The AWE tubular exhaust manifolds cost $2995. The header flanges are fly-cut and come Swain Tech coated. They give a peak 10whp gain at high rpm and over 20 lb/ft down low over stock. You can earn a $200 discount when its ordered with AWE's K04 upgrade.
Exhaust specialist GHL Motorsports takes pride in Rath-Gibson aircraft quality 304 stainless steel, which is about three times more expensive than imported 304. It's cat-back system retails at $930 and the downpipes are $1435. The downpipes use 400-cell ceramic high-flow cats, but GHL can include two extra test pipes to replace the cats if desired.
A Milltek cat-back and downpipe combo runs $2998 purchased through Stratmosphere. The German-made, platinum-coated, 200-cell cats give 85% open cross-section.
If you want to build your S4 heads, Matrix Engineering offers Supertech intake and exhaust valves, stiffer springs, titanium retainers, locaters, and solid lifters for $2457.
$3000-$4000
APR stage 2+ includes its $599 stage 1 software, as well as its stainless downpipes and cat-back, bringing the total to $3097. The downpipes remove the factory pre-cats off the turbos. They include two pipes off each turbo, a set of pipes with cats and test pipes without cats. The cat-back starts with twin 2.25" piping and merges to a single 3" muffler. You can expect around 270whp with this setup on stock intercoolers.
For the S4 that just blew its turbos, factory K04s is the way to go for both price and power. Any GIAC dealer can sell you the stage 3 "minus" software, aimed at people who can't afford a full stage 3 kit yet. All you need is a Bosch 5bar FPR and the turbos, which you can get from companies like AWE, Torque Factory and Evolution Motorsports. It works with the stock intake, injectors and exhaust. The setup will run about $3800 and will work on Tiptronic cars. Expect around 290whp.
$4000-5000
If you've got the coin and want Oettinger products, CEC imports the full, hand-built, stainless steel exhaust system. The cat-back retails for $2500. The downpipes go for $2000, plus an extra $3000 if you require the 100-cell motorsport cats.
Dahlback Racing reports it hits over 295whp from the stock K03s when using its 3" downpipes with 100-cell cats, specific software and RS4 intercoolers. The kit retails for $3495 and includes all hardware, but you'll need a cat-back as well. Keep in mind you'll also probably need a new clutch, which Dahlback has for $2100, including disc, pressure plate and single steel flywheel.
Stratmosphere's stage 3 system retails for $4300 and includes Milltek downpipes, 3" cat-back exhaust, Hyperboost diverter valves, 5bar FPR and O.CT stage 3 software. Expected power is 265whp with 320 lb/ft as early as 2300rpm.
$5000-6000
ASP offers a stage 3 K04 Motorsport turbo kit. It includes: K04 turbos, fueling kit with 60 lb/hr injectors and phenolic fuel rail spacers, 85mm MAF housing, coolant temp sensor, in-house software, plus hardware and gaskets for $5200. ASP recommends its downpipes, RS4 clutch and intercooler with this kit.
$6000-$8000
Give AWE $5250 and you get a RSK04 turbo upgrade, turbo inlet piping and installation hardware to the downpipes. This kit also includes AWE's optional fuel system with balanced Bosch injectors, spark plugs, GIAC software, AWE MAF housing with adapter, silicone hoses and all hardware. For '00-02 models, the Bosch MAF must be converted to a Hitachi unit, adding $300. AWE reports it doesn't use OE RS4 injectors because the spray pattern doesn't suit US-spec S4s. AWE also claims its downpipe ($1595) and cat-back system ($995) are needed to run this kit. The firm further recommends its intercooler upgrade ($1396), Samco hoses ($400), DTS bar ($349), RS4 engine mounts ($260) and RS4 clutch kit ($495). Tiptronic S4s need a level 10 transmission upgrade to run this kit.
Dahlback's stage 3 K04 upgrade includes turbos, fuel pump and injectors, RS4 intercoolers and downpipes. It costs $5195, but a cat-back system is also required. In testing, it's shown 370whp and 365 lb/ft on pump fuel, with 430whp and 440 lb/ft on 100-octane.
$8000-10000
Stratmosphere's stage 4 upgrade uses K04 turbos with special turbine wheels to provide K03-style spooling, but K04 top-end. Everything's included for $9400, making 300whp with 325 lb/ft from 2300-4200rpm. The kit includes Milltek downpipes and 3" cat-back, Hyperboost DVs, throttle body boot, special plugs and 5bar FPR.
ASP GT2871 upgrade kit providing...
