These manifolds were designed for high horsepower 350 cid to 427 cid engines that are making 350 HP Plus. Manifolds have a 1 1/2" I.D. runner with a tall D port entry. The outlet is 3" with a stainless steel air straightened plate to direct exhaust into riser pipe for maximum scavenging. Manifolds will work with angled spark plugged heads or straight spark plugged heads. Also you can use tall valve covers for various rocker arm assemblies and stud girdle. Black manifolds are hard anodized prior to powder coat, to ensure against corrosion.Weight savings of 65 lbs over cast iron and allows for twin installation down to 30" centers.Small Block std. Stainless steel “SA” riser pipes are designed for Mercruiser Silent Choice, Corsa Quick and Quiet (with modifications), and IMCO’s Sound Advantage systems. They can also be used with our Aluminum “S” pipes part number 02-1225 for under swim step applications.