The mysteries of Horsepower and other things explained
There seems to be a lot of confusion about the mysteries of engines by some on this site so I will post some links and add some text to try and clear some of this up.
First some terms:
Torque: A twisting force.
Horsepower: A unit of power equal to 550 foot-pounds per second (745.7 watts). The power of an engine measured in terms of this.
Duty cycle : The cycle of operation of a machine or other device that operates intermittently rather than continuously.
Since torque is simply a measurement of a twisting force one can't determine how much work is being done from just that term. I could say I am applying 1000 ft/lbs of force on a pipe wrench and if the joint doesn't break loose I’m not doing any work and that would be 0 HP. If the joint breaks loose and I can swing that pipe wrench 10 RPM's then math tells us I'm producing 1.9 HP.
Now; same situation: Someone tells me they can produce 1.9 HP. Since HP is a timed base formula I know they are doing work unlike if I just have a torque number. What I don't know is if they are making a lot of torque with very little RPM (Ex- 1000 ft/lbs of torque and 10 RPM = 1.9 HP) or if they are producing little torque (Ex- 10 ft/lbs of torque and 1000 RPM = 1.9 HP) or anything in between. As one can see, we have the EXACT same amount of work produced whether I have a lot of torque or not.
In the above example, if one has a contest between the two, math will show us the result will be a dead even tie.
It's simple to make torque or more torque:
One thing that a lot of people don't understand is that anybody can double or triple or whatever the amount of torque they want to make to their wheels if they have enough gears.
EX: If I have 100 HP and have 500 ft/lbs of torque to my wheels with a 5 to 1 final drive and want to double torque output all I have to do is double the final drive gear to 10 to 1 and now I have 1000 ft/lbs of torque to my wheels. Unfortunately, I still only have 100 HP but I have twice the torque value to my wheels. I will still only be able to climb a hill at X speed because; remember; I still only have 100 HP. (Remember, I'm not doing any more work or making anymore HP because HP is a timed based formula, all I'm doing is changing the amount of torque that gets to the wheels to keep the engine at its best operating curve.)
You can't change power with a gear change.
Remember that timed based formula? It proves by math that a gear change can't make more HP. (Ex: if we can lift 100 lbs of weight with a 2 to 1 gear and do it in 1 minute of time it will take twice as long to lift 200 lbs of weight with a 4 to 1 gear. Again, HP is the same. (the math says it has to be.)
The more HP an engine has the less gear you need:
This is a hard concept for some to grasp. I have seen time after time after time people on here say it's stupid for manufactures to only offer a high gear ratio for their diesel trucks. They don't understand that gears are nothing more than a round lever. (Ex: A big strong guy may need a very short lever to lift 500 lbs (tall gear) where a weak guy will need a very long lever to lift 500 lbs (short gear). (In the above ex, the strong guy will be making more HP even though the weak guy can still lift the load but will take longer doing it)
Now where a short gear will help is with duty cycle. Remember, even the big guy will get tired and if he has a long lever, he will be able to work longer and with less effort. (It will also take longer to do his work with a longer lever)
Duty cycle goes down as HP goes up.
Two different engines with the same HP can have WAAAAAY different duty cycles.
EX: My 2006 Duramax has 360 HP. The 3.5 Ecoboost has 365 HP. On a hot day my Dmax has a WAAAAAAY higher duty cycle than the Ecoboost. It all has to do with how an engine is designed. It has nothing to do with it's a gasoline or diesel engine thing. Diesels "usually" have a higher duty cycle although there are industrial gasoline engines out there that have extremely high duty cycles compared to a light duty diesel pickup engine. Again, it's a design thing and not a diesel/gas thing.
A truck has a 3.00 rear end and you change the gear set out to a 4.56 rear end. Which gear set will make more HP to the ground?
The answer is the 3.00 will make more power (slightly). Yes I know it's splitting hairs and yes I know; that doesn't seem right, but it is. Remember, torque does not make HP and the shorter the gear the more friction and loses it has hence the "slight loss" in HP.
