Ummm I disagree with that. Red caps , or spring will set rpm open point. For instance, 4k rpm to 5k rpm range. The vacum assist is just a more " leak restistant " method to pop valves vs exhaust pressure. Hold throttle at half pos and you can feel valves kick in and out like a power band. Shifting red tops can alter annoying on-off on engine surges during situations where towing tubers requires steady throttle response. Just my experience.