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The V-8, 5.0L Mercruiser will offer better lower end torque (out of the hole performance) and a small increase in total horsepower over the V-6, 4.3L Volvo Penta.
Mercruiser Pros...
1. Two year factory warranty on the engine/drive package.
2. Far more Mercruiser dealers available for service/warranty work compared to Volvo Penta dealers.
3. Far more Mercruiser certified technicians compared to Volvo Penta certified technicians.
4. Parts more readily availble on Mercruiser products.
5. Warranty support is far superior compared to Volvo Penta, customer relations warranty work (items covered out of the initial warranty period) is possible with Mercruiser in some cases and is virtually non-existant with Volvo Penta.
Volvo Penta Pros...
2. Very reliable drive unit, utilizes a cone clutch. Mercruiser Alpha drives utilize a clutch dog, the Bravo drives utilize a cone clutch similiar to the Volvo Penta drives.
3. Sea water pump impeller replacement is less expensive than Mercruiser but is recommended that it is replaced annually (Mercruiser's sea water pump impellers will typically last two years).
There really isn't too many cons with either choice, both are fine engine/drive packages, if the price difference is negligible, I would lean toward the Mercruiser engine/drive just for the increase in engine size/horsepower and the far superior warranty support.
I work for a dealership that services both Mercruiser and Volvo Penta engine/drives, based on my personal experience with both companies, the warranty support alone would make me choose the Mercruiser engine/drive package over the Volvo Penta engine/drive package.
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Service Manager, Master technician
Service Manager, Master Mercruiser, Master OMC Cobra and Certified Volvo Penta Technician