Agreed one for each side, run one side one pass then the next side. Lots of things can cause a need for jet stagger from side to side (intake/heads etc). I run mine at 9 and 3 O Clock on both sides. The car is street/strip so rather than mount it in the header I mounted it in the collector/reducer. I believe I read somewhere that the 02 sensor needs to reach a certain operating temp to work correctly, so put your bung hole as close to the collector as possible. Funny thread ...