dave54000

I have a 1985 sf repowering from my old 260hp alpha 1 to bravo 3. My question is what should I use for the best fuel economy at cruising speeds a used 5.7mpi or used fuel injected 454. I use it for fishing so fuel economy is main concern. Some people say that the bigblock will be just as efficient because of lower rpms needed. My cruising speed on old engine was 20 kts at around 3350 hoping to get atleast 25kts with repower. Longevity is also ? some say that 454 will last longer because it wont have to work as hard in the heavy boat. Thank u

mopower

First off , each gallon of gasoline contains a given number of BTUs. This produces horse power. Now , your boat requires a give number of horses to move it at any one speed. So you can run a 305 or run a 350 slower with more prop or run a 454 much easier with even more prop and gas consupmption should be little to no difference. Now the difference takes place when you exceed the speed of the smaller engines.
Running both a 350 and 454 at about 3350 of course the 454 will use more fuel BUT you will be going a little faster and it will be running for a shorter time so the over all difference ,agian, should be little to none.
Real live experience , I've repowered my last three boats. Running the same speed(less rpms due to a larger prop) or running the same rpms and faster speed but run a shorter period of time to go the same distance , the fuel was almost indentical. i calculated this going to my favorite lunch spot wich is 52 miles each way , so this was pretty much an identical test each time.
Longevity??? The 454 will be running he easiest and probably give you the logest life. Unless of course you get hooked on the speed and run it harder at wich point it would be no different than a small block.