The GT3, named after the FIA GT class for which it was intended, has a 3.6 litre naturally-aspirated six cylinder engine, based on the unit used in the Porsche 962 and Porsche 911 GT1 race cars.
A number of variations, designed for both road and track, have been added to the range since its launch in 1999. The current range (997) includes two road and three racing models. In addition to this, Porsche is currently developing a hybrid version that uses two electric motors and a Kinetic Energy Recovery Systems, was initially developed for Formula One.
The GT3 has had a successful racing career in the one make national Porsche Carrera Cup series, the international Porsche Supercup, and also winning numerous championship and endurance races, including the GT class of the American Le Mans Series seven times, the 24 Hours of Daytona outright and the 24 Hours Nürburgring five times.
The engine of the GT3 sets it apart from most of the other 996 models although it shares the same basic 3.6 liter displacement of the standard 996 type so-called "integrated dry-sump" flat-six engine. Along with those of the GT2 and Turbo, it is actually based on the original air-cooled 911's versatile, true dry-sump crankcase, with an external oil tank. The original version of the GT3 had 360 PS (265 kW; 355 hp), compared to the 300 PS (221 kW; 296 hp) of the regular 996.
In GT3 configuration, this so called "split" crankcase (meaning the parting line of crankcase is on the crankshaft centerline) uses, instead of a fan and finned cylinders, separate water jackets added onto each side of the crankcase to cool banks of three cylinders with water pumped though a radiator. Thus, the GT3 engine is very similar to the completely water cooled 962 racing car's engine, which is also based on this same crankcase.
The 962 differs, however, by using 6 individual cylinder heads while the GT1/GT3, like the air and water cooled Porsche 959, uses 2 cylinder heads, each covering a bank of 3 cylinders. The GT3 engine could thus also be thought of as similar to a 959 engine, but with water-cooled cylinders.
Up to early model year 2004 GT3 production, the basic casting used for the crankcase of the GT3 was exactly the same as the air-cooled engine and one could see the "964" casting number on the bottom of the crankcase and areas normally machined in the air-cooled application that are not machined for use in the water-cooled application. The crankcase casting was changed in mid-2004 to a "996" casting number crankcase to eliminate these external air-cooled remnants, but internally it is the same.
This engine gives the GT3 a distinct racing heritage that dates back to the Porsche 904/6 of the mid-60's, up to the Carrera Cup and 997 Super Cup and RSR racing cars of today.
Because the 911 air-cooled crankcase uses the Porsche 356 engine to transmission mounting flange configuration, the GT3 uses a manual gearbox also of air-cooled 911 heritage. This gearbox has interchangeable gear ratios and is more durable making it more suitable for racing than the standard 911 type 996 gearbox.
At 450 hp (336 kW), the 3.8 litre flat-six engine in the 997 GT3 RS is the most powerful six cylinder naturally aspirated engine in any production car with a 118 hp (88 kW) per liter output.
source: http://en.wikipedia.org/wiki/Porsche_911_GT3