Merc 4.5L 250HP

[Dave R]

Originally Posted by 8gv

Torque gets you going. Horsepower keeps you going.

When it comes to engines, torque is meaningless without knowing the RPM. When you combine torque and RPM, you can then calculate HP. Even with a tiny amount of HP, you can use a lever or gearing to multiply the torque. For example, if I use a 36" breaker bar, I can easily apply 500 ft-lbs of torque to a bolt. That's more torque than the 454 in my boat makes, but obviously I don't make more HP than my boat engine.

If I have an engine that makes 300 ft-lbs of torque at 4,000 RPM and I run it through a 2:1 reduction gear, it will make 600 ft-lbs of torque at 2,000 RPM (minus friction losses of course) at the output of the gear set. If I have an engine that makes 400 ft-lbs of torque at 3,000 RPM and run it through a 1.5:1 reduction gear ratio, it will make the same 600 ft-lbs of torque at 2,000 RPM as the smaller engine.

Originally Posted by 8gv

There's no replacement for displacement.

Using my example above, you can see that's not really the case. Displacement can be replaced by RPM. An 8 liter engine spinning at 3000 RPM moves the same amount of air as a 6 liter engine running at 4000 RPM (assuming the same volumetric efficiency). This is why GM no longer makes big block engines. They get all the power they need from small blocks that just spin faster and they simply adjust the gearing to get the required torque to the drive wheels.

This is taken to the extreme in sport motorcycle engines. My 1.8 liter car makes 130 HP at roughly 6,000 RPM. My .9 liter motorcycle makes 144 HP at roughly 12,000 RPM. You'd think the motorcycle engine wouldn't last long, but with 110,000 miles, it still runs like new.

Originally Posted by TiltonBB

The technology has come a long way for efficiency and power. If only that would translate to boats.

It actually has, Volvo Penta uses very modern engines with VVT and high WOT RPM these days and Evinrude has used direct injection for years. Marine diesels have used turbos and common rail injection for years too. Boats are always going to get crappy gas mileage though, they simply require a lot of power to move compared to cars due to drag.