[olddog]

Let me further expand on my Huge ports require less duration theory. Out of curiosity I was looking over Diesel designs. We all talk about getting away with running more compression with long duration cams do to the dynamic compression being lower than the static compression. Well the Diesel engine cannot start if the dynamic compression is to low, as the lack of compression will not get the air hot enough to light the fuel. So Diesel engines cannot allow the intake valve to stay open much past BDC. Running turbo chargers at high pressures limits valve overlap. So today's typical Diesel has an intake duration of 175 deg @ 0.050. The aftermarket performance cams are not going over 180.

Now these are huge port low duration engines, but they are still able to rev to 4000 rpm and make good torque.

I have also noticed over the years that big block gasoline engines in dump trucks tended to go the same way. These engines run out of torque by mid rpm, but I'm not sure if it is the cam or if they are limited by the carb size. I know they purposely limit high rpm high power in favor of low rpm torque, but I also believe they want to limit power to keep the heat down. A truck could work hard long enough to melt it.

On a poor flowing head the cylinder is not full at DBC and leaving the valve open longer allows air to continue to fill the cylinder. At higher rpm this really pays off. However ports that flow well are getting the cylinder full and leaving the valve open too long is just pushing the air back out of the cylinder, as it was already full. So my theory is to leave the valve open no longer than necessary to fill the cylinder at the desired peak torque rpm. If that is around 5500 rpm, a good flowing head does not need a radical cam.