HP vs. Torque?
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Elder User
Joined: Aug 2006
Posts: 501
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From: Eastern Ohio
HP vs. Torque?
After reading quite a few articles in Diesel Power I'm having a hard time understanding how one truck may only make 350hp but 1100ft/lb torque and the next one makes 650hp and the same torque. Which way is better to make the power? Does one make it higher in rpm and thus less for pulling vs one that is set up more for speed? I hope this doesn't sound dumb but I am fairly new to the diesel game and believe that I am probably not the only one wondering this but the one willing to ask. Thanks in advance!
Lead Driver
Joined: Jul 2007
Posts: 7,868
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From: Rhode Island
Horsepower is RPMxTorque/5252. A truck that makes say 1100 ft-lbs of torque and only 350HP, means that it makes 350HP at 1671 RPM.
Say that engine makes 1100ft/lbs @ 1200RPM, and then after RPM that it begins to drop off. So say that engine is makign 350HP@2800RPM, that means its only putting out 656 ft-lbs of torque at that RPM. After a certain RPM on any engine, torque beings to drop, but since RPMs multiply more then torque, horsepower can still go up.
So if you have a 350HP 1100ftlb engine, and a 650 HP 1100ft-lb engine, that means both engines have the same peak torque, but that 650HP engine maintains that torque longer and higher into the RPM range.
A simple and very crude way of saying it is, Horsepower lets you go fast, but torque lets you pull a load. This is why you see 400-500HP, 2000 ft-lb big rigs pulling 70,000 lb loads up a steep hill. Sure maybe at only 40MPH, but the twisting foce the engine puts out lets it do that. It just won't do it fast.
Say that engine makes 1100ft/lbs @ 1200RPM, and then after RPM that it begins to drop off. So say that engine is makign 350HP@2800RPM, that means its only putting out 656 ft-lbs of torque at that RPM. After a certain RPM on any engine, torque beings to drop, but since RPMs multiply more then torque, horsepower can still go up.
So if you have a 350HP 1100ftlb engine, and a 650 HP 1100ft-lb engine, that means both engines have the same peak torque, but that 650HP engine maintains that torque longer and higher into the RPM range.
A simple and very crude way of saying it is, Horsepower lets you go fast, but torque lets you pull a load. This is why you see 400-500HP, 2000 ft-lb big rigs pulling 70,000 lb loads up a steep hill. Sure maybe at only 40MPH, but the twisting foce the engine puts out lets it do that. It just won't do it fast.
Last edited by Lead Head; Dec 21, 2007 at 10:47 PM.
Senior User
Joined: Dec 2005
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it depends on what u want to do. pull or go fast. to make hp u need large amounts of fast moving air. but to make torque u need small amounts of slow moving air. if u have long intake runners youll make lots of torque but short runners will make hp. i drive a 1988 F250 7.3idi with a sidewinder. those idis made max torque at 1400rpm. even the "torque monster" cummins doesent make max untill 1900rpm. for me i want bottom end torque.because we use it for farming and about twice a year to pull our 5th wheel.
