Hi i own an opel astra f 1600cc 8 valve engine . About three years i tuned the engine 1; ported c/ head . 2; 4.2.1 exhaust. 3; set dhla 40 carbs. 5; kent cam nv063 cam kit . 6; f13 gearbox diff from 3.7 to 4.1. But the problem that i have is the distributer piching . I retard the distributer i feel it good but on the top end i dont feel the same power . Tyres are 17" 205 x 40. My standard engine was 1600 nz single point injection.