I swiped this from another forum, but it's the best article that I've seen that explains the relationship between the two. Much thanks to the person that wrote it on "Team Integra.net". Tried to get the really neat graphs to come with the article but they wouldn't transfer over. So, the entire article with graphs can be read here:http://www.team-integra.net/forum/blogs/su...wer-curves.html
HP and Torque. Analyzing power curves.
Posted 09-30-2002 at 01:00 AM by SurferX
Shady "tuning" shops will sometimes do anything to give the customers the dyno numbers they want. Alot of people will see a big peak number and be satisfied with that, not really caring to look at the actual dyno curve. Hopefully after reading this you will know how to determine what is a good power curve and what is a bad one.
The relationship of horsepower and torque
1 HP= 550 lb-ft per second.
HP is directly related to torque with respect to time. To put it into terms we can actually use, we can convert that second into an engine RPM value. One revolution would be equal to the circumference of a circle (2pi)r, "r" being a radian or one revolution, and one minute would be equal to 60 seconds.
550 lb-ft/[(2pi)r/60]= 5,252r
And the resulting equation to convert lb-ft (torque) with respect to RPM to lb-ft per second (HP) ends up being:
HP = (Torque x RPM) / 5252
Engrave this equation into your head. Note that dynomometers only measure torque, they do not measure horsepower. The horsepower curve is purely a calculated value using the above equation.
Changes in the torque curve (yellow) are very much reflected in the horsepower (blue) curve. Even though the torque curve may be flat or going slightly downhill, the engine's horsepower can continue to rise because of rising engine RPM. Of course it can only last for so long, as somewhere along the RPM range the motor's torque output will drop faster than the RPMs can climb, finally resulting in a loss of horsepower.
The torque and horsepower curves here are on the same axis. A tric alot of dyno operators play is putting the torque curve on a different axis to either make it look like you need more tuning or make it look better than it is. When the HP and torque curves are overlayed on the same scale you can beter see the direct relation between them.
In a nutshell there's only two ways to make horsepower, by increasing torque or increasing RPM. Our low displacement Honda motors can make a relatively large amount of horsepower with a low torque output because of our ability to maintain that torque output at higher RPM levels.
Now that the basics are out of the way, the next page will dive into why peak numbers don't always tell the whole story.
Peak horsepower vs. Average horsepower
Peak HP: 142
Average HP: 117.2
This is a sample dyno chart of a stock Integra GS-R. Too many people look solely at the peak power number without bothering to figure out the average horsepower. A strong powerband is dictated by the "area under the curve". The car with the most area under the curve will show to be the fastest in the real world. Peak horsepower queens will often fall short of performing like their peak number would make them think. That's probably why they stick to dyno competitions and not any real driving competitions. The average horsepower and torque gives a better idea of the area under the curves and how strong the car's powerband actually is.
Stock GS-R: Peak HP= 142. Average HP= 117.2
GS-R 1: Peak HP = 160. Average HP = 112.4
GS-R 2: Peak HP = 152. Average HP = 125.8
The car making the "most horsepower" actually is making the least power over it's powerband, even less than stock. In a driving competition the 160whp GS-R will have trouble keeping up with the stock GS-R whenever it can't stay in it's narrow powerband.
Peak power numbers mean little in how much total power the engine is making but for some reason seem to offer the most bragging rights. I have wondered why dynos don't display the average horsepower or area under the curve when it's something that could easily be calculated. I'm guessing because then nobody could trick customers into making them happy with big peak numbers but weak powerbands.
The next page shows two GS-R's which are making the same peak power, and how to tell which is stronger without needing to overlay charts.
Analyzing your power curves.
So what about looking at our own dynos? What if we had no stock baseline dyno done on our car and nothing to overlay and compare it with?
Two seperate people are making 200whp with their GS-Rs. By themselves without comparing each other it may be tough to decide if one is making a "strong" 200hp.
The trick to a strong powerband is to reach peak torque at a relatively early point then hold that consistant torque output to get a good horsepower number. It's really a compromise of having a big peak number and reaching peak torque at an early point, since low end and high end power often don't mix.
So the secret is really in the torque curves, since as we know hp and torque are directly related by RPM. Looking at the two seperate dynos up there, the first one reaches it's peak torque sooner and holds strong while the second torque curve doesn't finally reach it's peak until much later.
Overlay those two charts together and see what happens.
Even though both cars can be said to be making 200whp, GSR1 is going to be a whole lot faster. Notice the peak torque number is larger GSR1 too.
In the high RPM's it doesn't take much torque to make more horsepower, so when dealing with motors of similar peak HP numbers, you can bet the one with the larger peak torque number is going to have the stronger powerband.
Powerbands of stock Integras on the next page.
Stock Integra dyno comparison
I'll leave you with this dyno com of the 3rd generation Integras in stock form.
You've heard it all the time everyone saying how the LS has the best low end power so now you can actually see why. The B18 series motors were built for different purposes in mind giving torque output at different points of the powerband to achieve a certain horsepower curve.
Hopefully now you know how to tell a good dyno from a bad one and can go to the dyno with confidence knowing how to interpret your graph and not fall for bad tuner and dyno operator tricks of making you a retur customer with the best peak number. Good luck.