why at the lemans 24 hour race they use porsche 911 gt3 instead of 911 turbo

My guess would be that turbos require more maintenence than naturally aspirated cars, therefore the GT3 would require fewer stops and would encounter fewer problems than a 911 turbo.

Because the 911 GT3 gets it is name from the GT3 Endurance race category, and designed to comply with the rules for that category.
http://en.wikipedia.org/wiki/FIA_GT3_European_Championship
http://fiagt3.com

mybe it has more horsepower

First, the car that is racing at Le Mans in the GT2 class is not a street car, but a dedicated race car that is ''based'' on the Carrera GT3. The GT3 is made specifically as a model for the race car to be based on. All GT class cars in sports racing r based on a production model where the model in question must have a certain number of street models produced in order to be able to run in that class.

As such, the GT3 is aimed at optimizing a car for the set of rules. the Turbo is not. First, the Turbo is AWD, which is not allowed under the current regulations. Second, in order to run a Turbo, they would have to create a new engine with a much smaller displacement. or they would have to enter a turbo based model in the GT1 class, where it would not be competitive, falling a well short of the hp produced by the larger engined cars in that class.

If u wanted to compare just the street model GT3 against the Turbo, you'd find they r both very good performers, but the Turbo is a much heavier car that is more compromised for comfort than the GT3. While both would be fast, a good driver should be able to turn faster times in the GT3 than the Turbo on a dry track (though, when a 997 based GT2 appears, that will move a turbo engine into the same basic platform as the GT3 & yield something of a monster).

And just to add, in regards to the new GT3 Class - looking back at the development of the 996 GT3. Porsche homogolated the GT2 class GT3RSR against the special street version GT3 RS. The car that was able to run in the GT3 class was the GT3 Cup car, which is essentially a race prepped version of the street car (320hp or so vs. the 480hp of the GT3RSR, & many many other major differences). The GT3 Cup car is designed as a cost effective form of racing for teams that find normal GT classes too expensive to be competitive in. The Turbo is not developed here because it falls even further away from the rules, & it is cost to operate would be much higher, making it poorly suited to such a class.

You may see an occassional Turbo entered in something like the Speed World Challenge GT. though they r generally not competitive while the GT3 Cup cars r (developing a car to race form can be very costly & time consuming).

1st: theyre natrually aspirated. the turbo is not. Turbocharged cars arent allowed.
2nd: theyre much lighter than the turbos.
3rd: the turbo is the premium 911. the GT3 is the track car. It only makes sense to use the GT3 platform for the GT3 RS and GT3 Cup.
4th:theyve been developing and refining the car for years and years. using a new model would be like starting from scratch. Why fix what already wins every race?

l believe the ACO regulations do not allow turbocharging in the GT2 class or if it does it would require a smaller intake restrictor or displacement. Porsche most likely decided they could get more power from the larger intake than from a turbo and a smaller intake.

im not a rich boy but u should put some turbo in that porche

l was going to give an answer here, but Paul S has absolutely nailed it. The GT3 and GT2 cars are built for racing, l do not think Porsche has seriously campaigned a turbo since the 935.