Engine 1, a turbo diesel puts out 200 HP and 420 ft/lbs of torque; engine 2, a gas puts out 345 HP and 375 ft/lbs of torque. Which one will be first to the top of the hill in a towing contest.
All being equal (weight, sea level etc) who will win a towing race up a hill?
The HP formula teaches us it won't even be close. The gasoline engine will smoke the turbo diesel up the hill in the above example. But what about all of that big torque the diesel is putting out? Remember, torque is not power and never will be!
(These are true numbers from my 6.5 turbo diesel and a 5.7 Hemi)
An Ecoboost has 365 HP and a Dmax has 360 HP; who will win a towing race up a hill?
The HP formula says it will be a very close race at the end, even though my Dmax has a ton more torque than the Ecoboost. In all likelihood the Dmax will win the race even though the Ecoboost has slightly more HP.
"But wait; you said above that HP always wins; I told you torque was King!!" Sorry but that is not the reason the Ecoboost will lose the tow up the hill.
The reason is, HP curve. It takes time for an engine to build up RPM and since a diesel takes less time to get to peak HP it will be quicker off of the line "so to speak." Like I said, it will be close though because of the HP equality.
So what does all of this mean?
Lets wrap this whole thing up.
Number one on the list is: Torque alone is not power or work and never will be.
Torque plus RPM IS power and work. We have another name for it. It's called Horsepower and it can tell you how much work is being done in how much time!
You can't make HP with gears; you can only make more torque and torque is not power.
The shorter gear you have the (higher numerically) less HP you will get to the rear wheels. Again, this is splitting hairs because the amount is very small but it is true.
Just because it's a diesel doesn't mean it's more powerful.
Just because it's a gas engine does not mean it can’t be faster up a hill than a diesel with a lot more torque.
Here is some good reading to back up some of the statements made:
Really good reading about engine myths.
nmReally good reading from CAT about RV's and HP. (notice CAT doesn't even talk about torque)
Gears and HP to the ground.
First some terms:
Torque: A twisting force.
Horsepower: A unit of power equal to 550 foot-pounds per second (745.7 watts). The power of an engine measured in terms of this.
Duty cycle : The cycle of operation of a machine or other device that operates intermittently rather than continuously.
Since torque is simply a measurement of a twisting force one can't determine how much work is being done from just that term. I could say I am applying 1000 ft/lbs of force on a pipe wrench and if the joint doesn't break loose I’m not doing any work and that would be 0 HP. If the joint breaks loose and I can swing that pipe wrench 10 RPM's then math tells us I'm producing 1.9 HP.
Now; same situation: Someone tells me they can produce 1.9 HP. Since HP is a timed base formula I know they are doing work unlike if I just have a torque number. What I don't know is if they are making a lot of torque with very little RPM (Ex- 1000 ft/lbs of torque and 10 RPM = 1.9 HP) or if they are producing little torque (Ex- 10 ft/lbs of torque and 1000 RPM = 1.9 HP) or anything in between. As one can see, we have the EXACT same amount of work produced whether I have a lot of torque or not.
In the above example, if one has a contest between the two, math will show us the result will be a dead even tie.
It's simple to make torque or more torque:
One thing that a lot of people don't understand is that anybody can double or triple or whatever the amount of torque they want to make to their wheels if they have enough gears.
EX: If I have 100 HP and have 500 ft/lbs of torque to my wheels with a 5 to 1 final drive and want to double torque output all I have to do is double the final drive gear to 10 to 1 and now I have 1000 ft/lbs of torque to my wheels. Unfortunately, I still only have 100 HP but I have twice the torque value to my wheels. I will still only be able to climb a hill at X speed because; remember; I still only have 100 HP. (Remember, I'm not doing any more work or making anymore HP because HP is a timed based formula, all I'm doing is changing the amount of torque that gets to the wheels to keep the engine at its best operating curve.)
