Ol' Ron
Well-known member
There are many ignition systems that have no vacuum component in their advance curve, and the engines run just fine. However, for a street engine that spends 90% of it time in cruise, the vacuum advance is very critical when it comes to power. It's part of the tuning process to retard the spark under power when a richer mixture is introduced into the engine. In cruise the lean mixture need more advance, thus the vacuum advance. All modern ignition systems from the early 30's have it. Most racing ignition systems don't need it.