There are countless enthusiasts who have converted street cars to track cars for their own pursuit, this author included. Few of them are professionals, though (this author included), and consequently buying a used one is always a bit of a mixed bag of dealing with shortcuts, ill-conceived modifications or poor planning. At their base, these cars were also often used street cars initially, with years of miles on the road softening their chassis and electrical connections. Shifting gears, though, there are special cars built by enthusiasts that really stand out. These are close to factory builds; pro teams who take brand new street models and convert them to race cars for specific series. Today’s 2007 Porsche 911 Carrera was one such car; built for the popular production-based Koni Challenge, it’s an upgraded version of the already potent 911 that’s available for a fraction of the cost of the original build: