This workshop service repair manual covers Mercury Mercruiser 496 Mag HO, 8.1S HO, and 8.1S Horizon HO models, applicable to serial numbers 0M000000 and up. These V8 engines, featuring 8.1-liter displacement and Electronic Fuel Injection (EFI), are engineered for high performance and reliability in marine applications. The manual details support for Velvet Drive 5000 Series and Hurth transmissions, along with MCM (Sterndrive) and MIE (Inboard) drive configurations, ensuring comprehensive coverage for these robust powerplants. This comprehensive English-language manual provides detailed, illustrated, step-by-step instructions for maintenance, troubleshooting, and repair. It covers essential systems including starting, ignition, charging, fuel delivery, exhaust manifolds and elbows, and power steering pumps. Owners, mechanics, and technicians will find invaluable information to ensure the optimal performance and longevity of their Mercury Mercruiser engines through clear guidance on scheduled service and engine repair procedures.