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subba
03-28-2007, 12:59 PM
This is for the 3rd generation Legacy:




We recently performed a road test of the Australian market Subaru Liberty B4 ["Subaru Liberty B4 Twin-Turbo"]. Now we'll give you a technical explanation of the new super Subie - like, how does the sequential twin-turbo system work?

Engine Structure
The B4 is powered by a 'phase 2' all-alloy DOHC, 16-valve, intercooled turbo 2-litre boxer four. According to official Subaru literature, there are several advantages to the boxer design. Due to the balance afforded by horizontally opposed cylinders, the crankshaft requires less weighting and there is no need to install balance shafts. This results in reduced noise, vibration and less power loss. Strength is also an advantage of the boxer design - the crankshaft is sandwiched between the left and right hand crankcases and is supported by 5 main bearings. The low and wide engine structure also lowers the vehicle's centre of gravity and improves mass distribution.

Working from the crankcases out, the 'phase 2' engine sees the crank thrust bearing relocated to the rear of the shaft - this reduces transfer of natural frequencies to the transmission, resulting in improved NVH. Pistons are all-new in the B4. Despite being made from forged aluminium, the pistons are heavier (162 up from 152 grams) and incorporate solid skirts, reduced piston pin offset, a molybdenum coating, reduced top-land to cylinder clearance and flat-top combustion surfaces. Improved off-boost torque comes from a raised static compression ratio - 9.0:1 compared to the Impreza WRX's 8.0:1.

The two DOHC, 16-valve heads incorporate a fuel-efficient, low-emission design, with a 'tumble swirl air' motion created by a specially shaped intake port and increased valve angle and lift. The belt-driven camshafts act directly on hollow valve stems. Intake valves are hollow (reducing their mass from 51.6 to 48.4 grams) and the exhaust valves (46.7 versus 50.5 grams) are sodium-filled for more efficient heat transfer.

Sequential Turbo Staging
The 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 (190kW) is seen at 6000 rpm. Note that Japanese-market B4s - running on 100-octane fuel - are rated at 206kW.

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 Turbochargers
Despite 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.

GrimmSpeed
08-03-2007, 01:37 PM
Nice read, very interesting.


Josh

sheepdog
08-03-2007, 03:39 PM
Sweet. I'm going to add this to threads of note.

deadlydave
08-03-2007, 05:41 PM
fascinating. Seems pretty complex.

subba
08-03-2007, 08:03 PM
i though i would share because i though it was intresting.... i posted it almost 5 months ago lol.

Plays_with_Toys
08-03-2007, 10:15 PM
i though i would share because i though it was intresting.... i posted it almost 5 months ago lol.

you lie... :lol: I'm surprised it went unnoticed for that long...