why do cracked manifolds produce KR??
and how

I don't have all the answers, but know of one reason how cracked manifolds can produce KR.

As you know, KR stands for knock retard, which is your PCM retarding the timing when it "hears" knocking/pinging that indicates spark knock. Sometimes a knock sensor will react to sounds other than those produced by detonation. A noisy mechanical fuel pump, a bad water pump or alternator bearing, or a loose rod bearing can all produce vibrations that can trick a knock sensor into retarding timing.

You may know that cracked manifolds will make pinging sounds that produce sound waves that the knock sensor "hears" as spark knock.

Cracked exhaust manifolds don't flow efficiently. Therefore the gasses don't escape the cylinders as efficiently.

Same thing applies due to inefficient (cracked) exhaust manifolds. Combustion temps rise as the velocity of the escaping gas slows down due to leaks.

This is a problem for totally stock cars as well. This is also not a performance and brainstorming type question. This is typical and normal preventive or reactive maintenance.

Moving to General Chat.