Some revealing information about the Subaru turbo in the WRX and STi Impreza after it was introduced in the Version III in 1997 can be discovered in the Subaru Performance Handbook.
In true form for an industry that from time to time doesn’t let truth get in the way of a good story, the capabilities of the stock TD04L Subaru turbo fitted to the WRX are often overstated.
Since its introduction in ’97 with the Version III, the Subaru turbo spec has remained essentially unchanged on 2.0ltr models including the 2007 WRX STi. The boost characteristics of the 2007 WRX STi turbo are skewed more towards mid-range torque rather than out and out top-end grunt, courtesy of it’s comparatively size.
Peak power is reached at around 6,000 RPM, or a full 1,500 RPM short of redline, pointing towards a degree of breathlessness at high RPM. Small stature aside, the TD04L 2007 WRX STi turbo is a plucky performer all the same, responding well to all the normal changes as long as end goals stay in perspective.
Changes to air intake, exhaust, top mount intercoolers and engine management all reap usefulpower gains, but don’t expect much of a change of the point at which max power is achieved, remaining at max at around 6,000 RPM. Happily whistling away at peak boost pressures of 1.1-1.2bar the TD04 2007 WRX STi turbo is in its optimum efficiency range. Increasing boost by more than this only results in more intake charge heat and turbo exhaust backpressure creating a loss in power overall.
FMICs, exhaust headers, etc are nothing you should worry about unless a bigger turbo is planned in the future. Well modified 2.0ltr engines with a factory installed 2007 WRX STi turbo is capable of around 170kW ATW watch out for injector capacity though, as at this power output the stock top-feed injectors will be completely out of flow on some later models.
The Impreza STi is not immune to overenthusiastic power claims or the laws of physics either, but the fact remains, it produces in reality more grunt than other models.
Why is it so?
Detail mechanical differences aside, basically it boils down to the STi being a couple of shoe sizes bigger in the turbo department than the WRX. Being bigger the STi puffer moves more air generating more urge. It comes at a cost, as low-end performance suffers noticeably when compared to its less well endowed sibling.
The Australian model STi in particular is roundly criticised for doughy low-rpm performance and a relatively large amount of turbo lag. Subaru North America solved this issue, using the V8 brigade’s motto – there’s no substitute for cubic inches by specifying a big-block 2.5ltr engine for its STi go-fast model Subaru.
Additional cubes makes it possible to run a turbo for good top-end performance on lower-octane fuel, without all the penalties of lag etc faced by the 2.0ltr engines. As with the WRX, Subaru STi respond well to intake changes, exhaust and engine management upgrades. This time though, the larger turbo gives a better return on dollars spent fitting items such as FMICs and exhaust headers.
A well sorted 2.0ltr STi will make around 190-200kW ATW, on 98 Octane pump fuel running 1.4 bar of boost. USDM STi’s with the right combo of bolt-ons punch out around 380Hp (US numbers at the flywheel) running a touch less boost owing to the lower octane rating of American gasoline. Better served by bigger stock injectors, at these power levels the stock fuel pump will be struggling to keep up so have your fuel system checked as well!
Understanding how the 2007 WRX STi turbo works is vital to getting top performance from your car.