ASP GT2871 upgrade kit providing 560 whp
$10000-12000
The $7249 APR stage 3 kit includes two RSK04 turbos, plus the MAF, airbox, Y-pipe, turbo inlet piping, fuel pump and injectors from the RS4. It falls into this price bracket because downpipes and a cat-back are needed, adding $2498 and nets you 340whp. You should also add APR's intercooler upgrade to be around 360whp with 360 lb/ft for a grand parts total of $11245. A clutch is highly recommended as well.
If you took ASP's $5200 stage 3 K04 Motorsport turbo kit and added the recommended downpipes ($1550), cat-back exhaust ($1000), intercooler ($2200) and RS4 clutch ($800), you'd reach $10750, with a reported 400whp and 415 lb/ft on 93-octane.
If you're in need of a new engine, it's a good time to consider bumping up to 3.0 liter. ASP offers its 3.0 short block, bored-out and fitted with a forged ASP crank, rods and 8.5:1 pistons. The built short block costs $6500, but requires 2.8 liter heads with an upgraded valve train to bump the rev limiter to 8500rpm, bringing the total to $10000. ASP reports this upgrade is good for anybody after 900whp!If you dream of converting your S4 to RS4 performance using factory components, ECS Tuning has what you need. Starting with the engine, it's got the RS4 intake cams ($594), intake manifold ($420), fuel rail, pump and injectors ($1969), timing covers ($90 pair), head gaskets ($110), intake manifold and throttle body gaskets ($18), airbox and air mass sensor ($820), intake tube ($60), pressure pipes ($840), throttle body ($820), Y-pipe ($650), complete RSK04 turbo kit with inlet pipes ($2750) and intercooler ($750). If you include the RS4 engine cover ($740) and oil cooler ($1099), your total is $11730. The "380hp" RS4 puts out over 300whp stock.
$$$ Rich and Shameless
If your S4 is bone stock, you can double your power with AWE while maintaining good spooling. You need their RSK04 turbo kit with stainless and coated tubular manifolds and fueling system, plus the side-mount intercooler kit ($1396), downpipes ($1595) and cat-back ($995), bringing the total to $9236 ($10145 for '00 models converted to Hitachi MAF). Additionally, you'll need the RS4 clutch ($495) and a turbo-back system ($2590) to fully benefit, for a grand total of $12321. AWE has seen 380whp and 370 lb/ft on 93-octane, and 416whp and 428 lb/ft using 103. Thanks to the exhaust manifolds and GIAC tuning, 20psi is reached by 3000rpm.
ASP's cream of the crop is its $15500 GT28RS turbo kit. Along with the Garrett turbos with modified housings for better fitting, you get in-house software, ASP tubular and coated headers, downpipes with high-flow cats, 38mm Tial wastegates, split wastegate tubing, stainless lines, hoses, gaskets, 90mm MAF housing, 60 lb/hr injectors, 4bar FPR, Bosch Motorsport fuel pump, platinum spark plugs, phenolic spacers and ASP's FMIC kit. You'll also need a clutch kit ($800), which is sprung for a factory feel. Additionally, you'll need a cat-back ($1000) to fit ASP's downpipes, bringing your new total to $17300. The result is a whopping 560whp with 590 lb/ft on 93 octane, and 635whp and 611 lb/ft on C16 race fuel.
If you want a really fat torque curve, Stratmosphere's highest power upgrade is its $17959 stage 6. It includes quick-spool K04 turbos with hardware, low compression gaskets, MAF assembly, RS4 fuel pump with O.CT injectors, 5bar FPR, plugs, Milltek turbo-back, O.CT software, RS4 intercooler and oil cooler, Stratmosphere stage 2 clutch and flywheel, silicone induction kit and two diverter valves. The kit puts out 345 lb/ft of torque from 2500-4500rpm, only dropping to 300 lb/ft by 5700rpm. Peak power is 345whp.
Known for its fast cars, Dahlback's daily-driven S4 makes 520whp and 464 lb/ft on pump fuel using company pistons, rods and heads port-match to the RS4 intake manifold. The K16/24 hybrid turbos flow through RS4 intercoolers and throttle body. It also uses diverter valves, injectors, Bosch fuel pump and downpipes. Dahlback will install this engine for $35000 plus a cat-back exhaust.
Although primarily AWE dealers, Wicked Motorsports offers its "stupid-power" package. It includes a built motor to reach 800whp on race fuel with a custom GT2871 turbo kit, Innovate Systems headers and downpipes, built 2.8 liter
O-ringed motor using JE Pistons, Cunningham rods, nitrated crank, main and head studs, a worked head with oversized valves and custom cams, RS4 gaskets, Aeromotive fuel pump, fuel rail and 65 lb/hr injectors, RS4 intake manifold, Evolution Racewerks 4" side-mount intercoolers and Clutch Masters clutch and flywheel. The system is controlled by 034 Motorsports engine management. This setup will run over $26000, but don't forget 45-plus hours of labor.