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K03 vs K04 Turbo for the Audi & Volkswagen 1.8T

What’s the last time you saw an Audi on the road older than a first-generation A4? The B5-chassis A4 was a turning point for the brand, and it’s probably the reason Audi is still around today. It was also the first vehicle to use the 20v 1.8T engine, which soon found its way to the rest of the VW/Audi line, and created a tuning platform that’s still popular today.

There are many variants of the 1.8T with slight revisions over the course of several years, and each variant has its own 3-digit engine code, which can be find either on a label on the timing cover or stamped onto the engine itself. The biggest difference depends on if the engine in your vehicle is mounded longitudinal (north-south in your engine bay) or transverse (east-west). The Audi A4 and VW Passat are mounded longitudinal, meaning the cylinders are in line between the firewall and the radiator. On the Mk4 VW Golf, GTI, Jetta, New Beetle and Audi TT, the engine is mounted transverse, meaning the cylinders are in line between the strut towers. These two changes are done for packaging reasons; the A4 and Passat are larger vehicles overall, so they have more space under the hood. Even if the engines themselves are mostly the same, they will almost always use different turbos and belt-driven accessories.

No matter what vehicle your 1.8T came in, they all used a BorgWarner K03 turbo, from their unfortunately-named KKK series of turbos. One of the most popular modifications for a 1.8T is to replace the stock turbo with a K04; it’s slightly larger, but still uses the same mounting points as the K03, so it can still bolt in like OEM. The beauty of the K04 upgrade is in its simplicity; because it’s not a huge turbo, it will still run well on an otherwise stock vehicle, with a stock tune. If you live somewhere with strict smog checks or vehicle inspections, it even looks OEM.

If your original K03 turbo needs to be replaced anyway due to normal wear, and you have any interest in going faster, the K04 upgrade is easy. Once it’s installed, the vehicle’s MAF sensor will read that more air is coming in, and the injectors will spray more fuel to compensate, meaning a slight bump in power with no extra work. If you start adding on other common mods like a freer-flowing intake and exhaust, the system will be able to flow even more air and create even more power. The next step would be a front-mount intercooler to keep the pressurized intake air cooler and denser, and if you want to keep your vehicle daily-driver reliable, that’s probably about as far as you want to go on a stock tune. The next level would be bigger injectors, so the fuel supply can keep up with the air being forced into the engine, and at that point, you’re going to want a custom tune that accounts for these modifications, better controls the new injectors, and keeps everything working efficiently and reliably.

There aren’t many downsides to the K04 swap. They’re a little more expensive to purchase than a stock replacement K03, and your fuel economy will take a little bit of a hit, but that’s really about it. While we’re focusing on the 1.8T, certain applications with the newer 2.0T FSI engine have K04 upgrades available as well, and all of this applies to those vehicles as well. With either option, you’ll still need to replace the oil feed line to the turbo as well to ensure it receives a reliable oil supply. We have a full line of brand-new K03 and K04 turbos available here:

Vehicle Application Item Warranty Link
1997-2006 Audi A4, 1998-2005 VW Passat (1.8T, B5/B6 Chassis) BuyAutoParts K03 One Year Available Here!
1997-2006 Audi A4, 1998-2005 VW Passat (1.8T, B5/B6 Chassis) Stigan K03 Two Years Available Here!
1997-2006 Audi A4, 1998-2005 VW Passat (1.8T, B5/B6 Chassis) OEM BorgWarner K03 One Year Available Here!
1997-2006 Audi A4, 1998-2005 VW Passat (1.8T, B5/B6 Chassis) BuyAutoParts K04 One Year Available Here!
1997-2006 Audi A4, 1998-2005 VW Passat (1.8T, B5/B6 Chassis) Stigan K04 Two Years Available Here!
1997-2006 Audi A4, 1998-2005 VW Passat (1.8T, B5/B6 Chassis) OEM BorgWarner K04 One Year Available Here!
2002-2005 VW Golf/Jetta/Beetle, 2001-2006 Audi TT (Engine Code AWP) BuyAutoParts K03 One Year Available Here!
2002-2005 VW Golf/Jetta/Beetle, 2001-2006 Audi TT (Engine Code AWP) Stigan K03 Two Years Available Here!
2002-2005 VW Golf/Jetta/Beetle, 2001-2006 Audi TT (Engine Code AWP) OEM BorgWarner K03 One Year Available Here!
2001 VW Golf/Jetta, 2001-2004 Beetle (Engine Code AWW) BuyAutoParts K03 One Year Available Here!
2001 VW Golf/Jetta, 2001-2004 Beetle (Engine Code AWW) Stigan K03 Two Years Available Here!
2001 VW Golf/Jetta, 2001-2004 Beetle (Engine Code AWW) OEM BorgWarner K03 One Year Available Here!
2000-2001 VW Golf/Jetta/Beetle/Audi TT (Engine Code ATC/AWD) BuyAutoParts K03 One Year Available Here!
2000-2001 VW Golf/Jetta/Beetle/Audi TT (Engine Code ATC/AWD) Stigan K03 Two Years Available Here!
2000-2001 VW Golf/Jetta/Beetle/Audi TT (Engine Code ATC/AWD) OEM BorgWarner K03 One Year Available Here!
2000-2001 VW Golf/Jetta/Beetle/Audi TT (Engine Code ATC/AWD) BuyAutoParts K04 One Year Available Here!
2000-2001 VW Golf/Jetta/Beetle/Audi TT (Engine Code ATC/AWD) OEM BorgWarner K04 One Year Available Here!

If you’re still unsure about what’s best for your vehicle, feel free to contact BuyAutoParts for knowledgeable assistance in getting the best turbochargers for your vehicle. We carry guaranteed exact fit turbos, intercoolers, and more from known brands like Garrett, Holset, Mitsubishi, Stigan, IHI and more. All our parts professionals are here and ready to help!

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