How Does an Engine's Number of Cylinders and Their Configuration Influence Header Efficiency?

See all 1 photos

An important header-system design consideration is an engine's number of cylinders and their geometric layout—inline or V-style. To achieve optimum exhaust scavenging on a four-stroke engine, each cylinder should fire every 180 degrees of engine rotation. That amount of firing-pulse separation creates enough system vacuum to help pull the exhaust from the next cylinder. This is so efficient that collector outlet diameter needs to only be about 20-percent larger than the primary-tube diameter (about two tube sizes, in terms of cross-sectional area).

A 180-degree firing sequence is standard on inline four-cylinders. Not so on 90-degree V8s, where successive cylinders fire only 90 degrees apart. Engineers try to alternate firing pulses sequentially from one bank to the other, but regardless of the firing-order scheme, two cylinders on each bank must fire in succession.

The increased gas volume in the collector caused by two successive pulses only 90 degrees apart reduces the scavenging effect and may even promote reversion back to the adjacent cylinder in the firing order. It also mandates a larger collector than in an equivalent four-cylinder for optimizing the power peak, often resulting in a loss of low-speed and midrange performance. There are workarounds for this problem on high-level engines, and we'll get into them next time.

Contacts

Hooker Headers (A Holley Performance Brand); Bowling Green, KY; 866.464.6553; Holley.com/brands/hooker/