For Post-1986 Mercruiser GM V8 -4 inch elbow -3 inch spacer In the V8 GM Small Block, GLM's fully tuned design guides exhaust gases directly to the riser without turbulance and back pressure. This increases the engine's performance up to 10%. These GLM manifolds also feature an additional 40% increase in water cooling area that decreases the manifold's overall body temperature significantly. In GLM's Mercruiser replacement 4 inch riser, there are two additional water jackets. This allows a higher volume of water to flow through the riser, keeping the engine cooler. *All manifolds and risers are powdercoated black for maximum corrosion protection* **Cast Iron manifolds and risers come with a 3 year warranty**