Power, Torque, and fuel consumption

Also, ive always wanted to know about fuel consumption vs. horsepower, but how does torque play? Im pretty sure i know, but not quite sure. Its my understanding that a good efficient engine makes peak torque down low and more toque = better consumption of fuel.

Lets say we have 2 chevy 383s-my favorite.

Engine 1 is built for torque
310 hp 435 ft. lbs. torque

Ebgine 2 is a stock motor not really concentrated on torque, but more on power, like most in wake boats.
360 hp 375 ft. lbs. torque

Which will get better fuel economy, the one with more torque, or more hp?

Horsepower = Torque X RPM / 5252
So for any engine the torque and horsepower are equal at 5252rpm. That's just the formula.
Horsepower applied over time is related to BTU's or energy. So the more Horsepower you use for a period of time the more energy you use.
A boat requires some specific horsepower to travel at a speed with a load. The same horsepower regardless of the engine. Extra horsepower gives extra acceleration capability.

So the energy used is the same at any speed between engines and the fuel used is the same except for the efficiencies of the engines. If you use the extra horsepower you use extra fuel.

The trick is the horsepower available at the RPM you are traveling at. Your low power high torque engine makes more Horsepower at a low RPM than the high horsepower engine does with the specs you provided. That means it might get out of the hole faster than the hot motor and use more gas doing it. It can't give the same top speed (all assuming the same boat)

Usually the peak efficiency of an engine is at its peak torque RPM, or at an RPM that produces close to peak torque. So in your example probably the low power high torque engine gives marginally better fuel economy at 22 MPH. But not much.

thanks, now i understand. But wouldnt the tourqy engine give a good bit better fuel consumption and performance at wakesurfing speeds?

Yes, if they both had the same engine management. If the High output one was EFI and computer controlled and the torquey engine was carbed with points the computer engine would be better. You could also prop the High output one to a lower pitch to match the effective torque and still have more top speed potential.

(Message edited by rallyart on February 20, 2007)

what does it matter if its carbed or efi?

Just the ability to manage the engine efficiently. A better management system gives better fuel burn for more power and better economy. Ideally you want timed multi-port injection and multiple coils with engine feedback systems to keep it optimized in any weather or load.
Not a lot of those out there.

But still wakeboarding or surfing your never going past half throttle all that much so wouldnt it be logical to make the low-mid range power?(getting out of the hole and running at 22 mph)
Your saying that the high output motor coming out of the hole at somewhat high rpms would be the same as the torque motor coming out of the hole at relativly low rpms?
Are you also saying that even though the torque motor makes most power down low, it will burn just as much fuel as the high output motor will at a higher rpm?
I associate a wakeboat with a 1 ton pickup. The pickup has a torque motor to get the load moving(very much like a wakeboat moving a ton of ballast) If what your saying is true, why dont pickups have high output engines (lots of hp) and just gear the axle gears lower(which resemble the propeller pitch in this case). Some might think its a bad analogy but it makes sense to me.

Cost of the gears. It's easier to build a 3.5:1 gear than a 5:1 gear and it generates less heat.
It's cheaper to run at lower RPM because the parts have less load. An F1 engine runs almost 20,000 RPM but they cost much more than a 900 Hp. drag engine.
Your analogy is good. It's just cheaper to build a high torque engine at low RPM with the gears to match.
You could pull a semi load up a hill with a gas pick up at the same speed as a semi with the same Hp. but you are using gears and creating heat and running at much higher RPM. Horsepower is just the amount of work the engine can do.

But again at any given load any engine in the same boat will be making the same Hp. A higher output engine will be running slightly less throttle and at the same Hp.

just like if you had a lower hp engine you would be running more throttle, right

HP is how FAST the engine can do the work. Think of it this way, we can all walk up a flight of stairs (work). If you run up the stairs, you do the same work, but faster, more HP. If you and your tubby friend walk or run up in the same time, he is creating more HP because he is doing more work in the same time.

The simple answer to your question above is yes. This is due to the nature of a gas engine, unless you are at WOT, the engine is not at peak effiency. So you would throttle the engines different to produce the same HP.

Art is right, giving the same boat with two different motors at the same speed, weight, etc, they are producing the same HP. The one that is opperating with a more open throttle is more efficient (theoretically). However, there is way to many variables to make the blanket conclusion that one motor would be more get better mpg or gph than the other.

That is one reason why small 4-bangers get good mpg. To run 70 mph, their throttles are open and running at 70-80% of their peak effiency. Where as the same car with a v-6 running the same speed is running at less trottle, less efficiency poducing the same HP.

Bonus question, apart from the higher energy concentration from fuel, why is a diesel more efficient than a gas of the same size?

(Message edited by siuski on February 21, 2007)

bonus answer-because of the higher compression and the ability to have way more adjustment with fuel/air mix?

partially as there are several factors, but the main one is that there is no throttle (butterfly) as in a gas motor, so the intake pumping losses are less creating a more efficient engine under partial loads.