Project Audi B5 S4 - Eurotuner Magazine

In the previous installment (et 11/09) we took the squish out of our project S4, overhauling its chassis with some premium gear. This time we plan to make the go-pedal work better.

In the case of turbocharged Audis and VWs, faster is a simple matter of creating more boost. But boost and power doesn't necessarily equate to a fast car. A big, laggy turbo feeding lots of air with low boost might give big, unusable power for dyno bragging rights but a small turbo that feeds less air but produces a lot of boost will have the response and torque that makes a car fast on the street.

The design philosophy behind most stock turbos is fast spool, immediate response and good torque. And since the 2.7L V6 S4 engine was designed to rev to 7000rpm, we were better off tuning the turbos to operate effectively in that range, instead of fitting expensive big turbos that kick in late in the rev range.

First: Boost The easiest way to increase power from a turbocharged vehicle is to reflash the ECU with tuned software that provides more boost. So we opted for GIAC's reliable software package that's encrypted and not easily detected by dealerships. We selected the stage one K03 turbo package, which includes a pump-gas map with higher boost. With GIAC's switching we also got a race-gas map, stock, valet and kill settings, selected through a handheld Flashloader.

The software increased peak boost from 8psi stock to 19, with appropriate fuel and ignition maps so the car runs a little leaner than stock under mild load, contributing to small gas savings.

With the reflash, we raised the power and torque curves (while maintaining the same profile), gaining 32whp at peak and 45 lb-ft of torque at a useful 3200rpm.

Second: Intake Although we had gains across the board with the software, the power curve dropped off as the revs approached redline. This suggested we'd hit a flow restriction. Often, this is the result of an airbox built to minimize intake and recirculation valve noise at the cost of power at higher revs.

K&N Filters makes a bolt-on solution for the S4. Its 57i intake is a cone filter and intake adaptor that eliminates the stock airbox along with its long plumbing. However, the OEM cold-air ducting is retained to ensure fresh air is still directed to the cone. We found the slight increase in intake noise was an added bonus, too.

Testing on K&N's dyno showed the 57i increased airflow from 4500rpm upwards, increasing peak power by 17hp and torque by 6 lb-ft. These are good numbers for a filter mod and mean the power curve stays flat from 4500-7000rpm redline instead of falling away after 6000rpm.

Third: Exhaust With our S4 sucking a larger volume of air, we'd upset the overall airflow balance and constipated the car. We'd need a less restrictive exhaust to restore the balance. And since changing the exhaust on a B5 S4 can be a pain, we decided to replace the stock catalysts and downpipes at the same time.

Finding the right hardware for our stealthy street car wasn't easy. While flow is a good thing, we didn't want to be dragging a sewer pipe across every speed bump. Even a well-designed exhaust can have problems routing a large diameter pipe under the rear subframe. However, Techtonics offers a dual 2.5" stainless steel system that is able to use smaller tubing because the exhaust is split into separate streams for each bank of cylinders. Both tubes merge into a resonator and out of a Borla muffler. Techtonics also has a 2.5" downpipe with high-flow cats and a flex section to absorb engine movement.

With the system fitted by MD Automotive in Westminster, CA we found even more low-end gains on the dyno, which was our goal. We picked up 6whp at peak and gained a massive 34 lb-ft at 3200rpm. So our car develops peak torque when most turbos or NA engines are starting to suck enough air to make power. The car literally jumps off the lights and holds a good amount of power to redline.

Last: Intercooling With the response we wanted, now comes the tradeoff. By running our turbos at higher boost and turbine speed, we overloaded the stock intercoolers and increased the intake temperatures. In a daily driven scenario, the heat-soak would rob power and increase the chance of detonation.

Unfortunately, the S4's engine bay is tight and fitting a massive front-mount intercooler could suffocate the radiator. Audi knew this and designed dual side-mount intercoolers for the car, with two small units in either side of the bumper.

The best solution seemed to be following the same logic, but go bigger. Evolution Racewerks (ER) had created a dual side-mount kit that increases the cooling volume by 240% while still being able to hide behind the stock bumper and fender liners. This provides better cooling and less pressure drop. The SMIC kit also comes with optional carbon fiber shrouds to duct more incoming air through the intercoolers.

Because of their size, we needed help with the installation. The intercoolers fit well but it's a lot of work to squeeze them into place. So ER recommended Mike Ghaemi at Wicked Motorsports in Van Nuys, CA since you have to remove part of the front clip as well as doing some sheetmetal and plastic grinding.

The intercoolers could probably make power on a cold dyno run, but the real benefit comes from being able to maintain the same power level over a longer period of time. So we decided to test this theory by logging the intake air temperatures on a third gear pull (from 3000rpm to redline) on the highway, which would give us some real-world airflow. Our tests show that under similar ambient conditions, the stock intercoolers would gain more than 25°F over the run, even after initially dropping 15° as air moved through the intercoolers.

With the larger ER SMICs, the run started at a lower temperature and by the end of the pull had never exceeded the starting temp, meaning we had more consistent power delivery more of the time. What's more, the time required to make the run to the redline was shorter, meaning we were making more power over the distance than with the stock units.

Verdict If you're thinking this is only 266whp for several thousand dollars of bolt-ons, you clearly don't get the point. This car is fast on the street. There's enough grunt whether at a traffic light or cruising for our loaded S4 to pull away from the more powerful Nissan 350Z or Subaru WRX all day long. That's the beauty of torque and response. These mild modifications have allowed us to build a seriously fast street-sleeper that's both fun to drive and very reliable. And while we don't have huge horsepower gains to shout about, we created a car anybody could replicate and get good value for the money.