Header Collector Diameter Calculator
Collector size from the primary tube it is fed by. Collector area normally lands just under the combined area of the primaries.
| Ideal collector inside diameter | 3.074 | in |
| Working range to 3.227 in | 2.920 | in |
| Suggested collector | 3-1/4" | |
| Collector length to 18 in | 12 | in |
| One primary area | 2.061 | sq in |
| Combined primary area | 8.245 | sq in |
- target collector area = combined primary area × 0.90
- collector inside diameter = √( 4 × target area ÷ π )
- Primary inside diameter assumes 0.065" wall tubing.
When you need it
When specifying a four-into-one collector to match a set of primaries.
Why the area comes out under the sum of the primaries
The collector is not a plenum; it is where four pulses take turns. Sizing its area slightly below the combined area of the primaries keeps velocity up through the junction, which is what makes the scavenging work. Oversizing it kills the effect the collector exists to create.
Length moves the torque peak
A longer collector shifts the peak down the RPM range, a shorter one moves it up, and twelve to eighteen inches covers most four-into-one applications. It is a cheap thing to experiment with if you have access to a dyno.
Merge collectors earn their money
A properly merged collector, where the four tubes come together at a gradual taper rather than being butted into a can, is one of the better value gains in an exhaust system. The difference against a plain slip-fit collector is measurable.