7.4L mercruiser Enough Power - enough speed? 7.4L mercruiser Enough Power - enough speed?
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    7.4L mercruiser Enough Power - enough speed?
from Jeremy (  
8/24/1999 6:37:00 PM


 I bought a 1984 Mako 285 hull,without motors.27' LOA, 10'beam. It needs the transom replaced and I am thinking of putting a single big block Mercruiser engine in - 310 hp, with Bravo 3 drive, instead of the twin 200 - 250 hp outboards that used to be on it. My concern is how can I get some idea of what kind of speed I will get out of it before I have the work done. I am not worried about it being super fast, but I want to be able to cruise about 30 knots. It is a fairly heavy hull - but only an aluminum top above the deck. Any ideas?

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   Doubt it will cruise at 30 from Bayoudog (  8/24/1999 10:05:00 PM
 Jeremy, that boat is big, heavy, and beamy. The 454 is rated at 310 HP, but that's at wide open turning 4500 rpm. There is a difference in the way inboards and outboards ratings to be looked at. Outboards can be run at 90% of max most if not all the time without any problem. Inboards, like the 454 are GM automotive adaptations with valve trains that are not designed to run at more than about 3500 continuously without major premature wear. At that rpm, they are only putting out about 200 hp or less (based on the theory that output is proportional to the cube of RPM - some might dispute that and argue that it is based on the square of RPM). Regardless of which is right, that motor will not push it to 30 knots cruise unless if you want to run it wide open or close to it, which is not advisable. However, this is not to say that you should not go with the 454 - I think it is a good idea - only that your expectation should be downgraded a bit. If you got 22 knots at 3500 RPM, I would say for that size boat/motor combo it is not a bad deal. Also, you will get respectable mileage out of it - perhaps 2 mpg. Tight lines
   Bayoudog - what about the Mag 501? from Jeremy (  8/24/1999 11:22:00 PM
 Bayoudog - Thanks for the thoughtful response, which leads to the next question. The 454 engine comes in a Mag 501 (I think that is what they call it) which puts out 385 hp. How does it do that? I noted that it is designed with a top rpm of 4600 - 5000, is it just designed to be run at higher rpm, or is it also optimized with bigger intakes, etc? Would running the Mag version at it's higher range be less stressful on the engine, or does the same rule apply . I am wondering where the extra hp comes from, if not the higher top end.
   Big Block V-8 from Doug R. (  8/25/1999 12:05:00 AM
 Jeremy...To set things right, the 7.4L MPI sterndrive puts out 310 hp. The 454 Mag MPI puts out 485. The 502 Mag MPI puts out 410 hp. The 7.4's operating range is 4200 - 4600, the 454 & 502 operating range is 4600 - 5000. We have a 25' X 9' aluminum boat with a single 7.4L MPI Bravo II package. It hits the low 40's. With your boat this will probably be a little small for you. Try the 454 Mag MPI. The Bravo II drive is bullet proof and works well if you keep it below 45 mph. Another option is to ask your mechanic to see if they think you have room for twin small blocks. Once you have tried one of the new MPI engines you will be impressed. Doug R.
   Horsepower from Bayoudog (  8/25/1999 9:00:00 AM
 Engine manufacturers are in competition, and the most effective claim to superiority is horsepower rating. People are always impressed by horsepower figures, even though they are mostly meaningless.

When a manufacturer says "Operating range 4600-5000 RPM" take that to mean that "the propeller size is considered correct if the engine will turn no less than 4600 and no more than 5000 RPM @ WOT". That is all it means. It does not mean "engine is designed to run at" or "it is OK to run it continuously at that RPM", even though some people do that and wonder why their new engine did not last six months. Remember that those engines are all based on the "old fashioned" overhead valve & pushrods with an in-block cam, a design dating back to the early fifties.

Higher horsepower means more fuel and air being pumped into the motor. It is an accepted principle that engine longevity is related to the amount of fuel consumed over the lifespan of the engine. For instance, all other things being equal, a 454 will be considred dead upon consuming, say, 20,000 gallons of fuel. This can be in 2000 hours at the rate of 10GPH (producing 150 HP), or in 200 hours at the rate of 100 GPH (producing 500 HP) [figures arbitrary].

So, only you can decide which is the best power configuration for your boat, depending on your budget. If you can afford it, get the 385 version and run it at 4500 RPM and cruise at 35MPH. If you only use it 10 times a year for about 3-4 hours each time (as many people do), it will last a few years. If I was doing it, however (I run my boats about five times that) I would probably get the 310 version and run it at 3500 RPM for a 20-25 cruise speed, with the occasional short burst to 4500 RPM and a 30-35 top end to impress my friends. It's a hard decision, I know. Tight lines/Bayoudog



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