[Motor Daddy]

Let's say the peak pressure should occur at 14 ATDC for optimum performance. A chamber that burns slower and more inefficiently requires more spark lead to keep the peak pressure at 14 ATDC. Inefficient chambers have a slower burn rate, so you need to start the burn earlier, which means more spark lead is required. The problem with more spark lead is that it starts the burn earlier BTDC. That means there is more TIME for detonation to occur after the spark plug has ignited the mixture BTDC, which increases the risk of detonation, which occurs after the plug has fired and the mixture has been ignited.

In other words, an inefficient chamber with its required increased spark lead is more at risk to detonation, because the burn occurs over a longer time, which increases the risk of detonation. The optimum situation is to have a fast burn chamber, that requires less spark lead, so that the spark lead requirements are reduced to keep the peak pressure at 14 ATDC. Reduced spark lead with a faster burn chamber means a smaller amount of time of burn, which is more optimum for performance due to less negative work done BTDC after the mixture is ignited and pressure starts to build. It also means less risk for detonation while keeping the peak pressure at 14 ATDC.

Lean mixtures take longer to burn than richer mixtures. Lean mixtures require more spark lead to keep the peak pressure at 14 ATDC. Richer mixtures burn faster, require less spark lead to maintain the 14 ATDC peak pressure.

Higher cylinder pressures due to increased DCR's or increased VE% also required less spark lead, as higher cylinder pressure makes for a quicker burn.

Also, let's not forget, an intake port that flows a greater CFM will flow the same air at a lower boost than an intake port that flows less at a higher boost.

You may actually be flowing more air/fuel mixture into the cylinder (higher VE%) at a lower boost with the KRE's than you did with the iron heads at a higher boost PSI.

Just food for thought.