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2006 STI build - 373kW at the wheels! |
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 One of our most recent shop projects was a fairly comprehensive build-up on a customers 2006 Subaru STI, which included a full Cosworth engine. After the usual tithe of blood, sweat, and tears the end result was quite brilliant with nearly 300 kW at the wheels. Usually that would be quite enough. Usually. But just the other day, he called us up again, and wanted just a touch more power - 380kW at the wheel was his new goal for club sprint racing. I told him that it would not be easy, but it was possible.
First, for total reliability he would have to run race fuel as the available 98 octane fuel was too inconsistent. With that worked out, next would be upgrading the ignition system, the exhaust, the FMIC, and the intake. All these areas would need extensive fabrication to reach our goal. Once all those pieces were in place, we would set up a dual map on the LINK ECU so he could run 98 octane to get to the events, and race fuel for the club sprints. The deal was complete and the vehicle was delivered to us for the required modifications.
The AVO workshop started by removing everything from the vehicle apart from the Cosworth engine and Link Extreme G4 Engine Management System. Once that was done, we built a custom intercooler system that was even larger than the AVO one already on the vehicle. AVO cast aluminum endtanks were mated up to 600 x 300 x 90mm bar and plate core, with 2 ½” mandrel bent piping fabricated from the turbo to the entry of the intercooler. To keep maximum air flow, 3” mandrel bent piping was built from the exit of the intercooler to the throttle body. To match the black color of the car, all the piping was powder coated black.
Once all the intercooler piping was routed correctly, we fit the extractors back onto the engine and fabricated an up-pipe to reposition a new GT35R turbocharger. This is very important in regards to engine movement, and disbursement of heat. With the turbocharger properly mounted in place, the oil and water lines were redesigned and then fitted. Fitting the right exhaust housing is key to consistent power. Too large, and it won't spool up in time, too small and it will have issues with boost creep. Once we had that spot on for maximum performance with the Cosworth engine, we were on the home stretch. Finally, an 18psi adjustable AVO actuator was another upgrade that we knew would work best with the new setup, and once fitted handled the boost perfectly.
Next up was a 4” stainless inlet tube, designed with provisions for the breather tank kit and using a 4” AVO Sports Air Filter. The breather tank kit is an import piece of the reliability equation - too many people fit one after damage has already been done to the engine. Fitting one from the start helps prevent detonation from oil getting in through the intake system. Finally, an airbox was built around the inlet pipe and filter to keep it low key and also to keep hot air away from the air intake filter. A carbon fibre top was cut out of sheet carbon fibre to make the finish look very smart under the bonnet.
 Now that the turbocharger was situated properly, the next area of attention was with the exhaust system. For the desired power figures more flow was needed - a lot more - so we upscaled the size from the turbo back. Our experience with 400kW+ engine builds has taught us that the optimal design would be a 3 ½” mandrel bent stainless steel exhaust. An entire new system was tig-welded in our welding shop, with a new muffler and resonator. The vehicle previously had a AVO 3” stainless steel exhaust system which was quite good already, but if we wanted to reach 380kW, the vehicle would make the power much easier with the bigger system. The resonator and muffler were designed to keep it as quiet as possible while keeping back pressure at an absolute minimum.
With all of the mechanical bits squared away, it was off to the electrical. For maximum power we knew the standard ignition system would not be up to task. Two Bosch T-Mount Motorsport coils were to be mounted on each side of the vehicle to allow ease of running spark plug leads to each spark plug. Ice 9mm Spark Plug Leads run from them to the NGK Iridium Spark Plugs.
 Tuning can make or break the whole project, you cannot afford a single mistake when tuning past 300kW at the wheels in a Subaru. This is where our 40 years of experience really shows, and we didn't have a single issue during this process. Now that the mapping had been dealt with, the duty cycle had gone through the roof on the 850CC injectors that were already on the vehicle. Even the anti-surge tank with an 800HP Bosch Fuel Pump and larger AVO fabricated 3/8” bundy tube fuel lines were not enough to run this beast. Fitting a set of PFI fuel rails and Lucas 1200CC injectors brought the duty cycles down to an acceptable and safe range.
 Once we could pull out the ear plugs, we had a nice 373 kilowatts at the wheels [501whp] on 1.9 bar of boost running 113 Octane Unleaded race fuel on the sprint map. And the "safe" map for 98 octane produced 320 kilowatts at the wheels on 1.5 bar of boost. Even with 2.0 bar of boost the vehicle did not make any more power, so we are thinking the turbocharger is maxed out again. Lets see how the customer goes with this bunch of power and see if his hunger for more power is satisfied for the moment. |
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