The primary tubes extend from the Header Flange to The Collector. Each primary acts as the exhaust chamber for the cylinder it's attached to.
How are they measured?
The 2 dimensions that matter in header primary tubes are:
- Primary Diameter, and
- Primary Length.
The length of the primaries is determined by the Header Style you choose. However, you will often have a choice when it comes to the primary diameter.
How does it affect performance?
Primary size is the most important part of selecting headers for your engine.
- Longer primaries reduce the possibility of Exhaust Reversion.
- Equal length primaries time the pulses of exhaust into The Collector to maintain even flow.
- The diameter of the tube directly affects the speed of the exhaust gases traveling through it.
Exhaust travels faster through a small tube than a large one. When you change the size of the tube, you change the flow velocity of the exhaust. Changing the flow velocity changes The Scavenging Effect in the cylinder. The engine will react to different size primary tubes by changing the torque curve.
Primary Length affects the shape of the Torque curve.
- Longer primary tubes produce more torque before the peak. Torque will then fall off after the peak.
- Shorter primary tubes produce more torque after the peak. However, this means less torque before the peak.
**This is why full-length headers provide more low-end power than a set of shorty headers.**
Primary Diameter affects the torque peak, in relation to rpm.
- Larger diameter primaries will shift the torque peak higher in the rpm range.
- Smaller diameter primaries will shift the torque peak lower in the rpm range.
If your primary tubes are too small, they restrict airflow and rob the engine of power. If your primary tubes are too big, they will shift the torque peak too high in the rpm range to take full advantage of.
- Follow this link to see Recommended Primary Tube Diameters.