Re: 383 volvo upgrade?
Um... I'm pretty sure that's not entirely correct
Example:
http://www.nsxprime.com/FAQ/Technical/images/97nsxpowercurve.gif
You're right that many auto engine torque/hp graphs are that way, but not all of 'em, and not all marine engines either.
FYI it's not some kind of universal engine law that they intersect there, it's due to the desired power range of those engines which are made to be coupled to auto transmissions driving auto wheels.
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Actually it is a universal engine law that states they intersect there.
Horsepower is a function of torque. It is defined as "HP = (Torque * RPM)/5252". Thus, from looking at this equation, you can clearly see that whenever RPM = 5252, HP = Torque.
The reason it is not apparent in that graph you posted above, is because the torque and horsepower scales are not the same. If you were to rescale the graph with both torque and hp on the same scale you would notice that the two number do, indeed, intersect at 5252.
Just because the numbers intersect at this number does not mean this is peak torque. It simply means that before 5252rpm, torque will be less than horsepower, and after 5252rpm, horsepower will be greater than torque.
Also, as you can see from the above formula, horsepower is simply torque with a time factor added in (rpm), thus horsepower is a much more useful number at determining engine output.
For example, lets say you have an engine that creates an even 100ft/lb of torque across its entire RPM range (lets say 0-5000). On this engine, horsepower (and thus the power output of the engine) would increase in a linear rate proportional to RPM.
Many people love to argue that "torque is the real number" etc. etc. as if the two are separate things. Horsepower is just torque multiplied by the number of times per minute it is applied. Torque is instantaneous force, and without knowing the rpm it is applied at, it is a completely useless number. 1000ft/lb of torque at 1rpm won't move your boat very far at all, and only represents .19hp. The same torque at 2000rpm, however, represents 380hp, a very healthy power level.
Horsepower = for figuring out how much power you have at any given rpm
Torque = instantaneous force. Generally you want to watch torque when optimizing an engine on a dyno.
now, on topic:
peejcj8, I am planning also on upgrading to 383s (money permitting) on my 32' bayliner conquest this winter. It is currently powered by 260hp volvos, like your boat.
The important thing is your target cruising rpm, and your current engine's power output at this rpm, vs the power output of whatever 383 setup you are contemplating.
dyno comparisons of various 383 setups will provide very valuable as far as optimizing your power and fuel economy, as you will want to pick a setup that emphasizes torque at the correct RPM for your boat. You want to pick a setup (cam and heads are paramount) that will give you a nice fat torque curve from 1500 through 4000rpm (what happens after that doesnt matter).
My advice is to search various hot rod forums or whatever else for dyno graphs and attached spec sheets, and then when you find something with a nice looking dyno graph, try and replicate the setup head and cams wise.
I wish i could give you more specific advice, and i really have no idea on what effect the extra hp will have on your mph/mpg, as i am fairly new to marine stuff myself. My usual expertise is in high output (5+ hp per cu.in) small turbocharged gas engines and drift racing cars.
