[A Quiet Boom]

Larger ID or shorter length headers have a higher RPM HP peak and are generally more suited to racing while smaller ID or longer tubes are better for low to midrange power. That said it has ALWAYS been my experience when dealing with headers that a ID that is larger than the port ID will perform better due to the restrictions caused by bends and such. A best of both world would be large ID and long tubes which widens the torque and HP curves with a slight loss at peak HP. The area under the curve is what you want to increase not peak HP. For example lets say we have two motors with peak HP at 6000 rpms of 500. Motor "A" makes 200 HP at 5000 and 200 HP at 7000 while motor "B" makes 450 HP at 5000 and 450 HP at 7000 unless you had a variable speed transmission that kept the motor at peak HP motor "B" will always be faster. This is why drag racers running automatics use "high stall" torque converters effectively holding the motor near peak HP through each gear.

I'm running 1.5" headers on my 3.0 and they are definately on the mild side, with more cam and compression I'd opt for 1.625" or 1.75" pipes. My 600 HP smallblock drag car had the biggest pipes I could fit, 2" mandrel bent custom headers these gave me nearly 75 HP over the 1.75" headers I was previously running.