Jimmy S wrote:I thought the 2 turbo's were almost identical in size? and there is a valve in the headers that opens to allow flow to the secondary turbo at a set time? I would have thought you could somehow force that valve open at all times meaning that you would have a laggier car down low, but would be a bigger kick later on and no VOD(mostly because you are losing power down low?)...
Definitely different sized turbos. The primary (small) one is tiny, and you have pretty much got boost from idle.
From Wikipedia:
Sequential Turbo StagingThe B4 uses a sequentially staged primary and secondary turbocharger to deliver good throttle response and a wide torque spread. The primary turbo delivers boost in the low rpm and load ranges to deliver 278Nm at 2000 rpm, while the secondary turbo joins in above 4000-4500 rpm. With both turbos boosting, a 320Nm torque peak arrives at 4800 rpm and maximum power (190 kW) is seen at 6000 rpm. Note that Japanese-market B4s - running on 100-octane fuel - are rated at 206 kW.
The primary turbo (located at the left rear of the engine) delivers boost in the low rpm and load range, while the secondary turbo comes in to aid mid-to-high range breathing. During the primary turbo stage, boost pressure is controlled by a conventional arrangement of an ECU-controlled duty-cycle solenoid and an internal wastegate.
The secondary turbo remains inoperative during this stage, as a separate exhaust control valve (situated on the right side of the engine) remains closed. This valve prevents exhaust gasses entering the secondary turbine.
During the 4000-4500 rpm transitional stage, however, the exhaust control valve is partially opened, bringing the secondary turbo up to near-operating speed. The ECU - working with another duty solenoid and vacuum diaphragm - determines the amount that the exhaust control valve opens. The ECU calculates this amount of valve opening based on the input of a differential pressure sensor that takes feeds from the intake manifold and the outlet of the secondary turbo.
Any boost pressure produced by the secondary turbocharger during the transitional stage is redirected to the atmospheric side of the compressor inlet (between the turbos and air filter).
With the primary turbo continuing to supply manifold pressure and the secondary turbocharger essentially bleeding off the boost it makes during the transition, the ECU will determine when to close the pressure relief valve and fully open the exhaust control valve. Once this is done, the ECU will again look at the input from the differential pressure sensor and open yet another valve - the intake control valve (which is mounted between the secondary turbo's compressor outlet and the intercooler).
With the intake control valve open, boost pressure from the secondary turbo is allowed to pass through the intercooler (in addition to the boost supplied by the primary turbo). Manifold pressure during the second stage of turbocharging remains regulated by the primary turbocharger's wastegate - there is only one turbine by-pass valve in the whole system.
Subaru claims: "The 'staging' between the single and twin turbo operating range, which was quite noticeable to the driver on the previous model B4, has now largely been 'tuned out' by the careful selection of turbocharger size and the controlling mechanisms. As can be seen from this torque curve, however, under some driving conditions it is still possible to detect a slight reduction in the rate of acceleration in the preparatory or intermediate phase that occurs between 4000-4500 rpm."
But read our Liberty B4 road test to discover how much manifold boost pressure is lost during the transitional stage - it's more than "slight"...
The TurbochargersDespite being called twins, the B4's turbochargers are not identical. The primary turbocharger is an IHI VF33 unit, which uses a 46.5/35.4mm 9-blade turbine wheel and a 47.0mm/35.4mm 6 + 6 blade compressor. At idle, the turbo spins at around 20,000 rpm and it can go on to a maximum speed of 190,000 rpm. It has a 17mm diameter wastegate opening to bypass excess exhaust gas. The secondary turbocharger is an IHI VF32. On the exhaust side it uses a 46.5/35.4mm 9-blade turbine wheel, teamed with a 52.5/36.6mm 10-blade compressor wheel. It's rated at 180,000 rpm. Both the primary and secondary turbochargers use a floating metal centre bearing - not ball bearings.