Aren't 1-1/2" primaries a little small?
Here a Tube Diameter Formula from the internets:
Tube Diameter
To decide on the right primary tube diameter, there are numerous formulas used in the design of race headers, but for a street car, Don Lindfors of PerTronix (makers of Doug’s, Patriot, and JBA headers) says the following is a great rule of thumb: 200-325 hp look for 1.50-inch diameter primaries, 275-425 hp use 1.625 inches, 400-500 hp use 1.75 to 1.875 inches, and 500 hp and up look for 2 inches or bigger. If you’d like to get more specific, keeping in mind that smaller diameters set a lower limit on peak torque, a 300hp motor could use 1.5- or 1.75-inch primary depending on whether you want the torque at a lower rpm, or to allow it to keep moving higher at a higher rpm. Just a .25-inch increase in primary diameter will push the torque peak up by about 700-800 rpm.
We know many hot rodders like hard numbers, so if you’d like to get as specific as possible and try to size your primary tubes to work effectively within a specific rpm range, try these formulas out:
Primary Pipe Area (PPA) = (peak torque RPM ÷ 88,200) × 1 cylinder ci
To find the PPA, we use A=πr²
For a few common header sizes, this is:
1.5"=1.55
1⅝ = 2.07
1¾ = 2.19
1⅞ = 2.53
For example, for a 350ci engine with a peak torque goal of 5,000 rpm:
PPA = (5,000 ÷ 88,200) × 43.75 ci
PPA = 2.48
To see what the theoretical peak torque rpm of a certain set of headers is for your particular engine, just rearrange the formula:
Peak torque rpm = (PPA × 88,200) ÷ 1 cylinder ci
1⅝ (2.07) = 4,175
1¾ (2.19) = 4,415
1⅞ (2.53) = 5,100
So theoretically, our fairly high-winding 350 with a peak torque of 5,000 rpm should run a set of 1⅞-inch headers. But if your street-oriented 350 makes peak torque at a more typical 4,000 rpm, a set of 1⅝-inch headers would make more sense
