To allow with the Map reading through from the Blacktop engine (see “load metering”) the inlet manifold provides a wider encounter for that aspect of the head with an additional vacuum feed off each cylinder that results in an integral chamber for a vacuum signal to feed the Map sensor. https://4age-20v-for-sale36789.ampblogs.com/a-review-of-4age-blacktop-for-sale-76549910