Boat Motors vs The Stuff We Drive

[Drago]

Why is it that I can run my Yamaha F225 at near 6200 rpm all day and it just keeps on purring? If I did the same to my Honda Prelude or Ford 6.4 liter diesel, it would most likely let me down without much warning. What is it with outboard motors that they can stand this punishment? I've only been boating about 3 years and this ? has plagued me since the beginning. Does anyone except race car drivers and power boaters do this?

[chill'nthemost]

In the case of I/O's the engines are balanced to the max. I'm guessing it's the same for outboards. A balanced and blueprinted motor will have better performance throughout the rev range. Anyway, that's my guess.

[Bamaman]

All auto/truck engines are designed for specific use, and so are boat motors.

Even though a modern car based inboard outboard engine is running 4200-4600 rpm's, they have incredible torque load stresses on those engines. The same basic engine in an automobile might have camshafts that would let them redline @ 5500 rpm's, but those engines might not last long in marine use.

I once rode in a Winston Cup race car around Talladega @ 150 mph plus. Their engines are so smooth that 7000 rpm's felt like 4000 rpm's in a street car. RPM's are all relative to the job they have to perform.

When I used to fly single engine planes, my Piper rental plane had a 2300 rpm redline. Talk about a piece of junk. A blueprinted motor would make them much, much faster at higher RPM's. Light planes keep low rpm's as a reliability issue--and they also run at slow speeds but with high torque.

I had my Mercruiser motor apart, and I didn't find it balanced in any way. Mercury engine builders did custom fit each of the 5 bearings with different size bearings, as they had every size bearing there was in their parts bins. They didn't cut corners in the bottom end.

Mercury's race engine builders are absolutely tops in the engine world when it comes to assembling big cubic inch engines--like the four valve 9 liter (552 cu in) 1350 hp engine @ 6500 rpm's. It's hard to imagine that many rpm's and horsepower coming from an engine that large--short of being supercharged like a drag car. These monsters are all CAN/BUS engine, and their horsepower comes from wide open heads and high technology. Most go into offshore race boats as twins--2700 hp, and they'll run as long as there's fuel to feed'em. I'd just hate to think how much they cost.

A Yamaha 225 @ 6000 rpms is almost a toy motor by comparison to a Mercruiser 1350--in relative terms. There again, modern 4 stoke outboards are pretty incredible animals, and getting much lighter in weight, as we now see in the Yamaha SHO up to 250 hp.

[chill'nthemost]

I'm surprised to find out a Mercruiser isn't balanced. I've always thought all boat motors were balanced and set up to run in their torque curve. Never had one apart and hope to keep it that way

[GregF]

I am not sure about what they do now but a 135hp Mercruiser used to just be a garden variety straight 6 Chevy truck engine.