I recently installed a Forge-Milltek exhaust on my S4. My local Audi dealer (Don McGill Audi in Houston) was kind enough to grant me access to one of their lifts on a Saturday so I could do the install. It went so easily I probably could have done it with the car on jack stands but doing it on a lift is soooo much easier. Plus, I had Dan Severs available to assist me when 4 hands were required. Dan was a recent competitor in the 2000 Audi Tech Challenge and earned the distinction as the number one technician in North America based on his extensive Audi vehicle knowledge. He'll be going on a week long trip to Germany to be presented with his award. Way to go Dan!
The Forge-Milltek cat-back exhaust, distributed in the US by Stratmosphere, is a 3" stainless steel system that replaces the stock exhaust from the joint just behind the second pair of catalytic converters all the way back. The stock exhaust pipe just unbolts so, thankfully, no cutting is required to remove it. Compared to the stock pipe, the Forge-Milltek 3" pipe is downright huge. The body of both mufflers have a polished mirror-like finish (if that kind of thing gets your blood pumping) and the rear muffler has very nice dual chrome tips with "Forge" etched on top. The construction looks very good although for an exhaust priced at $1200 some of the welds could be cleaner but it's merely cosmetic. Fitment on my car was perfect and painless! All the exhaust hangers hang straight (as they should) and nothing looks stressed or tweaked. The exhaust is also tucked up close to the body so it shouldn't snag anything if I decide to upgrade my suspension some day. I've installed many exhausts over the years and I'd have to say that, without a doubt, this is was the easiest bolt-on exhaust I've ever done. No alignment problems, no problems with it contacting the body or any of the suspension / drivetrain components. Alignment of the tips with the rear valance was spot on too! Outstanding fitment.
So how does it sound? First, let me say I'm very picky about noise. My absolute ideal exhaust would be one quiet as stock with better performance. That said, the best way to describe it is quiet and very civilized. I've heard a few other S4 exhausts and there is no way I'd even consider them except, maybe the Neuspeed/Borla. The others are way to noisy for my taste. It's hard to remain stealth with the exhaust announcing your presence all the time. I've yet to hear an MTM system in person. Side-by-side the Neuspeed/Borla exhaust is a bit louder and harsher sounding. The Forge-Milltek is nowhere near the noise level of the VM1 or VM2 formerly available from UUC Motorwerks. At idle it has a mellow burble that any car enthusiast couldn't help grinning about. Driving at a steady speed there is no annoying resonance inside the car and above about 2,100 rpm at a steady cruise it is inaudible above road noise. If you listen closely at a steady speed you can hear it quietly humming if you ease on and off the throttle. Step into the throttle and it will to talk to you a bit but it's still very reserved. Standing outside, the sound of a full throttle departure is a mild V6 wail--subdued, very sweet. It also totally eliminates what many refer to as the "sandblaster" noise that the stock S4 exhaust exhibits when run with more than stock boost pressure. If you want an exhaust with more personality than stock but not in your face loud all the time the Forge-Milltek should certainly be on your short list.
Here are video clips from behind with a stock exhaust and the Forge-Milltek departing at full throttle. You'll need good speakers to fully hear the deep tone of the Forge-Milltek exhaust.
So how's the power? My butt dyno notices better midrange and better power up top. I hardly ever hit the rev limiter in 1st gear prior to the exhaust but now I seem to be bumping into it frequently. 3rd gear continues to charge forward where the stock exhaust felt like the car was running out of breath. I'm fresh out of court for a speeding ticket so I didn't do any high speed 4, 5 or 6th gear charges to see how it performs. One ticket is enough, thanks. I'm very pleased with the difference.
My butt dyno has failed me in the past so I decided to do some horsepower runs with my AP-22 performance meter. I also have a G-Tech but the AP-22 offers the ability to plug in CdA (aerodynamic drag), rolling resistance, vehicle weight and tilt factor which takes into account how much the vehicle squats while accelerating. It also does data logging for horsepower plots while the G-Tech only gives peak numbers. It's a very handy tool and the manufacturer claims a typical margin of error around 1% out of the box and less than 1% with drag strip calibration.
The horsepower runs were all done on the same car in the various configurations. I used a vehicle weight of 3750 lbs (car plus driver) and did coast down measurements for CdA and rolling resistance. I used the manufactures guidelines for the tilt factor. Weather was humid and about 60°F. Note that the numbers on the graph represent wheel horsepower. Also, I think it's important to note the differences between the various runs rather than focusing on the actual peak numbers. The AP-22 is new in my tool box and I don't have 4-wheel dyno numbers to confirm the accuracy with actual horsepower numbers although I think they are reasonably close.
The horsepower numbers came out much better than I expected. So much, in fact, I repeated them thinking I'd botched something along the way. Everything checked out. I paid close attention to fuel load (as accurate as the fuel gauge would allow) so I don't think it's an issue. All the runs were done with my intercooler misters running. Ponder the results--I did. Impressive if they hold true... 21 horsepower at the wheels is nothing to sneeze at. Still, I remain puzzled and slightly pessimistic.
Overall, I'm very impressed with the exhaust. It's well put together, fitment on my car was absolutely perfect, it has a quiet nature (so as not to attract attention of the men in blue) while still giving some driver feedback and offers horsepower gains whether you choose to run stock or higher boost pressure.