Let's say that a standard marine engine made 125hp@7,500rpm. Flat-2 engine.

Let's say we have an engine that also produced 125hp, but at 2,000rpm instead of 7,500rpm. This engine has 5.7l of v8 goodness.

If everything was set up right, would the 5.7 produce the same amount of thrust and propulsion as the 125hp@7,500 rpm engine?

If so, would that mean the 5.7 would get better fuel economy?

Technically, yeah. but car/bike engines in marine applications tend to fail.

You've really got a couple questions there, but anyway..

First of all, to compare them properly, you need to know more about the Flat 2. Displacement would be the most useful. Without the specifics enough to go into theory of any great accuracy (but this is still going to be most likely correct, otherwise it'd be pointless typing it. that was just a disclaimer) we'll assume the Flat 2 is 1.5L, so it displaces the same amount of air/fuel mix per minute.

Yes, they will both output the same amount of propulsion. That's a no brainer (125hp is 125hp), however the V8 is much more reliable (inherantly) and has the capacity to put out much more than the flat-2 is capable of.

Efficiency? Well, that depends on a number of things.

It could (again, that word just used as a disclaimer) also be a heck of a lot cheaper, in purchasing the engine itself, buying performance goodies, and buying spares because they're so commonly used that theres shit-tonnes of it all, and it's all been done before a million times.

There are reasons why inboard speed boats have used 350ci (usually built up heaps)Chevrolet V8s since day one and will still continue to do so...
..if outboards hadn't improved so dramatically as to make inboards a very niche market.


P.S. 125hp is a low output for an engine of such volume, by any standards.

P.S. 125hp is a low output for an engine of such volume, by any standards.

P.S. its only doing 2000rpm

This.

Important parts bolded.

P.S. its only doing 2000rpm

Fair call govner. I overlooked that momentarily (while translating that post script thought into text).

Fair call govner. I overlooked that momentarily (while translating that post script thought into text).

Thanks for the great post.

If you guys are wondering what I'm doing, check out my DIY thread