Executive Decisions

By Staff Writer
May 13, 2009 | BMW | Posted in Exhaust | From the May 2009 issue | Never miss an article

Story By Per Schroeder

It’s easy to make a car cheap and fast. It takes real brains—and added expense—to also make that quick machine pleasant and reliable. We think the potential payoff is worth the additional time and money, however. As some of us get a little older, we start to really appreciate a fast car that can also motor around town without disturbing the neighbors.

Our BMW 335i build fits that bill perfectly: It wouldn’t be a success if our search for more performance meant trading Clark Kent manners for Incredible Hulk finesse. We need to keep the car civil while increasing its performance.

The 335i’s big brother, the latest M3, is our benchmark for this project. The M3 is simply stellar, providing supercar performance in a smooth, daily-driver package. It’s the modern executive-level track car. Unfortunately, it costs $15,000 more than the 335i. We’d also rather have something a bit more personalized than an off-the-shelf hotrod; there are already enough cookie-cutter cars out there.

Our plan is to modify the cheaper car to kick some M3 butt—while only spending half the price difference. And for those looking for an even better deal, let depreciation be your friend: The first-year 335i can now be purchased for less than $30,000. Although a total investment in the neighborhood of $40,000 might sound very un-grassroots, this DIY approach of using our brains to solve a problem and save some money is at the very heart of our philosophy.

Our initial dyno testing has shown that we’re not too far off the mark in terms of comparative performance. In stock form, even though the 335i is giving up 73 horsepower at the rear wheels to the M3, we’re getting an additional 36 lb.-ft. of torque to the ground. Our car also weighs 40 pounds less.

When we compared the two models on track at Gainesville Raceway, our time deficit in the cheaper car was only a second and a half. We think we can narrow that difference—and maybe even tip it in our favor—while upping our car’s handling to make it even more capable and fun.

We’re Exhausted

Our performance quest started at the exhaust system, a place where the factory often has to make a few compromises. Not only does the stock system have to direct the exhaust gases from the engine to the atmosphere, but it must also keep the engine noise in check.

We were willing to make the exhaust note a bit louder if it meant more horsepower and torque. In keeping with our goals, however, we weren’t going to settle for a cheap, cobbled-together exhaust. It would take some real effort—usually spelled “time and money”—to develop something that is as powerful, quiet and well-built as the factory system.

Finding the right solution would also mean that some testing was in order. Of course we wanted to improve performance, but we didn’t want the car to drone on the highway or disturb our neighbors. We also wanted to keep an eye on the price—it simply wouldn’t do to spend a significant portion of our budget on a set of shiny pipes.

So, we gathered five different exhaust systems to test at the Superchips dynamometer facility in Sanford, Fla. Their in-ground Dynojet is planted right in front of a two-post lift, making exhaust swaps a breeze. The systems were all photographed, analyzed, weighed and tested in a matter of hours; fortunately conditions here in Florida were nice and consistent, making for a valid power comparison. We used a sound level meter to measure noise levels during the testing, with each reading taken 10 feet to the side of the exhaust outlets.

All of our exhaust systems were constructed from high-quality stainless steel tubing and mufflers. As one would expect nowadays, each unit would likely last the life of the car. The days of needing replacement mufflers are passing—honestly, for many of us on staff it’s been close to 20 years since we’ve had to replace a muffler because it was rusted or blown out. While this is good news, it does make our job of finding the best exhaust a bit harder.

After we took objective measurements for each system, we also made some subjective judgments as to their sound and build quality, scoring them from bad to good on a scale of one to five. Read on to see how our different exhausts stacked up against one another.

Stock (baseline)

The BMW 335i’s exhaust stream exits the turbos in two separate downpipes, each with its own primary catalytic converter. After the cats, these downpipes each end in a flange. These flanges mark the point at which the main exhaust system—and our testing—starts.

Like many new BMW and MINI products, the car’s exhaust gases then pass through a secondary set of catalytic converters located at the start of the main system. From there, the gases travel through an X-pipe that allows the pulses from each bank to mix with each other before finally reaching the rear mufflers.

The stock BMW exhaust system—from the downpipe flange to the exhaust tips—weighs in at a hefty 78 pounds. There’s a vacuum-operated valve within the left-rear muffler that diverts some exhaust flow to the right-rear muffler under full throttle. This vacuum line was plugged after the stock system was removed.

BMP Supersprint (axle-back)

The Supersprint axle-back system requires that the stock system be cut off right before the factory X-pipe. We used a reciprocating saw to cut through the thick factory stainless steel; it took a nice, fresh blade to get the job done. The Supersprint system then simply slips onto the stock pipes and clamps in place using pinch bolts that are built into the pipe itself.

Supersprint’s performance exhaust systems are renowned for their longevity and build quality, and this one is no exception. The Italian-made Supersprint is designed to meet all European Union requirements. As a result, it’s nearly as quiet as the OE system through the entire rpm range. The exhaust was barely audible over the tires on the dyno.

Using this system with the front half of the stock system was a weighty combination, however, tipping the scales at 89 pounds. On the plus side, retaining the factory secondary catalytic converters meant that this was one of two systems tested that were emissions legal in 50 states. If you’re a Stock-class autocrosser or have a strict visual state inspection, you’ll want to retain those second cats.

We saw gains of 8 horsepower and 6 lb.-ft. of torque with the Supersprint. That’s impressive considering we also retained the full emissions legality and nearly stock levels of sound. The downside, however, is that the Supersprint suffers from the current dollar/euro exchange rate and thus retails for $2500, more than twice the cost of some of the other units we tested.

