In previous issues we visited South Bay BMW in Torrance, CA to install the BMW Performance body kit and carbon parts for the exterior and interior. These official tuning parts are available from your local dealer or online at accessories.bmwusa.com The parts are of impeccable OE quality and won't adversely affect your warranty. What's more, you can finance the parts on a new car, helping to spread the cost.
We thought it was time to get into the engine tuning, and so we're starting with the BMW Performance exhaust. The system is designed to reduce back pressure, allowing the engine to operate more efficiently and sound more exhilarating.
The $1095 BMW Performance stainless steel exhaust comprises a replacement rear muffler that uses the same housing as the stock system, guaranteeing a perfect fit. However, the tailpipes are changed for polished chrome tips and the muffler has different internal baffling.
A short section of pipe is also included to replace the center resonator. Clamps are provided to join it to the system.
By allowing the engine to rev and breathe more freely, BMW claims the exhaust contributes an extra 5hp at 6600rpm and 3 lb-ft of torque.
Armed with the system, we visited our friends at GT Pro in Fullerton, CA. The tuner is developing a big-turbo kit for BMW's 3.0 twin-turbo motor, so is very familiar with it.
GT Professionals
Fitting and testing was carried out by company president Brian LaFuente, shop foreman Duane Dahl and tech Rick Keyser. We started on the company's all-wheel drive DynoJet 424X dyno to establish a baseline.
We'd heard from several tuners that BMW's claimed 300hp and 300 lb-ft was conservative for this engine, and ours was no different: "This is the most powerful we've ever seen on our dyno," claimed Brian, who has a catalog of previous dyno runs to verify his statement - our 135i delivered 284hp to the wheels at 5700rpm and 294 lb-ft at 3400rpm!
Fitting the exhaust was straightforward, as you can see on the next page. Removing the rear muffler could be done at home on jackstands. However, cutting out the resonator requires a little more thought. The guys at GT Pro used an exhaust tube chain cutter with the center pipes in situ. Alternatively, you could remove these pipes and cut them off the car - they're bolted to the downpipes.
We were surprised to see the replacement pipes didn't sleeve over the stock system. Instead, two circular clamps seal the joints as they're tightened. This means you have to get the cuts straight and the clamps tight to avoid gas leaks and excessive noise.
The stock rear muffler has a vibration damper as well as a flapper mechanism to assist engine warm-up and low rpm response. The damper transfers to the replacement muffler but not the flap, for some reason, so the vacuum hose should be clamped off.