HP limit with 1-5/8 headers

FWIW,

The larger the tube diameter on headers the higher in the rpm range the torque peak occurs. The length of the tubes will rock the torque curve around the torque peak. Longer tubes enhance the power below the peak, shorter tubes above the peak. To a point longer collectors will enhance power below the peak and larger diameter will enhance the power above the peak.

Read an article in Popular Hot Rod about a month ago where they tested 8 different styles of headers on a stout 454 big block Chevy. The reported average power over the test rpm range which as I recall was 3k to 6500 rpm. Anyway, the average power for all the styles of headers was within 1 or 2 HP of each other. They tested tube diameters of 1 3/4", 1 7/8", 2" and 2 1/4" and step headers.

What this means is when you pick up power on the top end you loose it on the bottom end and visa vers but the average's essentially stay the same. So just like picking cams, carb sizing and what type of intake to use you have to know your operating range. I would venture to say if you won't be spinning the engine past 6000 rpm that the smaller tubes would be a better choice but if you plan on looking for max power above 6000 rpm the larger tubes would be the way to go.

Remember, Horsepower is just the torque the engine produces, times the rpm, times a constant.