Torque or Power ?

[pirhana6]

Quite a few automotive enthusiasts here. I'm wondering what you value most from an engine for normal road use - torque or power?

[BiPoLaR]

Quite a few automotive enthusiasts here. I'm wondering what you value most from an engine for normal road use - torque or power?

Horse power creates torque

[saggs]

Horse power creates torque

True, but the amount of torque you get out of each HP depends on the engine design.  For example you can have a new Chevy Corvette V8 with near 640 HP and maybe 600 lb-ft of torque.  Or you can have a Cummins inline 6 diesel with only 280 HP and over 650lb-ft of torque.

HP is a measurement of power or work / time, and torque is a measurement of torsional force or energy x distance of lever arm, so...  

It all depends on what the demands of your vehicle are.  If you want a sports car or muscle car that accelerates fast, and has a high top speed and cruise efficiency you need both.  In simplest car terms; HP= top speed, torque= acceleration/pulling/towing.  Which is why sports cars usually have near = amounts of both.  While work vehicles like trucks, and tractors have lower HP but gobs of torque.  With the same torque but only half the HP that Corvette would get off the line just as fast, but start to run out of power the faster it went, and have a much, much lower top speed.

PS.  when I got bored one day studying electricity in school I figured out that my cell phone has .027 HP 

[321BAR]

True, but the amount of torque you get out of each HP depends on the engine design.  For example you can have a new Chevy Corvette V8 with near 640 HP and maybe 600 lb-ft of torque.  Or you can have a Cummins inline 6 diesel with only 280 HP and over 650lb-ft of torque.

HP is a measurement of power or work / time, and torque is a measurement of torsional force or energy x distance of lever arm, so...  

It all depends on what the demands of your vehicle are.  If you want a sports car or muscle car that accelerates fast, and has a high top speed and cruise efficiency you need both.  In simplest car terms; HP= top speed, torque= acceleration/pulling/towing.  Which is why sports cars usually have near = amounts of both.  While work vehicles like trucks, and tractors have lower HP but gobs of torque.  With the same torque but only half the HP that Corvette would get off the line just as fast, but start to run out of power the faster it went, and have a much, much lower top speed.

PS.  when I got bored one day studying electricity in school I figured out that my cell phone has .027 HP 

very well written and agreed to all points saggs

[Strip]

Saggs maybe a bit over generalizing....

As an extreme Formula 1 cars have very little torque compared to horsepower yet they are one of
the highest performing piston powered cars on earth. A more real world example would be a high
performance rotary engine or inline four cylinder, very high rpm with little torque yet quite a bit of
HP. There are plenty of examples that buck the trend Saggs has posted, a ton of European sports
cars come to mind, especially the ones with low (relative to number of cylinders) displacement high
rpm V-10/12s.

My  1,900lb dragster (548 CID big block Chevy) runs 7.70's at 170+ mph in the 1/4 mile and the
converter is to stall above the peak torque. We don't use torque to accelerate the car, we use
horsepower, if not the car would slow down quite a bit.

Another wrench, the drive train does not care how torque is manufactured, whether by raw power
or mechanical leverage. You can take a high rpm engine and put 5-8 gears behind it and create all
the wheel torque you want. Create a lot of horsepower in very narrow high rpm band, put as
many gears as you can row and you will get somewhere very fast.


As far as normal every day use torque moves you car along though, specifically high torque at low
RPM which will yield the best MPG as well.

Strip

[RTHolmes]

much prefer the torque curve to peak high up the rev band - nothing like an engine with a bit of top end fizz

[SFRT - Frenchy]

Horse power creates torque

Well not quite ... My 73 trans am has 250hp and 370tq. Its because its a small epa cam into a 455 cu. Diesel engines might be the same, need that tq for towing.

[MrRiplEy[H]]

Both are important but for day to day use I'd choose a high torque engine even if it sacrifices some power. Max power usage is very rare in normal traffic in the end while you constantly need torque accelerating out of turns etc. in while cruising peacefully.

My Jeep Commander has a 3.0 crd engine that puts out around 220hp but a nice 510nm torque on a wide rpm range (1500-4500). It moves that chunk of steel surprisingly fast. With 6200lbs gross weight that thing is far from a rocket but the torque makes hauling it around very effortless all while getting mpg that the gas versions can only dream of.

My E320 Merc is certainly faster when pushed to kickdown but in regular traffic it's lower torque makes it feel much more sluggish despite the fact that it's 2000lbs lighter and has also 220hp engine but only with 315nm torque at much higher rpm. It has to be pushed hard to reach the kind of takeoff the diesel puts out with half throttle. Nobody wants to drive the car pushing kickdown in the middle of city, right?

[Captain Virgil Hilts]

Horsepower = Torque x RPM / 5252


Torque is the actual measure of the rotating force generated by an engine. Horsepower is merely the speed at which that force called torque is produced.

Horsepower does not "make torque".

What is really most important on the street is throttle response.

Most people find that low speed and mid range torque work best on the street. You simply do not have a place to use the high RPM power. Nor do you have the gearing required to do so in most street cars.

The really serious hardcore street guys, however, do use high RPM horsepower, mostly because that high RPM power comes at the cost of low speed torque, and making that exchange allows them to get traction.

Diesel engines make massive amounts of torque by their nature, most of them have a very long stroke compared to their bore. The weight of their internal components precludes high RPM use, the forces created by moving that amount of weight fast are not easily contained. Further, the nature of diesel fuel, and the design intake and exhaust is ideal for low RPM use. Diesel has a considerably higher BTU content than gasoline. And the ports in a diesel head are designed to create their best cylinder filling efficiency at well under 3000 RPM, and in the case of big diesels, such as those found in heavy trucks, that RPM can be as low as 1500 to 2000 RPM. Look at the formula for horsepower above, and consider how much torque is necessary to create 650HP at 2000 RPM, as is done with the old Detroit Diesel 8V92TTA.

