other half of the gas flow to drive the larger diameter part of the impeller to drive the rotation speed and provide the ultimate boost. spacing out the exhaust pulses also makes them much more efficient in transferring the energy from the exhaust. You get the same characteristics without the expense of 2 turbo units or the complexity of control systems.
My 1993 Impreza WRX with a TD05 turbo made 300bhp (a TD05 is good for nearly 350bhp) from a 2.0 litre engine. The power delivery was nothing-nothing-BAM! IIRC, it didn't make positive boost until over 3k rpm, and full boost wasn't until 3750. My later 2001 WRX had a TD04 and made 280bhp but was a lot smoother (and less exciting) to drive but that turbo was at it's absolute limit. My Golf R makes 310bhp from a 2.0 litre engine and you barely know it's got a turbo, they're making boost just off idle, and that's not even a twin scroll design.