Want a laugh? "Expert" technical engine article...

[oz]

ashford is 100% correct. I was going to say the exact same thing. The area under the torque curve in the operating RPM's is the amount of work done. The most sucessful racers are the ones that build their engines to make the most TORQUE in their useful RPM range.

HP is just a way to quantify the rate at which work (torque) can be done based on time. In this case, the time is 'minutes' - as in rotations per minute. HP is a direct result of Torque and can only be calculated - it CAN NOT be measured. As I said before it is a calculation that allows people to gauge how fast work is being done.

 

[Hack]

Area under the torque curve? Umm, I don't think so.

Try this. Two engines in identical cars. 2,000 rpm "usable range" in each engine.

One makes 100 ftlbs of torque between 1,500-3,500 rpm (perfectly flat for ease of calcs). Average horsepower = 48

One makes 80 ftlbs of torque between 4,500-6,500 rpm (same). Average horsepower = 84

Which is faster? Hint: not the one with more area under the torque curve. Yes you have to calculate horsepower. That doesn't mean it isn't important. Power is the rate at which you do work. A lot of work done quickly is what you want to accelerate a car.

 

[65ShelbyClone]

HP is just a way to quantify the rate at which work (torque) can be done based on time.

I thought horsepower was the work and torque is the ability to do that work....or thereabouts. One "horsepower" is the ability to lift 550lbs one foot in one second.

Also, how does work over distance differ from work over time? It seems to me like a diesel would do more work over distance(lots of work, low rpm, ie, crankshaft roatational distance) whereas an F1 engine would do more work over time with the high RPM. Is that even close to being right?

It's weird, I can understand how semiconductors work on a molecular level, I know what caused Minamata disease, but I'm still figuring out just what torque is. I know how to use it to calculate power and I know why I like it, but I couldn't explain its function if someone asked me.

 

[oz]

Hack,
I see your point but I know there is more to it than this.
It really comes down to symantics - torque can be measured while hp can not. As far as which car would be faster, there is always an advantage to making more torque at a higher rpm although there is a point at which the car with more torque and less hp in your example would beat the lower torque, higher hp car - otherwise we'd never beat the ricers...

65ShelbyClone,
You've got it backwards. Torque is work - literally measured as how many pounds of force (lbs) applied at what distance (feet) from the centerline of the crankshaft. Just like a torque wrench. When you crank down a bolt to 100 ft lbs - that's exactly the same as when an engine puts out 100 lt lbs. It's measurable. hp is torqe x rpm/5252.... and yes, torque and hp are ALWAYS equal at 5252 rpm.

Torque is work, HP is the rate of work ... hp tells you how quckly you will be able to use the work.

Say you have two buckets of water...
#1: Completely full of water, small hole in the side for water to run out.
#2: 1/2 full of water, large hole in the side for water to run out.

Place #1 above a water wheel so water runs onto the wheel. The wheel will turn slowly for a long time. Work is being done.
Place #2 above the water wheel. The wheel will turn faster for a shorter time. Less work is done (only half as much water) but at a faster rate (larger hole in bucket).

Bucket #1 is the high torque, low hp engine.
Bucket #2 is the low torque, high hp engine.

Ideally, we want the best of both... a full bucket with a big hole in it
...since this is not possible, we conduct trade offs to get the engine that makes the most torque in the highest rpm band in which the engine will survive. This gives us the largest area under the torque curve and the highest torque numbers (as rpm increases so does hp).

If you run a series where maximum rpm is limited (Indy car, etc), you build your engine to have as much torque as possible in the rpm band when the race will be run. It's that simple.

 

[Edbert]

You cannot ignore either figure guys, building a 400rwhp motor that can't make much torque will make for a poor performing street car. Also building a big and flat torque curve which cannot make any power at the top end wont win you any races.

But in general (which means wrong for specifics...LOL) torque is what you want for street and hp is what you want for track.

 

[rbohm]

Area under the torque curve? Umm, I don't think so.

Try this. Two engines in identical cars. 2,000 rpm "usable range" in each engine.

