I asked a similar question and didn't really get an answer.
Now it's partially answered.
I have an engine comination I'd like to build or have built and wanted to know if the HP and Torque were good and what RPM to run it at.
Here's a link to it.
383 ci. SBC Engine

So a relatively flat torque curve is better? is that why big cubes is better so that you do not have to run a radical cam? Man 2 strokes are easier arn't they :-)

I think there are many other factors playing in here. First you have to define what you mean by what is better. Better means what? Longest lasting, fastest top speed, best acceleration???

Your turbodiesel is configured that way for more than just the ability to tow. I could come up with a number of smaller displacement, higher reving gas engines that, when properly geared, will out pull that diesel. The problem is that they will get 2 gallons to the mile and grenade at 30k miles.

Continuing to use the turbo diesel as an example, why don't we see these in to many smaller style offshore boats? Power to weight is one, but the biggie is that that volume of low end torque is not needed, hence their use in large offshore cruisers.

I'm ducking now!

I guess when I said better I meant for top speed.

Jon

Maybe you need to do some talking to JDNCA1 and Wettevette. They are on the fringe of high rpm, high torque-curve-in-a-boat recreational configurations. I think Wette's is a vee and JDNCA's is a tunnel. I have my suspicion that you do not need the gobs of low end torque if top speed is what you are after as long as you can make enough torque to get up the load curve. More than you need just means you get there faster.

Think of it this way. Your boat is always going up hill towing a fully (usually overloaded) trailer. Having a flat torque curve is best but also remember that curve has to be enough to get the boats butt out of the water and not spin so fast as not to be practical.

Read Tom Z's post on "engine out of tune". He has a relatively flat torque curve but the engine has to really spin to get to the HP.

I use the CompCam Extreme Marine cam in my 409's. I have a peak HP of 425 at 5000. Which is still in the effective RPM of my RPM Peformer intake, and 432 FT. LBS. of torque at 2000 and 485 max at 3500. I have a very flat curve. The boat has tons of power out of the hole, and rockets from 3000 to 5000. Torque is where its at. But in a boat you always need as much as you can get regardless of RPM, hence a flat curve.

You keep making that nice flat torque curve that falls off at 4000rpm while Wette Vette rockets past you on to 7000rpm! You would probably give him a run out of the hole up to 50mph, then he's gone, on to 100mph, with about 800hp out of a similar displacement engine.

Flip side is that while you're out enjoying endless days out on your boat and a good idle, he (and me) is (are) changing lifters every 50 hours and probably getting only 200-300 hours out of our motors, if we're lucky.

I'm not disputing that a flat torque curve is the best, but unless you are planning on going with an electric motor, you have to trade off something with an internal combustion engine. You choose to have a maximally flat curve up to ~4000rpm and give up peak hp. Wette chooses to give up the bottom end umph for major gains on the top end. I give up neither with a positive displacement blower! I have no less than 800 lbs torque from off idle to top rpm! Of course all of my **** is always broken, so who cares?

My first engine was configured much like yours, and it was the most all around fun package I have had to date, regardless of top speed (the wife constantly reminds me of this fact).

I'm just trying to make a distinction between various definitions of what is considered 'better' or 'Ideal'.

I know what you mean about the wife. Mine yells anytime I'm over 3500.

I wonder how long my Alphas would last at 7000. I think about 3 seconds. Then its alpha bits.

I all reality, its all in how you plan to use the boat. Mine is the family truckster with a kick when the kids and the wife are gone. It nice to have the idle and reliability...Knock on wood. Mine got built the way it is because the old engines were held together with bubble gum and bailing wire. I now get good performance for the cost of a normal 350 260 HP rebuild. I wonder how squirrley my Nova would get at 100. Anyone ever see a Nova with a canopy?

At least in the small block arena (where torque is not as monstrous in comparison to the big-cid big block crowd) I think a flat torque curve is where it's at. I'm probably going to either go back to the XM-262H that I used in the last motor (and have just yanked out of the new motor thinking that the XE-274H would be a better way to go) or go with Comp's recommendation for a custom grind. The 274 makes a lot of upper mid-range to hi rpm power, but I think it has a hard time overcoming getting the boat out of the water in an efficient manner. All the prop calculators and such say that I should spin a 21P prop at least. So far a 21P has been a dog where my old reliable 17P has shined (though it has put a MAJOR dampener on top end). I would love to be back to the numbers that jr posted with his 409s. 400+ ft pounds at little more than idle speeds with a small displacement engine (in comparison) is just awesome!

Later,

My theory is to keep the torque high and flat to about 6000 RPM and prop to run on the end of that flat curve. In order to get the RPM a relatively loose prop must be run which will still make the boat accelerate hard on the bottom end. I run a 29 Spinelli 6300 RPM through 1.5 gears for speeds in the high 90's. I feel I could probably gain a few MPH possibly if it was propped for max RPM at say 5800 RPM. I would also loose a bunch of acceleration especially from 3500 to 5500 RPM. I feel for a racing type engine this is the best way to go. For less wild pleasure boats peakier torque is just fine and good speed can still be obtained, with some acceleration sacrificed. Oval port heads, hydraulic cams, small carbs, dual plane intakes, and manifold type exhaust will work great on low RPM motors and will contribute to a peaky torque / HP curve. Rect heads, solid roller cams, big valves, dominator carbs, tunnel rams and single plane manifolds, and tube headers help flatten out the torque curve at higher RPM. Blower motors are another game, but these concepts still apply. I chose a NA motor because my 21 foot Baja would have to be called death ride if it were any fastrer than it already is.