You can't change power with a gear change.
Remember that timed based formula? It proves by math that a gear change can't make more HP. (Ex: if we can lift 100 lbs of weight with a 2 to 1 gear and do it in 1 minute of time it will take twice as long to lift 200 lbs of weight with a 4 to 1 gear. Again, HP is the same. (the math says it has to be.)
The more HP an engine has the less gear you need:
This is a hard concept for some to grasp. I have seen time after time after time people on here say it's stupid for manufactures to only offer a high gear ratio for their diesel trucks. They don't understand that gears are nothing more than a round lever. (Ex: A big strong guy may need a very short lever to lift 500 lbs (tall gear) where a weak guy will need a very long lever to lift 500 lbs (short gear). (In the above ex, the strong guy will be making more HP even though the weak guy can still lift the load but will take longer doing it)
Now where a short gear will help is with duty cycle. Remember, even the big guy will get tired and if he has a long lever, he will be able to work longer and with less effort. (It will also take longer to do his work with a longer lever)
Duty cycle goes down as HP goes up.
Two different engines with the same HP can have WAAAAAY different duty cycles.
EX: My 2006 Duramax has 360 HP. The 3.5 Ecoboost has 365 HP. On a hot day my Dmax has a WAAAAAAY higher duty cycle than the Ecoboost. It all has to do with how an engine is designed. It has nothing to do with it's a gasoline or diesel engine thing. Diesels "usually" have a higher duty cycle although there are industrial gasoline engines out there that have extremely high duty cycles compared to a light duty diesel pickup engine. Again, it's a design thing and not a diesel/gas thing.
A truck has a 3.00 rear end and you change the gear set out to a 4.56 rear end. Which gear set will make more HP to the ground?
The answer is the 3.00 will make more power (slightly). Yes I know it's splitting hairs and yes I know; that doesn't seem right, but it is. Remember, torque does not make HP and the shorter the gear the more friction and loses it has hence the "slight loss" in HP.
Engine 1, a turbo diesel puts out 200 HP and 420 ft/lbs of torque; engine 2, a gas puts out 345 HP and 375 ft/lbs of torque. Which one will be first to the top of the hill in a towing contest.
All being equal (weight, sea level etc) who will win a towing race up a hill?
The HP formula teaches us it won't even be close. The gasoline engine will smoke the turbo diesel up the hill in the above example. But what about all of that big torque the diesel is putting out? Remember, torque is not power and never will be!
(These are true numbers from my 6.5 turbo diesel and a 5.7 Hemi)
An Ecoboost has 365 HP and a Dmax has 360 HP; who will win a towing race up a hill?
The HP formula says it will be a very close race at the end, even though my Dmax has a ton more torque than the Ecoboost. In all likelihood the Dmax will win the race even though the Ecoboost has slightly more HP.
"But wait; you said above that HP always wins; I told you torque was King!!" Sorry but that is not the reason the Ecoboost will lose the tow up the hill.
The reason is, HP curve. It takes time for an engine to build up RPM and since a diesel takes less time to get to peak HP it will be quicker off of the line "so to speak." Like I said, it will be close though because of the HP equality.
So what does all of this mean?
Lets wrap this whole thing up.
Number one on the list is: Torque alone is not power or work and never will be.
Torque plus RPM IS power and work. We have another name for it. It's called Horsepower and it can tell you how much work is being done in how much time!
You can't make HP with gears; you can only make more torque and torque is not power.
The shorter gear you have the (higher numerically) less HP you will get to the rear wheels. Again, this is splitting hairs because the amount is very small but it is true.
Just because it's a diesel doesn't mean it's more powerful.
Just because it's a gas engine does not mean it can’t be faster up a hill than a diesel with a lot more torque.
Here is some good reading to back up some of the statements made:
Really good reading about engine myths.
nmReally good reading from CAT about RV's and HP. (notice CAT doesn't even talk about torque)
Gears and HP to the ground.