One makes 100 ftlbs of torque between 1,500-3,500 rpm (perfectly flat for ease of calcs). Average horsepower = 48

One makes 80 ftlbs of torque between 4,500-6,500 rpm (same). Average horsepower = 84

Which is faster? Hint: not the one with more area under the torque curve. Yes you have to calculate horsepower. That doesn't mean it isn't important. Power is the rate at which you do work. A lot of work done quickly is what you want to accelerate a car.

bad example hack. a better one would be you have two engines, doesnt matter the displacement of either engine, but the cars they go in are identical.

the usable rpm range of both engines is 2500-6500 rpm.
one engine has a much broader but lower torque curve, the other engine has a high peak torque, but in a very narrow range. which engine will run harder? the one with the broader torque curve. the reason? it is always making more torque in all but a very narrow rpm band.

 

[Hack]

bad example hack. a better one would be you have two engines, doesnt matter the displacement of either engine, but the cars they go in are identical.

the usable rpm range of both engines is 2500-6500 rpm.
one engine has a much broader but lower torque curve, the other engine has a high peak torque, but in a very narrow range. which engine will run harder? the one with the broader torque curve. the reason? it is always making more torque in all but a very narrow rpm band.

I don't think it's a bad example, because it shows that an engine making less torque can beat an engine producing more torque. Yes I agree that having a broad torque curve is good, but the example makes my point clear.

 

[Hack]

Hack,
I see your point but I know there is more to it than this.
It really comes down to symantics - torque can be measured while hp can not. As far as which car would be faster, there is always an advantage to making more torque at a higher rpm although there is a point at which the car with more torque and less hp in your example would beat the lower torque, higher hp car - otherwise we'd never beat the ricers...

65ShelbyClone,
You've got it backwards. Torque is work - literally measured as how many pounds of force (lbs) applied at what distance (feet) from the centerline of the crankshaft. Just like a torque wrench. When you crank down a bolt to 100 ft lbs - that's exactly the same as when an engine puts out 100 lt lbs. It's measurable. hp is torqe x rpm/5252.... and yes, torque and hp are ALWAYS equal at 5252 rpm.

Torque is work, HP is the rate of work ... hp tells you how quckly you will be able to use the work.

Say you have two buckets of water...
#1: Completely full of water, small hole in the side for water to run out.
#2: 1/2 full of water, large hole in the side for water to run out.

Place #1 above a water wheel so water runs onto the wheel. The wheel will turn slowly for a long time. Work is being done.
Place #2 above the water wheel. The wheel will turn faster for a shorter time. Less work is done (only half as much water) but at a faster rate (larger hole in bucket).

Bucket #1 is the high torque, low hp engine.
Bucket #2 is the low torque, high hp engine.

Ideally, we want the best of both... a full bucket with a big hole in it
...since this is not possible, we conduct trade offs to get the engine that makes the most torque in the highest rpm band in which the engine will survive. This gives us the largest area under the torque curve and the highest torque numbers (as rpm increases so does hp).

If you run a series where maximum rpm is limited (Indy car, etc), you build your engine to have as much torque as possible in the rpm band when the race will be run. It's that simple.

I don't think it is semantics. Edbert is right; you can't ignore either.

Comparing the area under the calculated horsepower curve will always tell you which engine configuration is preferred, whereas the area under the torque curve doesn't. My above example shows why. Decent torque produced at higher rpm beats better torque produced at low rpm (always assuming that you've geared the car to take advantage, of course).

Just calculate horsepower and look at the area under that curve. That's what tells you what you want to know.

The bucket analog only applies if engine number 2 is peaky (produces good power only in a very narrow rpm range). I completely agree that producing power only in a very narrow rpm range is not good. If you look at the area under the horsepower curve this will show up.

 

[oz]

Hack,
You need to accept the fact that hp is a function of torque... to get technical, integrate torque over time (RPM) and you get hp... there is no reason to calculate 'area under the hp curve' because hp is a function of torque. And thus, 'semantics'... hp is calculated from torque!!!

As an engine builder, you'd be an idiot to make hp your ultimate goal. Hp is a result of torque. As an engine builder the important factors are TORQUE and OPERATING RPM. hp is the RESULT of this --> maximize your torque in the usefull rpm range and hp follows (as a result of the calculaton hp=rpm x torque / 5252). It is symantics... you are saying the same thing as I am but you are citing the result - hp, while I am citing the cause - torque and rpm band.

