How do you interprete the Horsepower VS Torque Graph

manualisbetter SG

Believe many of you have seen the PS/HP vs KW/Kgm graph before. Actually how do you interprete for the intersection when the 2 lines meet. Does that mean the optimum RPM which you should switch your gear getting the maximum efficiency from engine?

Also what could you deduce with a graph where the 2 lines never intersect?

[email protected] SG

oh this, ok i’ll try explaining without a graph. basically, one line shows the change in bhp with increase in rpms, one line shows the change in torque with increase in rpms. torque is needed for acceleration, bhp is needed for speed. tuning should be done so that the bhp curve intersects the torque curve somewhere near the peak. and once that is done, u shift your gear so that your rpms is always between the 2 peaks

ditzy SG

In addition to the 2 intersecting lines, a more specific graph might give you efficiency curves. This curves cut across and give you an approximate efficiency at which you are running at a specific horsepower and torque. Very useful more for rpm vs horsepower graphs.

[email protected] SG

would try to find a graph n put up for clearer understanding

dansplace SG

Try checking silverrims.com… saw an article there

STiYaKuzA SG

ahh got a graph of the WRX STi twin turbo version

in the graph, the red line represents the horsepower in kW which shows a maximum rating of around 206kW(280hp) at 6000rpm. the blue line shows the torque rating in newton metre, which shows a maximum of 394Nm(40.2kgm) at 4400rpm. for power curve, it would be good if the max. power is at a higher rpm, but for torque, u would want the rpm to be lower. if the rpm is higher for the power curve, it means a higher top speed. however, if the torque curve rpm is high, it means the car would take longer to accelerate to its max. rate of acceleration. therefore, a lower rpm would be good for the torque curve(to bring the car to its max. rate of acceleration faster and therefore bringing it to the top speed faster). so if u have an engine which has 1000hp but 1kgm of torque, it would take ages to reach the top speed Mr. Green . as u can see, the point of intersection does not indicate the best rpm to shift gears, as at that time the car would have been almost deprived of torque(by right, the car will not be able to exceed 6000 rpm at gear 6 & high speed due to air resistance and other reaction forces. in lower gears like 1 & 2, it would still be possible to exceed 6000 rpm as the reaction forces are not that great yet). therefore i would say that the optimum rpm to change gear is around 5500rpm as the torque is still quite plentiful. and i doubt there is any graph that does not show any intersection. even if they do not show, the two would intersect sooner or later, as it is impossible for the two to stay at optimum forever Mr. Green . btw this is quite a bad engineering, as it forces u to change gears at an rpm which is not optimum yet... usually NA cars like the NSX would have better intersection(almost near peak of both curves). i would try to find one and compare the differences

bismarck SG

ok….my physics is screwed…

Theatre Of Dreams SG

hard on u, thanks for such great info.

[email protected] SG

thanks! thanks for helping me find the graph n explain. hee i can relax liao.

ditzy SG

Good job, nearly wanted to take my fluids textbook go and scan. Cool

manualisbetter SG

Good explanation but i believe the optimum rpm to change gear is at 4400 (using your graph) since it is the time when max torque is released.

And when i mean both lines do not intersect, what i mean is within the limit of engine, i.e. before the redline kicks in. So far i have noticed a car which engine displaying such pattern:

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I will believe in this case, i will be able to obtain

Red Lobster SG

I've always been confused about the concepts of torque & horsepower, so did some research on google.

Torque and hp are not really intuitive to grasp, and here are some of the easier-to-understand links I've found:
http://www.nissanperformancemag.com/november98/torque.shtml

http://www.allpar.com/eek/hp-vs-torque.html

http://vettenet.org/torquehp.html

Essentially, the articles come to the same conclusion as many of you have mentioned - torque is responsible for acceleration (how hard your head is thrown against the seat when you floor it), while hp is a representation of how much torque your engine can produce at high rpms - and would be more important for top speed.

They say that torque and hp are mathematically related by the formula:

Hp (bhp) = Torque (lbs.ft) x rpm / 5252 (if torque is given in US units) OR

Hp (bhp) = Torque (Nm) x rpm / 7023.5 (if torque is given in Nm)

I think this means that the max torque & hp specs we read so often are basically 2 snapshot views of an engine's torque curve, at different rpms. The max torque shows the engine's peak torque, which happens at a certain rpm level, while the max hp indicates how much torque it can pump out at a higher rpm.

Eg. Car A has max torque of 145Nm at 4000rpms & max hp of 100bhp at 6500rpms. From the formula, when the car is generating max hp at 6500rpms, it is pushing out 108Nm of torque.

Car B has same max torque of 145Nm at 4000rpms but higher max hp of 120bhp at 6500rpms. From the formula, Car B is pushing out 130Nm at 6500rpms. So Car B can accelerate harder at 6500rpms than Car A, although they accelerate just as hard at 4000rpms.

The formula also means that all hp-torque charts will always intersect at 7023.5rpm, IF the units are given in bhp and Nm, AND the scales are the same, regardless of car, engine, tuning, TC or NA.

So for the WRX Sti graph posted above, which shows 206kW = 280bhp at 6000 rpm, the formula predicts that at 6000rpm, the torque should be 328Nm. See where the 6000rpm line intersects the blue torque curve and yup, its around 328Nm.

The articles go on to recommend that its better to upshift at redline, rather than anywhere else based on the shape of the curves.

What do you guys think of the analysis given in the links?

[email protected] SG

Originally posted by Red Lobster:
I've always been confused about the concepts of torque & horsepower, so did some research on google.

Torque and hp are not really intuitive to grasp, and here are some of the easier-to-understand links I've found:
http://www.nissanperformancemag.com/november98/torque.shtml

http://www.allpar.com/eek/hp-vs-torque.html

http://vettenet.org/torquehp.html

Essentially, the articles come to the same conclusion as many of you have mentioned - torque is responsible for acceleration (how hard your head is thrown against the seat when you floor it), while hp is a representation of how much torque your engine can produce at high rpms - and would be more important for top speed.

They say that torque and hp are mathematically related by the formula:

Hp (bhp) = Torque (lbs.ft) x rpm / 5252 (if torque is given in US units) OR

Hp (bhp) = Torque (Nm) x rpm / 7023.5 (if torque is given in Nm)

I think this means that the max torque & hp specs we read so often are basically 2 snapshot views of an engine's torque curve, at different rpms. The max torque shows the engine's peak torque, which happens at a certain rpm level, while the max hp indicates how much torque it can pump out at a higher rpm.

Eg. Car A has max torque of 145Nm at 4000rpms & max hp of 100bhp at 6500rpms. From the formula, when the car is generating max hp at 6500rpms, it is pushing out [b]108Nm of torque.

Car B has same max torque of 145Nm at 4000rpms but higher max hp of 120bhp at 6500rpms. From the formula, Car B is pushing out 130Nm at 6500rpms. So Car B can accelerate harder at 6500rpms than Car A, although they accelerate just as hard at 4000rpms.

The formula also means that all hp-torque charts will always intersect at 7023.5rpm, IF the units are given in bhp and Nm, AND the scales are the same, regardless of car, engine, tuning, TC or NA.

The articles go on to recommend that its better to upshift at redline, rather than anywhere else based on the shape of the curves.

What do you guys think of the analysis given in the links? [/b]

dunno about the link but i think u've offered a pretty comprehensive explaination about torque n bhp. well done!