As some will be aware following on from a discussion on one of the boards I decided to drop Borgwarner an e-mail asking them some simple questions regarding the K03 turbocharger and what some have called the K03S. The Question [ QUOTE ] I type this e-mail to you on behalf of many Audi enthusiasts, and myself as an Engineer, who ponder on the same question, as to the interchange-ability of the K03 and K03S turbocharger units. As a member on one of the largest Audi forums on the web (www.audi-sport.net), as well as a member on many other Golf and Audi forums, I have often seen this subject raised but with no definitive answer ever given, due to a lack of information available. Audi dealers are not technically inept enough to offer an answer, Audi UK are not interested in answering and most aftermarket tuners opt for either the K04 or other brands of turbo so appear not to of looked into the subject. So the questions are simple; Can the K03 turbo (5303 988 0011 or 5303 988 0053) be directly replaced by the K03S (5303 988 0052 or 5303 988 0026) in terms of the intake and exhaust housing dimensions? Is K03S the correct designation for the latter turbo mentioned above? Obviously if a K03S was fitted an engine map could be changed to suit, but would a standard turbo map for the K03 be suitable for running a K03S on? or would a loss of power be expected? and are the characteristics of the two very similar? I hope you can assist with the above and any information you could supply would be gratefully accepted. It's taken me a couple of weeks to get a response, but the response was well worth it, I think yóu'll agree. [/ QUOTE ] The Answer [ QUOTE ] Dear Sir, thank you for your interest in our products. First of all, we don't produce any turbo that we call K03S. But I hope I can answer your questions with the following explanations. All the following turbos have the same installation dimensions and thermodynamical performance. The differences are only in the actuator that opens the turbine bypass valve: K03-011 (5303 988 0011) 150 hp, 65 N actuator K03-026 180 hp, 85 N actuator K03-035 180 hp, 85 N actuator K03-044 150 hp, 65 N actuator K03-045 156 hp (Ibiza Cupra), 85 N actuator with 2 ports K03-049 150 hp (Sharan/Alhambra), 65 N actuator The 180 hp versions have an actuator with a higher opening force due to the higher exhaust gas pressure (which is a consequence of the higher boost pressure). Otherwise the valve would be pushed open by the exhaust gas pressure. The following turbos are a further development (since 2000) and have an improved and slightly larger compressor while using the same turbine (still with the same installation dimensions): K03-052 180 hp, 85 N actuator K03-053 150 hp, 85 N actuator K03-058 150 hp/180 hp, 85 N actuator Consequentially, putting a 180 hp turbo on a 150 hp engine will not bring about any change in performance, but putting on a K03-052/053/058 instead of the older versions will bring a slight improvement in engine efficiency. Additionally, with re-mapping the ECU you can achieve about 215 hp without danger of overspeeding the turbo. With the older turbos, 195 hp is the limit. With the K04 that's also commonly used (5304 950 0001) the power output should not be more than 220 hp. That means, changing a K03-052/053/058 against a K04 does not make a lot of sense. I hope that these details answer your questions. If you need more info, one of our service distributors will be glad to help you. Their addresses are on our website www.turbodriven.com. Mit freundlichen Grüßen/Best regards, BorgWarner Turbo Systems GmbH [/ QUOTE ] Big respect to Christian at KKK for taking the time to respond, especially to an end user like myself.