Headers only improve flow at higher RPM than most stovebolt engines ever see, so they're mostly for the "cool" factor, like dual carbs. Even then, they would need to be tuned to produce better flow at a specific RPM range, short tubes for high speed HP, longer ones to improve scavenging at low speed to boost torque. The only gains I've been able to measure on a dyno run involved header tubes of equal length for every cylinder, and only at a pre-selected RPM. The 700+ HP Dodge 440 in the pull truck we ran needed header tubes from each cylinder nearly 8 feet long to get the low end torque we were looking for.
Jerry
