Straight flow is the most ideal for turbo cars but it's noisy. S-flow is better than stock in terms of performance but less superior compared to straight flow. So I went for straight. In fact many Evo/Subbie/Skyline owners go for straight flow rather than S... if you google pictures on the web. In US, some of them hated it and called it 'fart can'. Who cares when turbo whines and smoke the hell outta ya, right? :)