Thats because you have a T2 turbo and big cams and/or are running too much boost.
S4's are good turbo cams. You start getting into the C series that have more overlap you need to have a good flowing turbo setup aka T3 or bigger.
Same reason why sr16 cams are not that good of an idea on a t2 powered car. Yes you will see gains in places and topend but you increase heat left in the combustion chamber during high backpressure situations at higher boost levels.
Only time I would recommend using a large overlap cam on a t2 setup would be if your running E85 or water/meth injection that will cool the cylinders and air temp down. Other than that. Big cams on a small turbo setup is asking for trouble.
S4's are good turbo cams. You start getting into the C series that have more overlap you need to have a good flowing turbo setup aka T3 or bigger.
Same reason why sr16 cams are not that good of an idea on a t2 powered car. Yes you will see gains in places and topend but you increase heat left in the combustion chamber during high backpressure situations at higher boost levels.
Only time I would recommend using a large overlap cam on a t2 setup would be if your running E85 or water/meth injection that will cool the cylinders and air temp down. Other than that. Big cams on a small turbo setup is asking for trouble.