Q: i want to make a bmw m3 coupe even lighter without touching the interior.
A: A good way is to of course have a clean car. Dirt does weigh it down. Another thing is you can take your back seats out if they are hardly used. I have seen some people go as far when racing them taking the carpet out. Use light alloy rims, if you dont use the A/C take the compressor out, that alone weighs a good 30 to 40 lbs. You could do carbon fiber body parts. Carbon fiber is so light and if you can get body kits which I know you can for a price, it will help out a lot. Remember, for every 10 lbs you take out you gain 1hp. Have fun!