Let's be honest: Increasing horsepower is always the most enjoyable and anticipated part of a project. In Project Street MINI's case, it's even better because we're playing with a supercharged car, and forced induction generally responds to power tweaks better than standard induction systems.
For this chapter we paid attention to the backside of the engine, testing two exhaust systems--a unit from Magnaflow and another from Supersprint--with the help of the technicians at evosport.
The car was strapped to evosport's Dynojet 248C two-wheel dynamometer, and to ensure the systems were tested in a fair manner, all runs were performed on the same evening and as close together in time as possible, eliminating temperature variations. Also, intake and coolant temperatures were monitored using evosport's Snap-On Graphing Scanner during testing.
First was a baseline run, a third-gear pull on straight 91-octane fuel. The little guy ripped a best 155.5 hp to the front wheels, with 136.3 lb-ft of twist at 4150 rpm. Considering this car is factory rated at 163 bhp, we were duly impressed. (Maybe the manufacturers are starting to quote horsepower figures closer to the power that actually reaches the wheels.) Whatever the case, we were seeing less than a 5% driveline loss already (but don't imagine this car actually puts out over 190 hp at the flywheel with BS conversion factors, because that's very likely not the case).
We were concerned, though, that on every pull the car seemed to hesitate slightly from 6400 to 6800 rpm. On the graphs it was evident the stock ECU pulled back some timing due to possible detonation on the pump gas (it's a glitch we'll try and sort out in the very near future).
The Magnaflow exhaust was tested first. Magnaflow designs its products in its fully equipped R&D Center, complete with multiple engine and chassis dynos along with flow bench and audio test equipment.
The Magnaflow system features rust-proof 16-gauge steel construction. Two mufflers the firm specially developed for the MINI Cooper S were used to keep our ears from bleeding. The tubing of the system starts at 2.25 in. and widens to 2.50 in., eventually letting the gasses escape out 3.5-in. dual tiops. Magnaflow uses 400-series exhaust-grade stainless steel as opposed to the commonly used 304 grade. The company reports experience with the 304 grade "springing back" after it has been bent into shape, causing fitment problems.
The Magnaflow system replaced the entire stock exhaust from the catalytic converter back. With its trick, trumpet-like exhaust tips, this system had a much deeper rumble than the factory's unit. On the dyno, the roar of the exhaust system forced our hands over our ears--this exhaust system is loud. Even so, it sounds great. Magnaflow said it's working on a longer resonator to quiet the system down slightly, probably in hopes of targeting a more general market.
When the numbers came out, however, we were disappointed to see little gain in peak torque and horsepower figures, and there were even some slight losses in the higher rev range.
To be fair, we decided to exclude gains and losses through the 6400 to 6800 rpm rev range where the engine was detonating. Any power spikes there would be heavily influenced by the car's knock sensors.
According to the dyno numbers, the Magnaflow exhaust system offered a 2.2-rhp peak gain at 5450 rpm; the peak 2.4 lb-ft of torque occurred at 3300 rpm. Peak losses were charted at 6300 rpm: 1.4 hp and 1.2 lb-ft of torque.
The losses were pretty negligible, and the car seemed to have gained about a lb-ft of torque right up until 6000 rpm. What this seems to show is the BMW gurus who put this car together slapped a pretty nice factory exhaust onto it, or else the limiting factor comes at some other point in the air's path through the engine.
Without any expectation of significant horsepower gains as a result of the previous runs, we installed the Supersprint exhaust. Because this exhaust doesn't replace the entire one-piece factory cat-back system, evosport technician Josh Rickards had to cut into the factory unit to clamp the Supersprint system onto it. That might seem like a pain, but with a Sawzall it's very simple.
The Italian-made exhaust is built from 1.5mm-thick stainless-steel (factory dimension) mandrel-bent tubing, and it has welding seams for strength and prevention of the build-up of corrosive material. Supersprint manufactures exhaust systems for such European makes as Alfa Romeo, Audi, BMW, Mercedes-Benz, Volkswagen and Volvo. Each system uses stainless-steel internal components as well as long-lasting stainless wool and long-strand rock wool packing. The firm's muffler housings are 1.5mm thick stainless steel or aluminized steel, with chrome-plated stainless tips. Although the clamps supplied by Supersprint hold the exhaust firmly, Josh tack- welded some areas to prevent any movement.
On the dyno, the system performed similarly to the previously tested Magnaflow. We were still seeing detonation in the same rev range, but the Supersprint had gains down low from 2500 to 4000 rpm, with peak increases of 1.9 hp and 3.4 lb-ft of torque at 2900 rpm. Peak losses were no more than 0.9 hp and 1.0 lb-ft at 4850 rpm with this exhaust system.
The sound of the Supersprint was even a little louder than the Magnaflow unit, but it wasn't obnoxious. With a racecar-like rasp, it sounded like this Mini meant business when the motor was spun to higher digits. The two large chrome-coated DTM tips out the back leave an obvious clue to any performance upgrades on this car.
To release a few more ponies from the Supercharged four-banger, we tried a BMP Design intake, featuring a K&N-style cone filter and heat shield from the engine compartment. The installation was easy and took evosport about 15 to 20 minutes to perform. As is usually the case with BMP's products, there were no fitting issues and the car was tested again.
With the intake on, as well as the Supersprint exhaust, we finally saw some peak horsepower gains--161 hp at 7000 rpm and 139 lb-ft of torque at 4050 rpm. Looking at the graph made it obvious the addition of this intake system bumped the torque up from stock fairly consistently throughout the entire rev range, with no losses outside the troublesome range of 6400 to 6800 rpm. The peak gain in horsepower was 5.8 at 7000 rpm, and the torque peak was 4.6 lb-ft at a low 2900 rpm.
In order to compare the gains from the intake system alone, we compared the graph to the pulls done earlier with the Supersprint exhaust system alone. With 5.9 hp and 4.5 lb-ft representing the peak gains at 6970 rpm, the BMP Design intake system proved by far the best bang for the buck in this test, with consistent torque gains from 3300 to 6200 rpm and even greater increases throughout the peak horsepower range at redline.
With a simple bolt-on of an intake and exhaust, Project MINI is now just two road horsepower shy of the factory-rated brake horsepower. We did learn that the BMW engineers used an excellent exhaust, but we're still going to keep the Supersprint on Project Street MINI--we like the sound, and it did help performance a bit.
Stay tuned; we may see some sizable gains with boost and timing adjustments in the very near future.