A Perfect Example
Guy 1: My car has almost 300hp and weighs only 3000lb
Guy 2: Well, you will be in my rear-view mirror as I have almost 300 ft.lb torque and weigh the same!
Who will win this race?
The answer is, there simply is not enough information. Neither of the guys specified what RPM their power figures are at, what their gear ratios are, neither do we know what kind of race it will be.
HP and Torque are Twin Cousins
Yes, you read that correctly. What we must remember is that HP is computed from torque, and torque can also be computed from HP! How is that so? Well the formula for HP is described below:
HP= (RPM X torque) / 5252 where RPM = engine speed
A simple transposition and we can figure out torque from HP:
Torque= (HP*5252)/RPM
So you see, HP is derived from torque produced at a specific RPM.
Missing Information
Power band
By now you should realize that TQ and HP figures are meaningless without RPM describing it. As a matter of fact, stating the peak number is far less significant than stating what the entire power band looks like. Or maybe, showing a dyno sheet might be a better show of overall performance.
So, lets say both racers presented their dyno sheets:
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| Car 1 - 295hp and 213 ft-lb tq |
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| Car 2 - 240hp and 300 ft lb tq |
Now we can almost guess that the first car is all motor (N/A) and the second is forced induction based on their torque curve shapes.
Remember, horsepower is that final figure that describes the power moving the car. With the dyno sheet we can imagine the power delivery of both engines as it goes through the RPM range. Car 2 gives a kick in the back early out because of the sudden spike in horsepower. It is making about 210hp from about 3600 RPM. Comparing this to car #1 which only sees 125HP at the same RPM! Further up in the RPM we see the tables turning. Car #1 is now making 260HP at 6500 RPM, while Car 2 dies off with 200hp @ similar RPM!
So what does all this translate to? What does it mean when one car has more HP down low and another has more HP up to? Which will be the faster car? Let’s add one more piece of information: gear ratios.
Gear ratios
Your car has a gearbox to perform 3 functions:
1. Increase output speed (higher gears are faster)
2. Increase power (lower gears are more powerful for pulling loads, etc)
3. Change direction (reverse gear)
Let’s assume that both have the same gear ratios and will enter a drag race, what would this race look like?
Drag race
Provided both cars have no issue with traction, Car 2, with its early high HP would enjoy a catapulting take off if launched from a lower RPM. Car 1, if launched at similar RPM would not be happy with its low 125HP at that region. On the other hand, if Car 1 should launch at a higher RPM (say 5000 rpm), then it would most likely jolt quickly as well and as RPM increases its power does too.
As 2nd gear is engaged, and the RPM falls back to about 6500 RPM in Car 1, and 5500 in Car 2, we see that each car will be seeing very similar power outputs from their engines.
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| Car 1 - will be at around 260hp @ 6500 |
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| Car 2 - will be at about 260hp at 5500 |
However, Car 2, the high torque car, has no more additional HP as RPM increases, while the low Tq, high HP Car 1 still has another 25 HP to add. We can assume that the other gear changes will be similar, and therefore the high HP, lower peak torque car will win this race.
Sprint / mountain climb twisties race / Autocross
On the other hand, if both cars should challenge an autocros event, the results may flip. As each car exists the corners, the high torque engine may produce early HP for Car 2, while Car 1 will have to downshift and upshift very often and may be doing quite a few more gear changes. This is where earlier power band will always win.
Conclusion
Peak torque and HP values say very little about the car’s performance with stating the RPM it occurs at and how the car will be used. A low torque high revving high HP car is good for long straights, like drag racing, but for autocross and hill climbs with short straights, the car with early HP (high early peak tq) will always be the champion.
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