Horsepower & Torque as a function of tube diameter



It's no secret that a good set of headers is one of your best performance investments. In fact, most engine builders say headers are one of the first things you should change when you start modifying your motor. But before you plunk down your cash, remember that a couple of factors determine what's going to work best for your particular application.

First thing to consider is the intended use of your car. Are you building a daily driver, a sometimes bracket racer, or a dedicated quarter miler? The RPM range where you want the most torque, along with the displacement of the engine plus any other serious modifications (cam, blower, heads, etc.) are important factors. Stock manifolds are mass produced and are designed to clear all the accessories offered for any vehicle the block may go into. So naturally, performance is compromised by manufacturing requirements. Tubular headers are built for power and their design can be very specific. Let's look at how headers work, and clear up a couple of common misconceptions. First is the size of the primary tubes. it's easy to assume the bigger the tube the better, but that's not the case. Fact is, primaries that are too large actually cost you torque and horsepower by slowing down the rate at which the exhaust travels through the system.
Think of your engine as an air pump. Every time the exhaust leaves the combustion chamber, it's being forced into the primary tube for that cylinder.

Smaller diameter pipes flow less volume than large ones, but the exhaust in the smaller pipe flows faster. Until you reach the RPM where the sheer volume of exhaust gases require bigger primaries, smaller tubes scavenge far more efficiently. If you're using the engine in the 1,500-3,500 RPM range, which is typical for a street driven car, you definitely want 1- 1/2" to 1-5/8" primary tubes for a small block and 1-3/4" to 1-7/8" for a big block. Any bigger and you'll lose plenty of low end torque. Beyond 3,500 RPM it's a question of where you want the power peaks. As you can see from the chart above, small tubes don't lose their edge in horsepower or torque until you get above 5,500 RPM.

Even if you're running a radical cam and blower, you're better off sizing your headers smaller rather than larger, unless you plan to do most of your driving at full throttle. The headers we build are sized correctly for even the most heavily modified street motors.

John,
Trying to remember, was TTS the builder that had 1-5/8" primary tubes for the Vette?

I thought I also read of install problems?