Repowering with a Big Block V8
TOPIC: Repowering Your Boat That's Currently Equipped With a Mercruiser 470 Marine Engine.
We are regularly faced with the question as to what the option is for replacing your Mercruiser 470 marine engine. There really aren't any easy answers for this so the best option is probably to outline what the issues are and go from there.
History
Mercruiser's 470 (3.7L) engine was really one of the only, if not the only true inboard marine engine. Mercruiser used their world class die casting capability to produce a die cast block coupled to a Ford 460 cylinder head as the basis for the engine. Mercruiser was trying to design in several specific marine design features into the engine, namely a huge amount of low end torque for the relatively small engine displacement. The inline configuration of the engine made the cooling system and draining requirments much easier. The cast iron block in conjunction with the aluminum block required the Closed Cooling system. Over the years Mercruiser changed exhaust manifolds due to problems resulting from trapped air within the cooling system, and therefore now only offer the Aluminum manifold. They do however have the complete retro exhaust manifold kit available if you need to replace your existing cast iron manifold. So here are the problems faced when repowering a boat currently equipped with the 470 engine.
- The 470 engine made a considerable amount of torque, comparible to the 4.3L V6 or 5.0L V8 engine. When repowering you need to go with a V configured engine to get the same amount of torque. This causes issues with the boat stringers, etc when fitting in a wider V engine.
- The gear ratio used with the 470 may need to be changed when changing out the engine since the gear ratio and engine torque/Hp go hand in hand.
- If the boat was produced prior to 1982 the transom cut out will not be currrent and therefore a transom adapter plate will need to be used in order to match up to the current transom cut out (if the gimbal assembly is switched out).
Installation Drawings for reviewing specific dimensional differences (open is separate windows)
470 (1976 Model) Installation Drawing
470 (1978 Model) Installation Drawing
3.0L (Inline 4 Cylinder) Installation Drawing
4.3L (V6) Installation Drawing
5.7L (V8) Installation Drawing
Products in the Store
Exhaust
The bottom line is that replacing your 470 engine is not an easy job and will require a the project in detail. We suggest replacing the longblock rather than completely changing engines.