I tuned engines for racing on cars with carburetors back in the 80's. We would use visual observation of the spark plugs to look for signs of detonation, fuel mixture optimization and spark timing. It was part of the fun with racing. The ECU handles that now. Sometimes we would blend toluene or methanol into our fuel when we could get away with it. Combustion chamber design could help reduce detonation and empower you to tune in more power. It was fun.