In racing circles, they say the bullshit stops when the green flag drops. That is also true with the aftermarket tuners. After all, when you contemplate spending a small fortune modifying your engine, suspension, wheels/tires and brakes, you want to know that the finished car really will perform like it says on the box.

So every year for the last 10 years, German performance car magazine Sport Auto has sponsored an event for the makers of modified cars at Hockenheim Circuit around the end of May. In this Tuner Grand Prix, the best timed laps result in prizes for the top finishers in each of six categories.

The long straights of the 4.22-mile Hockenheimring Grand Prix Circuit are ideal for top-speed testing. But if your primary concern is assessing handling or just getting the feel of a car, then the 1.63-mile Club Circuit is the benchmark. Sport Auto is one of the two German magazines which incorporate lap times at Hockenheim's Club Circuit and the nordschliefe of the Nrburgring into its full road-test data. From that, we have control times for various high-performance cars, and we have an idea of how fast they are in relation to one another and the opposition.

In deference to the F1 drivers who complained about the gravel traps, which F1 cars tend to fly over, gravel was replaced by more concrete run-off areas on a couple of bends, and one corner was modified before the start of this year's season. However, Sport Auto's testers reckon lap times are unaffected, so the existing benchmarks stand.

Just to give you an idea of a quick time, a standard 321-bhp E36 M3 or M Coupe will lap Hockenheim's Club Circuit in roughly 1 min. 18 sec., compared to around 1 min. 13 sec. for a 380-bhp Porsche GT3. To put things in context, a proper race car such as 1999's 410-bhp Porsche GT3 R Le Mans contestant on slicks will lap the Club Circuit in around 1 min. 8 sec. The 2002 Supercup cars are reputed to be 3 sec. a lap quicker!

To keep things fair, cars are divided into classes. Class 1 is for small cars like the Renault Clio and VW Polo; Class 2 is for Compact cars such as the BMW Compact, VW Golf and Audi A3/S3; Class 3 is Coupe/Cabrio, which means the BMW M Coupe, Mercedes-Benz SL and SLK, Porsche Boxster and 911; Class 4 covers Limousines (and Estates) like the BMW M3, Audi A4, Audi RS4, Mercedes-Benz C- and E-Class, Mitsubishi Evo and Subaru WRX. Class 5 is known as Funcars and includes a Donkervoort, Lotus Elise and even an Opel Zafira Turbo MPV! The really powerful cars like the Porsche 911s and Ferraris run in Class 6, the GT category.

To make sure they get the best results, almost all the teams hire professional race drivers to pilot their cars. This year, big names such as Jochen Mass, Roland Asch, Kurt Thiim, Claudia Hurtgen and Wolfgang Kaufmann figured in the lineup. Given that professional race drivers should be within a second a lap of each other on the same track in the same car, the raw data also gives us a fair baseline to compare how much quicker a modified car really is over the standard issue.

However, at the end of the day, you have to put circuit lap times in context, and a car that is quick on a racetrack will likely not be super quick on a bumpy public road. In fact, on a wet bumpy public road, it might well be slower than the standard article.

It has only rained once in the Tuner GP's history, and that was in 2001. This year rain was not an issue, and it was a blazing hot 30C during Friday's practice session, and it managed to stay dry for the GP proper the next day, although light showers had been forecast.

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