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 ”A” risers are designed for stock Mercruiser Bravo exhaust tip locations. 3/4"npt fitting is used on front of pipe for thermostat housing that have (4) 1" hoses water to go to exhaust manifolds if needed click on (ADD BUNG). Shift bracket, ECM, or EFI brackets available as bolt on or welded to the risers. To order see PowerFlow accessories.