 

[Hack]

Hack,
You need to accept the fact that hp is a function of torque... to get technical, integrate torque over time (RPM) and you get hp... there is no reason to calculate 'area under the hp curve' because hp is a function of torque. And thus, 'semantics'... hp is calculated from torque!!!

As an engine builder, you'd be an idiot to make hp your ultimate goal. Hp is a result of torque. As an engine builder the important factors are TORQUE and OPERATING RPM. hp is the RESULT of this --> maximize your torque in the usefull rpm range and hp follows (as a result of the calculaton hp=rpm x torque / 5252). It is symantics... you are saying the same thing as I am but you are citing the result - hp, while I am citing the cause - torque and rpm band.

Yes you do need to look at the area under the HP curve. Yes, HP is calculated from torque and rpm - since HP takes both torque and rpm into account, it is the number you really want to look at.

I've shown that maximizing torque doesn't necessarily give you the best results. The 100 ftlb motor loses to the 80 ftlb motor in my example. If you just look at the average hp rather than the torque it's much clearer.

Maybe we are talking about the same thing, but you seem to disagree with my claim that maximizing torque without doing additional calculations doesn't give the whole picture. The area under the horsepower curve really tells you a lot more than looking at torque!

edit: Remember that since HP is calculated from torque, you aren't ignoring torque when you look at HP. The mistake to avoid is in looking at PEAK HP rather than maximizing the average HP.

 

[ashford]

sorry i fudged in thought its horsepower under the curve. i try to use torque under the curve to find the best setup within the same rpm range, not different rpm ranges.

hows this a 300 hp disel makes 300 horsehower at 3000 rpm but torque peaks at 2500 rpm at about 600 ft.lbs. ok not put a gearbox behind it 1:2 ratio now it makes 300 hp at 6000 rpm and 300 lb of torque at 5000.

 

[zookeeper]

I've said this before, but I think it's still true. If you think torque is more important than horsepower, try shifting your car at the torque peak the next time you're at the drag strip and see how it compares to shifting at the point of peak horsepower. The very most important decision in building an engine is to build it for what you intend to use the motor for, right? How many people have we all seen build a 302 with too large a cam and too big of a carb that may only see the drag strip once or twice a year? The other 363 days they spend putting up with a loose converter, overheating and poor mileage. Does that make sense? Also a motor built for open track days at road course is going to be much different from a motor built solely for 1/4 mile sprints. Anyone remember the Boss 302 T/A cars that used two Holley Dominator carbs? That's right they actually got the things to make killer horsepower on a dyno, but the power quality was like a light switch, either all or nothing. My point is a person can argue mathmatics all day every day, but you need to consider the application before you decide to carve in stone which is best.

 

[65ShelbyClone]

65ShelbyClone,
You've got it backwards. Torque is work - literally measured as how many pounds of force (lbs) applied at what distance (feet) from the centerline of the crankshaft. Just like a torque wrench. When you crank down a bolt to 100 ft lbs - that's exactly the same as when an engine puts out 100 lt lbs. It's measurable. hp is torqe x rpm/5252.... and yes, torque and hp are ALWAYS equal at 5252 rpm.

Torque is work, HP is the rate of work ... hp tells you how quckly you will be able to use the work.

Alright, I think I'm getting it now. I had to wait a day or so for it to thoroughly sink in. Tell me if I have it right; suppose a a hypothetical F1 engine makes ~350lb of torque at 10,000rpm and ~900hp at 20,000rpm. Is the peak horsepower such a large number in relation to torque because the "work" is being done so quickly at 20,000rpm?

I've said this before, but I think it's still true. If you think torque is more important than horsepower, try shifting your car at the torque peak the next time you're at the drag strip and see how it compares to shifting at the point of peak horsepower.

I think the cause of some confusion in here is that torque is being discussed in two different contexts; the scientific and the practical.

 

[reenmachine]

These are all fine examples of why I criticized the article in the first place. You have all nicely shown that the reason that the article is laughable is that this is a far more complicated issue than the author makes it sound. It goes to show that you just can't boil some things down to totally simple terms that are appropriate for all circumstances like the author implies.

That, and I love starting a torque/horsepower thread!!!

Great discussion ensues every time.