When can less, be more? During the course of re-mapping Simon North’s Subaru 22B engined Impreza GC8 TDi’s technicians noticed that they were approaching the point of diminishing power returns far earlier than they would have expected. Crucially the Impreza’s entire turbo system was having to work extremely hard, with high boost levels to achieve the power output. Clearly something was amiss with the turbo exhaust manifold and system configuration on the car, which was installed by another tuner.

TDI’s technicians, lead by Operations and Technical Director Sam Borgman soon established that by half way through a dyno run, there was a back pressure build up (over 160kpaA) in the main exhaust system post turbine, which was causing a gas flow restriction.

Ideally, post turbine back pressure should be absolute zero (O kpaA) or in other words a complete vacuum, but reality this isn’t possible, but TDI embarked on designing and producing a new exhaust system which aimed to get as close to this figure as possible.

Pleasingly, a post full power run on TDI’s Roto Test hub dyno revealed that Simon North’s Subaru Impreza’s exhaust back pressure now peaked at a mere 108kpaA, compared to an ambient pressure of 102kpaA.

Because exhaust manifold pressure for a given intake manifold pressure is now significantly reduced, there’s an increase in the pressure delta across the cylinder head. The Impreza’s exhaust gas temperature (EGT) readings are also significantly reduced, and as you might expect torque production is massively increased.

The results of all this exhaust development and the simultaneous fitment of a superior exhaust manifold design has been a large jump in the Impreza’s torque and power figures. “It’s as if the hand brake has been released in terms of engine efficiency,” says TDI’s Sam Borgman.

On TDi’s Roto Test hub dyno, Simon’s Impreza (which uses a large ‘twisted’ GT40 turbo, MoTec M800 ECU and Cosworth intake plenum,) produced a mighty 557bhp @ 6870rpm, allied to 454 lb.ft of torque at 5590rpm.

So what difference has this made to Simon North’s Impreza GC8 in reality? Prior to TDi’s work on the Subaru Impreza the best standing quarter mile time down the Santa Pod drag strip was 10.6 seconds. Crucially, this was achieved running a full 2.5 bar of boost and nitrous oxide injection. Last Sunday Simon matched that time on the unprepared concrete surface at Elvington airfield running just 1.8 bar of boost and no nitrous oxide injection.

By correctly identifying and resolving the parasitic pumping losses caused by the poor exhaust design on the EJ22 engine, TDi’s technicians have produced an ideal outcome – less boost, less engine stress, more power and more proven performance from Simon’s monster Impreza GC8. Proof positive that with the knowledge and facilities at TDi, less really is more!

Leading British automotive performance tuner Torque Developments International (TDI) plc today announces the release of a new, high flow intake plenum for the iconic Subaru Impreza STi.

Carefully developed to remove the performance compromises of the stock intake plenum, the Cosworth EJ257 version uses a larger internal volume allied to large tapered runners to offer a 10% flow improvement compared to the original Subaru unit.

Ideally suited to the needs of high power output EJ20 and EJ25 engines, the Cosworth EJ257 intake plenum is specially designed for use on Imprezas using an aftermarket front mounted intercooler. Easily installed as straight swap for the Subaru item, the Cosworth EJ257 intake plenum is directly compatible with 2004-2009 model year cars using the factory drive by wire electronic throttle body.*

Cosworth’s engineers have retained the stock throttle body mounting location, ensuring widespread compatibility with most leading brands of front mount intercooler kits, including Cosworth’s own recently launched FMIC kit for the GRB Impreza.

Key Cosworth Subaru Impreza EJ257 intake plenum specification highlights include:-

• Substantially increased plenum volume for high power applications

• Specifically designed for use with aftermarket front mount intercooler

• Large, tapered intake runners with radiussed inlets

• Tuned plenum volume, runner lengths and tapers

• 10% more intake flow compared to original Subaru version

• Integral vacuum manifold for easy connection to brake servo, boost gauge, MAP sensor etc

• Compatible with standard or forward mount throttle body installations

• Stock throttle body location retained

• Suits all Impreza STi’s 2004 onwards with drive by wire throttle body

• Direct replacement for original Subaru intake plenum unit

• Priced at £744.44 including VAT @ 15%

Carrying part number: 20006438, the Cosworth Subaru Impreza EJ257 high flow intake plenum is available now from principle Cosworth performance parts specialist and main distributor TDi-plc, priced at £744.44 including VAT @ 15%.

* With some rudimentary modifications the Cosworth Subaru Impreza high flow intake plenum could be utilised for EJ20 cable throttle body applications.

Torque Developments International are a UK distributor for the range of legendary Cosworth engine parts and components.

Derived from years of F1 engine research, the quality, effectiveness and attention to detail that you would expect from one of the worlds best Motorsport engine manufacturer is now available for you to insall in your car.

Among the components available from us are: Cosworth crate engines, Cosworth stroker kits, Cosworth crankshafts, Cosworth pistons, Cosworth connecting rods, Cosworth gaskets, Cosworth engine bearings, Cosworth oil pumps, Cosworth CNC ported cylinder heads, Cosworth camshafts, Cosworth intake manifolds, Cosworth supercharger systems.

Cosworth manufacture the above race and road parts for Nissan VQ35DE, Mitsubishi 4G63, Subaru EJ20, Ford Duratec.

To review the range of Cosworth components for different cars, please go to http://www.tdi-plc.com/