[MrRiplEy[H]]

The really serious hardcore street guys, however, do use high RPM horsepower, mostly because that high RPM power comes at the cost of low speed torque, and making that exchange allows them to get traction.

This is also why a diesel combined to full time 4wd is an excellent platform.

By the way, the true limiting factor on top diesel rpm is not moving masses and such, it's the maximum available rail pressure. Due to the nature of the diesel engine, the fuel needs to be sprayed the faster the rpm rises and yet you're supposed to pump out MORE fuel at the same time. This can be achieved with modern piezo-controlled injectors that can cycle very fast but there is a practical limit to the flowthrough you can get.

The fuel has to be sprayed through a needle thick nozzle in order to create a combustible spray (which is needed to create non-smoking low pollution cycle). This means it's not possible to just enlarge the injector when you need more power. Now, when you need to spray more fuel the only way is to increase pressure given the time for spray is constant. Current high power diesels deploy very high rail pressures to achieve power and rpm but it makes the engines vulnerable to leaks and the pumps very very expensive to manufacture (and repair). High rail pressure makes the engines sensitive to water contamination, too. Water doesn't compress like diesel fuel does so adding water drops into a diesel pump is near equivalent of inserting rocks to your mouth while chewing gum. A technician can visually see the damage done by water as crazy as it sounds.

[CAP1]

Quite a few automotive enthusiasts here. I'm wondering what you value most from an engine for normal road use - torque or power?

for a normal street car, decide what type of street driving you're going to be doing most. what will be your redline? where will you be shifting? then build the engine so that your horsepower and torque curves intersect near this rpm.

[Reaper90]

Horse power creates torque

Actually it's almost the opposite: Engines create torque, horsepower is calculated based on that torque and rpm. In a manner of speaking, torque creates horsepower!

In simple terms, torque gets the fun started (that seat of the pants kick when you stomp the pedal and the tires hook up), and horsepower keeps the fun going.

From an automotive point of view, what's best? Horsepower, as high of rpm as possible, so that you can take advantage of *gearing* to multiply that power. (edit: I make this statement from the perspective of a sports car owner and "part-time race car driver" - obviously if you're talking about towing or freeway cruising and not "high performance driving" then the magical "what's best?" answer changes...)

Case in point comparison. My 1990 Corvette versus my 1999 Corvette.

The 1990 featured the L98 engine, long tube runner intake design great for low-end torque, but unable to breathe above about 4800 rpm. Heavily modified, that car produced IIRC about 310 hp rwhp at about 4300 rpm on a chassis dyno, but over 525 lb/ft of torque. With so much torque and 315-wide Hoosier DOT-R competition tires it would hook up so hard coming out of corners that it would lift an inside front tire and twist the chassis of the car. It however couldn't rev and lots of shifting was required to keep it in it's very narrow low-end power band. Best 1/4 mile was a 12.99 at 105mph and a 3:33 rear gear.

The 1999 Corvette features an LS1 engine, a wonderful lump that breathes well right past 7000 rpm. Mine is once again heavily modified, 245 stage II heads, LS6 intake, long tube headers, very aggressive cam, VaraRam CAI, underdrive pulleys, a few other things I can't remember. Dyno'd 445 rwhp at about 6400 rpm and 456 lb/ft of torque. 3:45 rear gear. More hp than the 1990 but significantly less torque. Difference? best 1/4 mile of 11.54 at 119 mph at 295-wide cheap Firestone street tires. Would very easily be in the 10-second range with a 3:73 rear gear and better tires.

Horsepower RULES.

THIS LINK is all you need to read to answer all your questions re: Torque vs Horsepower

http://www.zhome.com/ZCMnL/tech/torqueHP.htm

[MrRiplEy[H]]

From an automotive point of view, what's best? Horsepower, as high of rpm as possible, so that you can take advantage of *gearing* to multiply that power.

That would be the case if unlimited rpm would be available without subjecting the engine to extreme wear and tear and passengers to excessive noise levels. If those weren't objects we'd be driving screaming 2-stroke monsters or jet engines instead of easy going soft 4-strokes.

In practice the more power you get with low rpm the better for passenger comfort and engine durability. Of course that's adversely true to the durability of the transmission but that's a whole another story there.

[Reaper90]

That would be the case if unlimited rpm would be available without subjecting the engine to extreme wear and tear and passengers to excessive noise levels. If those weren't objects we'd be driving screaming 2-stroke monsters or jet engines instead of easy going soft 4-strokes.

In practice the more power you get with low rpm the better for passenger comfort and engine durability. Of course that's adversely true to the durability of the transmission but that's a whole another story there.

I agree entirely, and there's a huge variable missing in the discussion if we're only talking about torque, hp, and rpm's.... as Carroll Shelby said "there's no substitute for cubic inches."

DISPLACEMENT

I'll take a bigger engine turning at lower rpm over a smaller engine turning higher rpm, equal horsepower, any day of the week.

But when we're comparing two engines of equal displacement, higher rpm horsepower is gonna be faster all day long. The grannmas and grandpas are gonna want low rpm, smooth, and quiet, but if we were talking about quiet and comfort instead of performance I doubt we'd be having a discussion about horsepower vs torque in the first place. 

[CAP1]

remember....he was talking street car. there's no reason to take a street engine much above 5500rpm, maybe 6,000. even that's a little high. build the power and torque in low in a street car to make it usable.