my nephew has got a corsa 1.2 8v (nearly beats the Z not lol he has got a sportex back box and a pvd air filter (pipercross) and he wants a bit more power but what he realy wants is it to pop when he backs off and i said it might do it with a larger injector but i ask u lot problem is its only singal point but he cant afford any thing faster cos he is only 17

Just create an air leak on the intake or perhaps exhaust manifold, that will usually do it !