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Why is it that most V6 engines are rated higher in HP than in torque, such as the SLK at 268 and 258 respectively, and most V8's are the opposite, here at 360 and 376.

The Boxter S is about 280HP and 226 ft lb torque, and the little 4 bangers like the S2000 have even a greater differential between higher HP and lower torque. Then the big MB V12's have 600HP with over 730 torque.

I understand that torque is generated by the engine and HP is the rate at which this torque is applied, and that these numbers are all peak numbers and may be at widely different RPM's, but I was just wondering why it seems to be so much more efficient in the smaller engines.
 

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Under 5250 RPM the torque number is always higher than the horsepower number. Above 5250 RPM the horsepower number is always higher than the torque number. And of course at 5250 horsepower and torque are identical. And as we all probably know, smaller displacement engines need higher rpms to develope their peak horsepower. Hope this helps.

By the way, it's easy to see from the calculation - HP = TQ X RPM / 5250
 

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NukeSLK - 5/2/2005 12:44 PM

Under 5250 RPM the torque number is always higher than the horsepower number. Above 5250 RPM the horsepower number is always higher than the torque number. And of course at 5250 horsepower and torque are identical. And as we all probably know, smaller displacement engines need higher rpms to develope their peak horsepower. Hope this helps.

By the way, it's easy to see from the calculation - HP = TQ X RPM / 5250
Wrong! That would mean that all engines of the same HP would have the same torque. It would also mean that they would have the same relationship between HP and Torque across the RPM band.
 

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Snowball, Snowball, don't be so quick with your statement. Are you saying my quote is wrong or my calc is wrong? Either way, they are both correct.

a car with 300 horsepower at 5250 rpm also has exactly 300 lb/ft of torque at that rpm. It's that simple.

If that same car was at 5249 rpm the horsepower would be 299.942 with torque at 300 and if it was at 5251 the horsepower would be 300.057 and the torque at 300 still. (Given the torque would remain 300)

Not saying a cars peak power is at 5250 rpm. What I'm saying is, a car with a peak horsepower above 5250 rpm will have less torque than that peak horsepower, period. It does not matter if the car has 900 lb/ft of torque, if it has that torque above 5250 rpm, then the horsepower will be higher than 900 and vice versa for below 5250rpm.

You owe me an apology. [:D]
 

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By the way, it is not as simple as that little calculation, but if you've ever seen a dyno chart, the horspower and torque lines cross each other and it's always at the same spot, you guessed it, 5250rpm.
 

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NukeSLK - 5/2/2005 4:36 PM

By the way, it is not as simple as that little calculation, but if you've ever seen a dyno chart, the horspower and torque lines cross each other and it's always at the same spot, you guessed it, 5250rpm.
If your equation is true solve for tq and plug in 300 hp. You have:
5250*HP/RPM = tq
1,575,000/RPM = tq
if RPM equals 1000 for example tq = 1,575 lb-ft. I dont think so.
 

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Okay, here we go, math 101.

HP = TQ X RPM/5250

HP = 300 X 5250/5250

HP = 300 X 1

HP = 300

To solve for Torque the equation changes to

Torque = HP/RPM X 5250

Torque = 300/5250 X 5250

Torque = .00571 X 5250

Torque = 300

I don't see the problem.
 

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Oh yeah, if you can make 300 hp at 1,000 rpm you do have 1575 lb/ft of torque. It's that simple. Math does not lie, sorry.
 

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Here's a dyno chart for a custom engine builder. See how the torque is lower than the horsepower at all RPM's because the only RPM's shown are above 5250. Also plug in one of those torque numbers or hp numbers to the calc and see if you get the same torque or hp that is listed. Math doesn't lie.