BMP Supersprint (full system)

Supersprint offers a modular approach to exhaust systems for the 335i, and the parts can be ordered a la carte. Supersprint also offers a pair of pipes that bolt up to the factory downpipe and remove the secondary catalytic converters, creating a more conventional aftermarket setup. Since this setup removes part of the factory emissions system, BMP Design and Supersprint only sell it for off-road and track use. However, we see it meeting any applicable sound limit.

The addition of these pipes helped the car gain a bit of power on the high end of the tachometer, but there was a corresponding small decrease in torque, especially at the low end. This was the only system tested that seemed to produce a slight drone in the mid-range. There was a quick rise in sound levels as the car reached about 3500 rpm, but it subsided as the car accelerated. The peak decibel readings were in line with the rest of the full systems.

UUC Corsa RSC335

Like the first Supersprint variation that we tested, the UUC Corsa RSC335 exhaust system is an axle-back system. Also like the other partial exhaust, this one required that the stock exhaust system be cut—this time at a specified 4 inches from the inlet of the rear mufflers. Once that’s done, the UUC system is designed to simply slip onto the factory pipes and clamp down.

Unfortunately, we had to swing by a muffler shop to have the inlets slightly expanded so they’d fit over the factory piping. That’s no big deal, though we did contact UUC to have them update future systems so this won’t be a problem for buyers. This flaw earned the system a fit and finish score of only four points instead of five.

Like the Supersprint axle-back system, the Corsa’s inclusion of the factory secondary catalytic converters means that this system is also emissions legal in all 50 states. The factory system could also be easily reinstalled with some sleeves and a bit of welding.

For sound control, Corsa uses a chambered muffler design and acoustic tuning, bypassing the use of any packing material that could blow out or degrade over time. It produced an exhaust note that was higher-pitched than the other systems, giving the 335i a thrilling, exotic sound on acceleration.

While the UUC Corsa axle-back muffler set didn’t post the horsepower gains seen with some of the other systems, it did produce the second-best peak torque figures. It was also the least expensive choice and was built very well, featuring two rather attractive double-walled tips. Aside from the pipe-sizing issue, we felt that this was on the top tier in terms of fit and finish.

Borla

The Borla full exhaust system is quite a bit different from the other units we tested. First off, it’s the lightest of the bunch, tipping the scales at a svelte 51 pounds. Borla uses a very high-quality variant of stainless steel that allows for a thinner wall tubing, and that means less weight. The Borla also uses a single dual-outlet, dual-inlet rear muffler that spans the area underneath the trunk.

The Borla yielded strong horsepower and torque gains. Add in its light weight, and the Borla system is a natural choice for the track day or autocross enthusiast who wants to shed a few extra pounds at the cost of a little bit of refinement. Where the other systems had fully polished tubing and thick flanges, the Borla made do with slip joints and brushed 300-series stainless—functional, effective and light, but not as jewel-like as the other examples.

On the sound front, the Borla was neither intrusive or booming, but it did have a deeper tone that was noticeable both at idle and during acceleration. For the price, the Borla offered a boatload of performance.

Autobahn Exotics

The Autobahn Exotics exhaust is the only system we tested that removes the X-pipe from the equation. While there are some pluses and minuses to separating the two pipes coming off the turbos, there’s no arguing against the dyno numbers. The Autobahn Exotics system had our second-highest horsepower reading and was nearly tops in torque as well. The power improvements were respectable at nearly all engine speeds.

The Autobahn Exotics system comes with nice, thick flanges that connect the various portions of the exhaust. While this takes slightly longer to put together than a typical slip joint, we have found that the flanged systems tend to remain more leak-proof over time.

The Autobahn Exotics system was the one that gave us the most traditional “hot BMW” sound from the straight-six without being intrusive. We noticed some increased noise at idle and around town, but there was no drone on the highway.

The Autobahn Exotics was a nice combination of power, price, weight and build quality. Deciding which exhaust to use was getting harder.

BimmerWorld HKS

Japanese performance giant HKS is expanding into the European market, and this 335i exhaust is one of their first forays into new ground. We sourced the system through BimmerWorld; James Clay, company president, is working with HKS to bring the product to the U.S. market and giving feedback for any necessary improvements.

The HKS system uses yet another design for piping, as it features an H-pipe. This allows for the equalization of exhaust gas pressures, but it doesn’t encourage lots of commingling of the streams.

The HKS also has a second set of resonators that help smooth out the exhaust note—and smooth it is. The HKS was the quietest of the bunch at idle, yet had a nice tone when accelerating hard. We found it difficult to tell that we had a performance exhaust on the highway, but around town it sounded just a bit throatier, much like a factory performance exhaust.

Another neat feature of the HKS is a bolt-on reinforcement plate that limits tip misalignment as the exhaust system warms up. You might never notice that little brace, but you would certainly notice misaligned tips. Unfortunately, those tips didn’t quite seem appropriate for a car of this type. The large-diameter, blue-tinged titanium tips might look more fitting on a Japanese sports car rather than a European sports sedan. Future HKS systems sold through BimmerWorld will feature more traditional-looking tips.

Despite the near-stock sound levels, the HKS exhaust system was the chart-topper for both horsepower and torque. We saw an additional 13 horsepower and 8 lb.-ft. of torque, with good gains at every rpm point above 2000 rpm.

Box It Up

Based on our test results, we decided to run the HKS system sourced from BimmerWorld on our car. This one offered the best gains with the least compromises in sound levels and build quality.

That said, there were no stinkers in this test. Every single system offered gains in horsepower and torque. We’d also say that the entire field yielded improvements in both sound and build quality.

Next on our quest to dethrone the mighty M3, we’ll be opening up the intake side of the twin-turbo engine while tweaking the ECU’s programming for even more power. And of course, we’ll be posting updates and results as they happen.