When Audi released the S4 in late '99, everybody went nuts. At that time, no other V6 in the States had a twin-turbo engine this side of the $120k Porsche Turbo, never mind all-wheel drive.
The 2.7 liter started with 250hp and 250 lb/ft at the flywheel, good for a 14sec quarter at nearly 100mph. With four doors and room for five, this ultimate hauler continued its reign until '02.
In this article, we're using 200hp at the wheels (whp) as our baseline, indicating 20% drivetrain losses. But gains vary from car to car. To calculate engine hp, divide whp by 0.8.
Also note, installation costs aren't included but should be considered since a turbo upgrade can take several hours once the engine is removed.
Furthermore, importers of European products like RS4 turbo kits are at the mercy of fluctuating exchange rates, so prices can vary.
$0-1000
As always, a decent intake system gives the best bang for your buck.
AWE Tuning's S-flow intake uses an ITG filter that sits in the factory airbox, intended for light mods with stock turbos. For $190, you can expect an 8whp gain with a 205whp peak.
ABD Racing has the Quick Flow intake for $270, which gives 5-6whp. ABD also has the Big Bore intake with 3.5" ceramic-coated tubing and silicone couplers for $165.
APR has a $599 software upgrade with an optional switch to toggle between stock, pump gas, race fuel, and valet modes. You should expect over 245whp on 91-octane with 292 lb/ft, or 255whp with 306 lb/ft on 93 at 1bar (14.7psi) boost. APR also has an O-ring sealed aluminum BiPipe for $349, replacing the boost-leaking throttle body boot.
Neuspeed cat-back exhaust...
Neuspeed cat-back exhaust
Neuspeed's $450 software, added to a K&N drop, should net approximately 50whp CARB-legal at 1-1.2bar.
Autospeed Performance (ASP) has stage 1 software, providing performance and stock codes, as well as valet and kill programs, all for $549. Expect 50whp and 90 lb/ft on 93-octane. There's also a shift light and launch control option. With ASP's $199 1.4 lb 6061 aluminum crank pulley (6 lb stock), expect 8-10whp. Throw in ASP's phenolic intake insulation spacer and you'll be saved from heat soak.
The $130 spacers reportedly reduce intake temps by 10.
Those wanting larger turbos should consider ASP's 85 and 90mm MAF housings for $330 and $389 respectively, allowing for custom tuning.
ASP phenolic intake insulation...
ASP phenolic intake insulation spacer
Other ASP components in this price range also reduce drivetrain losses. One is a $249 billet transmission stabilizer bar, which ties the tranny to the chassis through a rubber bushing. There's also $395 billet engine mounts with two stages of urethane bushings, making shifting easier so more power can be transferred to the wheels.
AWE sells its billet Drive Train Stabilizer (DTS) for $349. Since many S4s rack up the miles, AWE sells factory RS4 engine mounts for $130 each - something to think about when upgrading to K04 turbos.
Since the two factory diverter valves are weak links, Forge Motorsport sells $150 billet replacements that improve throttle response and maintain reliable boost.
AWE also sells $160 Bailey Motorsport diverter valves in either polished or black. While replacing a faulty DV, the Bailey valve revealed the old unit lost 8% horsepower during AWE's dyno tests. They also sell Samco hose kits for $445, replacing all boost hoses on high mileage engines.
Dahlback Racing has a $599 chip upgrade putting out 250whp and 315 lb/ft. With a decent cat-back, these figures can be increased to 260whp and 320 lb/ft, respectively.
ECS Tuning sells a $270 aluminum pulley kit, designed to add a few hp by reducing rotational mass instead of underdriving the accessories. This includes the crank, power steering and alternator pulleys, each weighing 683, 307 and 123 grams, respectively.
Evolution Motorsports (evoms) sell a CAD/CAM designed venturi-type intake that acts as a vacuum. The patented $299 V-Flo reportedly gives 10whp. Also available is a $289 cold-air intake system (CAI) with a thermal-coated aluminum heatshield.