RPM-----------------C TORQ---------------C PWR
BAND---------------FT - LB------------------CHP

--------------------------------------------------------------------------------
6000------------------870.3------------------- 994.3
6100------------------865.7-------------------1005.5
6200------------------870.1-------------------1027.2
6300------------------866.4-------------------1039.3
6400------------------860.1-------------------1055.1
6500------------------857.3-------------------1063.3
6600------------------854.4-------------------1071.1
6700------------------853.6-------------------1088.9
6800------------------856.4-------------------1108.9
6900------------------844.3-------------------1109.2
7000------------------844.5-------------------1125.5
7100------------------838.8-------------------1133.9
7200------------------833.2-------------------1142.2
7300------------------825.6-------------------1147.6
7400------------------819.1-------------------1154.1
7500------------------805.9-------------------1150.9
7600------------------796.4-------------------1152.5
7700------------------774.7-------------------1135.9
7800------------------764.8-------------------1135.8
7900------------------747.5-------------------1124.4
MAX TORQUE----870.3
MAX HORSEPOWER--------------------1154.1
 

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OK - You win. I just fond it hard to believe. I got this off the WEB

Horsepower and Torque:
We all talk horsepower, but did you know that dynos ONLY measure torque! Torque is the “twist�, but horsepower is a calculated number based upon the produced torque at a given RPM: (horsepower = rpm x torque / 5252). Since HP is derived mathematically, based on the torque produced at a given RPM, HP and TQ curves always cross at 5252 RPM because of this relation to time. If you see HP and TQ curves that don’t cross at 5252, you know you’re looking at a bogus set of curves. So why does a huge diesel engine, that produces 800+ ft lbs of torque, have only about 300 HP? It’s because it produced that torque at a low 1900 RPM. So a drag race engine that also produces 800 ft lbs of torque, but does it at 6000 RPM, is producing more than 900 horsepower! If YOU had to produce the twist with a lever, wouldn’t it be harder to maintain that twist as the rate of spin increased?
 

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We all learn something new everyday. It is nice knowing some of these interesting facts. I have built a few American V-8's and learning all about how to build power was half the fun. [:)]

Hopefully the thread starter got something out of it!
 

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NukeSLK - 5/2/2005 5:50 PM

We all learn something new everyday. It is nice knowing some of these interesting facts. I have built a few American V-8's and learning all about how to build power was half the fun. [:)]

Hopefully the thread starter got something out of it!
So what is the answer to his org. question about 4 vs 6 vs 8 cylinder engines?
 

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Snowball - 5/2/2005 5:54 PM


So what is the answer to his org. question about 4 vs 6 vs 8 cylinder engines?
I believe I answered it in my first post. Smaller displacement engines have to rev higher to make adequate horsepower and as we've just discussed, above 5250 rpm, the torque is always lower than the horsepower. Motors are just big air pumps. A smaller motor will have to spin faster to make a decent amount of horsepower.
 

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Discussion Starter #15
Why then does the C55 engine make 376 ft lb of torque, but is rated at 362 HP, and the SLK55 which also makes 376 ft lb of torque only generate 355 HP?

And, if V6's have to rev higher to generate more HP, why don't we rev the V8 higher to generate more HP too?
 

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jlangmd - 5/2/2005 7:30 PM

Why then does the C55 engine make 376 ft lb of torque, but is rated at 362 HP, and the SLK55 which also makes 376 ft lb of torque only generate 355 HP?

And, if V6's have to rev higher to generate more HP, why don't we rev the V8 higher to generate more HP too?
First question - The SLK55 makes it's peak horsepower at a lower rpm than the C55. Remember, this is not exact, and the torque - horsepower line is not linear to and from the crossover point (5250 rpm), but does give you a general idea of the relation between the two.

Second question - A V-8 or larger displacement engine that will spin to greater than 6500 reliably would cost a lot of money. Nascar spins theirs to 9,000 rpm and make 700+ horsepower, but not too reliable.
 

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Thanks for the insight!

NukeSLK - 5/2/2005 1:44 PM

Under 5250 RPM the torque number is always higher than the horsepower number. Above 5250 RPM the horsepower number is always higher than the torque number. And of course at 5250 horsepower and torque are identical. And as we all probably know, smaller displacement engines need higher rpms to develope their peak horsepower. Hope this helps.

By the way, it's easy to see from the calculation - HP = TQ X RPM / 5250
After looking at 100's of dyno charts I never realized this.
 

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NukeSLK,

Thanks for the information. I'm dumbfounded. All these years, all those motoring books and magazines and I never realised there was a direct relationship between power and torque. That it can be represented by such a simple equation blows me away. It has been an education [:D]
 
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