Here's one for ya. I just replaced the stock piston for a wiseco 12.5 high compression piston. After firing it up for the first time, everything sounds good and responds well except for a loud knocking noise coming from the engine. I can't tell exactly where it comes from but it's pretty loud. At first I was worried that the piston was knocking against the valves but I'm positive that the timing is correct. My theorys are that it could be pre-firing due to the high compression ( I used premium gas 91) or possibly I didn't set the valves right. With a 12.5 compression, do I need to run race gas or octane boost? If I did set the timing wrong or slightly off I'm sure it would make a lot more noise than this. Just want to know what to check first before I start tearing it apart again.