Hi there,the vacuam advanc is designed to increase the timing at cruise speed when the engine is at its highest vacuam and therefore no load and is safe to increase timing to increase efficancy ie fuel economy on usually factory set ups.They work well but they have there limits eg:if you were cruising and jumped hard on the gas on a modified engine it will want to rev and there may be a moment of detonation or pinging,so thats why some guys remove them.Hope this helps