When was the last time you saw an Audi older than the first generation A4 on the road? The A4 B5 chassis was a turning point for the brand, and perhaps the reason why Audi exists today. It was also the first vehicle to use the 20v 1.8T engine, which soon made its way to the rest of the VW/Audi line, creating a tuning platform that remains popular today.
There have been several variants of the 1.8T with minor modifications over the years, and each version has its own 3-digit engine code, which can be found on a label on the timing cover or stamped on the engine itself. The biggest difference depends on whether your vehicle’s engine is mounted longitudinal (North-South in the engine compartment) or transverse (East-West). The Audi A4 and VW Passat are mounted longitudinally, which means the cylinders align between the firewall and the radiator. In the Mk4 VW Golf, GTI, Jetta, New Beetle and Audi TT, the engine is cross-mounted, meaning the cylinders are aligned between the strut towers. These two changes have been made for packaging reasons; The A4 and Passat are larger vehicles overall, so they have more room under the hood. Even though the engines are mostly the same, they will almost always use different belt-driven turbos and accessories.
No matter what vehicle your 1.8T came in, they all used a BorgWarner K03 turbo, from their unfortunately named KKK Turbo series. One of the most popular modifications to the 1.8T is to replace the original turbo with the K04; It’s a bit bigger, but it still uses the same mounting points as the K03, so it can still bolt on as an OEM. The beauty of the K04 update is in its simplicity; Because it’s not a big turbo, it will still work fine in a stock vehicle with stock trim. If you live in a place with strict air pollution controls or vehicle inspections, it looks like an OEM.
If your original K03 Turbo needs replacement anyway due to normal wear and tear and you’re interested in going faster, the K04 upgrade is easy. Once installed, the vehicle’s MAF sensor will read that more air is coming in and the injectors will spray more fuel to compensate, which means a slight increase in power without any extra work. If you start adding other common modifications, such as a free-flowing intake and exhaust, the system will be able to flow even more air and create more power. The next step will be a front-mounted inter cooler to keep the pressurized intake air cooler and denser, and if you want your vehicle’s daily driver to be reliable, that’s probably as far as you want to go in a standard setting. The next level will be the larger injectors, so the fuel supply can keep up with the air entering the engine, and at that point, you’ll want a custom fit that takes these modifications into account, better handling the new injectors. and keeps everything running efficiently and reliably.
The K04 Exchange does not have many drawbacks. They’re a bit more expensive than the stock K03 replacement, and your fuel economy will suffer, but that’s about it. While we focused on the 1.8T, K04 upgrades are also available in some applications with the new 2.0T FSI engine, and that all applies to those vehicles as well. As with either option, you will also need to convert the oil feed line to the turbo to get a reliable oil supply. We have the full range of new K03 and K04 turbos available here: