As first project of 2011, we were testing the limit of factory top mount intercooler on a 2008 Subaru WRX. One of the most important concerns that the owner of the 2008 WRX had was if turbo upgrade was going to affect the fuel economy of the car badly. “I use the car for daily commute. So naturally I don’t want to increase my fuel expenses after the upgrade. But then again, I want to enjoy what the boxer engine has to offer with an upgrade,” the owner said.  The 2008 WRX already had aftermarket downpipe, cat-back and air intake but had never been tuned. The car on average had 22 miles per gallon (mpg) fuel economy on the current modification. Having heard all the concerns, Edwin Tang, owner and tuner of EDO Performance, suggested the Tomioka Racing TD05-20G turbocharger, set of power enterprise 800cc top-feed injectors, set of one heat range colder NGK Iridium spark plugs, Walbro 255LPH fuel pump and Tomioka Racing equal length  header with mild tune.

The Tomioka Racing TD05-20G turbocharger is rated at 400 horsepower. It’s a bolt-on turbocharger with 640 cfm air-flow rating. It’s quite an upgrade from the factory 2008 WRX turbocharger.  “The bigger 20g turbo coupled with the high flow exhaust manifold, 800cc injectors, and 255LPH fuel pump should easily overwhelm the factory top mount intercooler. We shall see how it will hold,” Edwin concluded. The next night, the owner of the WRX dropped off the car for the upgrade.

A stock 2008 WRX published peak engine horsepower is at 224 HP.  Conservatively, 17% of power will be lost when measuring power to the wheel. So realistically, a 2008 Subaru WRX wheel horsepower is at 186. Edwin decided to do a quick tune on the WRX with its current aftermarket catback, downpipe and air intake. With peak boost at 14.63 psi, the peak wheel horsepower was at 221 with peak torque at 225.8. Things were about to get serious on the next round.

First to go was the factory turbocharger, pulling the factory turbocharger and comparing it to the Tomioka Racing TD05-20G, the factory turbo was like its little brother. Despite its larger size, the clean polish on the compressor housing exhibited factory turbocharger characteristics and it sure looked very low profile for a 400HP rated turbocharger. While the turbocharger was being loosely fitted, attention shifted to the fuel injectors. The 800cc fuel injectors surely dwarfed the factory injectors. Twice the size and flow rate of factory injectors, the owner was right to concern on the new WRX fuel economy. But his worry would have to wait until the car was finished tuning.  Next, the one step colder iridium spark plugs were installed and it was time to put back the top mount intercooler. It is always a bolt-on package with Tomioka so the top mount intercooler fit flawlessly on the new and larger turbo. With that, under the hood modification was completed. It was time to move to under the car.

TD-0520G for 08-11 WRX

Next on the list was the Equal length manifold. The Tomioka Racing Equal length exhaust manifold looks superior to the factory header in every way and it is. The larger primary tube design increases exhaust flow to the smooth merge collectors making for nice smooth transitions on the way to the large diameter three bolt up-pipe. Tomioka uses a 3 bolt up-pipe flange to avoid blowing out the lower gasket as is a common occurrence with the two bolt up-pipes. Tomioka also includes a roll of heat wrap to keep all the exhaust heat where it should be and not in the nearby oil filter or oil pan. With the equal length exhaust manifold a Subaru will lose its “boxer rumble” slightly, but it will make up for it with a substantial in the gain in power.

Tomioka Racing Equal Length Exhaust Manifold

Last but not least, the fuel pump was upgraded to 255LPH (Liter per Hour) fuel pump. The install is a little more complicated than the earlier WRX’s but the experienced technician at EDO had no problems getting it installed properly.

With the installation completed, it was time to tune and release the full potential of the EJ25 engine. It was known that the TD05-20G would spool slower than the factory turbocharger and Edwin needed to limit the lag in spool. Tuning was going very well, at one point the WRX broke the 300whp mark, but the stock intercooler was showing signs of fatigue trying handle the increase in horsepower.  The final numbers, peak boost was at 19.36 psi with peak wheel horsepower at 299.7 and torque at 305.7. As expected the WRX was lagging in its spool rate with the larger 20G. It was an increase in lag of 200 rpm which we shall see its effect on the road.  Overall, the WRX gained both power and torque across the entire RPM range.

Dyno tune was completed and it was time to test the new WRX on the road. The biggest concern was finally answered. The final number on the fuel economy was 17 -19 miles per gallon on combined city and highway driving.  The owner didn’t feel as bad losing 3-5 mpg after a 78 horsepower gain. “The car was making more power and didn’t really feel the lag from the larger turbo,” the owner concluded.

After all the modifications, the 2008 WRX still looked stock under the hood. The owner of the WRX has plans to achieve close to 400 horsepower with this turbocharger. The WRX already has all the fuel and exhaust components in place to achieve that goal. There is a list of stock parts will have to be replaced to achieve that  power gain, the intercooler,  gear set, and cooling are just to name a few. This car and turbo have a lot of potential left so stay tuned for its progress!