I mean its not a requirement but its recommended. it will make your car knock more I was on a stock pulley with headers, intake and a tune and my dad put in 87 by mistake and my car knocked compared to 93 I would have no knock what so ever. don't get me wrong it was a small amount of knock but its doing more harm to the engine nonetheless. its also not required to change your oil every 3 to 5k miles but the difference after 100k miles is night and day haha