A collector with what feature increases velocity at lower engine speeds?

• A. Larger Diameter
• B. Shorter Length
• C. Thermal Barrier
• D. Smaller Diameter

Answer: (D)

Increasing exhaust gas velocity at low engine speeds comes from narrowing the flow path. The basic idea is flow rate equals velocity times cross-sectional area. If you keep the same flow but reduce the cross-sectional area, the velocity goes up. A collector with a smaller diameter boosts the gas speed, which helps scavenge the cylinder and reduces backpressure when the engine is turning slowly. That improved scavenging at low rpm is what makes a smaller-diameter collector the best choice for increasing velocity at lower engine speeds.

A larger diameter would slow the gas and reduce velocity, hurting low-speed scavenging. Shorter length mainly affects the timing and interaction of exhaust pressure waves rather than steadily increasing velocity, so its impact on low-speed velocity is less direct. A thermal barrier doesn’t influence gas speed or exhaust flow at all.