01mgvert said:
Help me to understand the difference between horsepower and torque. It seems to me that torque is what moves the car from a dead stop and horsepower is what keeps it accelerating.
A stock Mustang makes 260 bhp and 302 ft/lbs torque. With my current mods, my car dynoed at 250 rwhp and 290 ft/lbs torque. With an automatic transmission, that equals 310 bhp and 360 ft/lbs torque. A stock 2002 Firebird, with the 5.7l LS1, makes 310 bhp and 340 ft./lbs torque. What advantage do I have with 20 ft/lbs of additional torque?
The topic of torque vs. horsepower has been brought up and discussed a million times already, and that's on stangnet alone...
The problem, and the source of a lot of confusion, is that people only look at
PEAK torque and horsepower numbers and then try to draw conclusions.
"We got the same HP and same weight, but I got 50 more lb-ft of torque so I owned him from the start".
While I'm sure this has happened, it is just a coincidence. It doesn't mean that "torque moves you from the stop, horsepower keeps you moving", or that "horsepower sells cars, torque wins races".
As in any other issues that stir a lot of confusion, the answer is slightly more complicated that what people are ready to accept. "Torque accelerates the car" is
NOT the answer. "Horsepower accelerates the car" is not the complete answer either, but it is at least somewhat closer to the truth.
Here's my attempt of explaining this topic:
Acceleration is all about energy. To accelerate a car, you need to increase its kinetic energy. You do that by burning air/fuel mixture in your engine. The more air/fuel you can burn in a given time, and the more efficient you are in translating that energy into speed, the faster you will be accelerating.
Burning more air/fuel really translates into pumping more air through the engine, since you can fairly easily control the amount of fuel injected.
There's different ways of increasing the amount of air flowing through your engine. You can increase displacement and so have more air/fuel burned in each explosion in your cylinders. Or, you can increase the frequency of your explosions - you burn the same amount but more frequently. Or, you can increase the amount of air you stuff into your engine by compressing it.
So,
theoretically, you should be able to double the energy your engine produces by
a) doubling the displacement, with everything else being the same
b) doubling the RPM, with everything else the same
c) adding a turbo with 1bar (15psi) of pressure
In other words, a 4 liter engine revving to 5000rpm, a 2 liter engine revving to 10000rpm and a 2 liter engine at 5000rpm with a 15psi turbo should produce the same amount of energy and should accelerate a given car at the same rate. Which, amazingly enough, is not that far from the truth. These three types of engines will produce about the same horsepower in practice and will provide about the same acceleration for a given car.
Horsepower is the ability to do work. Increasing your kinetic energy (accelerating) IS work. So horsepower is the ability to accelerate.
This seems pretty clear. Horsepower should be the one and only measure of car's acceleration capability. So why all the confusion, and why so many examples of cars with same horsepower and weight but different torque, where car with more torque is slightly faster, epecially at low revs?
The problem lies in the fact that in practice, different types of engines behave differently and their efficiencies change as the revs change. The fluid dynamics of the air flowing into the engine and the gasses flowing out is
drastically different at 1000 rpms and at 9000 rpms. Even with modern tricks such as variable valve timing e.t.c., high revving engines suchs as S2000 have a problem maintaining the same efficiency throughout the whole rev range, so some compromises have to be made. In S2000's case, that's sacrificing the efficiency below 3000rpm to achieve maximum efficiency at 8000-9000 rpm. This translates into low (relative to peak) torque under 3000 rpm.
Big bore & big displacement engines usually don't rev very high, so their problem of maintaining high efficiency is not that pronounced. Also, these types of engines are usually designed to get maximum efficiency at low rpms at the cost of high-rpm efficiency.
So, a 4 liter engine revving to 5000rpm and making 200HP should (and will, most of the time) be slightly faster than a 2 liter engine revving to 10000rpm and making the same 200HP. Especially at low RPM's.
But this has nothing to do with peak torque. Yes, the 4 liter engine will have about twice the peak torque of the 2 liter engine, but
that's not what makes it faster. It's the
SHAPE of the torque curve (fat down low and slowly decreasing for the big engine vs. the mostly flat or slowly increasing for the small engine) that makes it faster. In fact, one could design a 4 liter, 200HP/300lb-ft engine with a bad torque curve and a 2 liter, 200HP/150lb-ft engine with a good torque curve and put them against each other. The 2 liter engine would be faster.
The reason that torque curve affects acceleration is because most of modern cars have gears. When accelerating through a given gear, car goes through an rpm range. The shape of the torque curve throughout that range
IS important for the total acceleration. If today's cars didn't have gears but a continous variable transmissions (CVT) -as more and more of them do, torque and the shape of the torque curve would
NOT matter for acceleration. A 200HP car will accelerate exactly the same as any other 200HP car of the same weight.
Another example: two cars, both weighting the same and both rolling at the same speed. Both punch the gas. Which one will pull away?
Answer: the car that's at a higher horsepower point at that time. That is, you look at both cars and you look at the RPMs their engines are at that speed. The car whose dyno has a higher horsepower at its RPM point will pull away.
Sorry for the huge post.
Cliff's Notes:
1. Horsepower accelerates the car. From a standstill and from a roll.
2. Peak horsepower is a decent, but not 100% accurate estimate of car's ability to accelerate, because:
3. Torque curve plays a role in acceleration, since the engine has to traverse this curve when running through a gear.
4. Peak torque #'s are absolutely meaningless for acceleration. People get confused and think that peak torque #'s are important because a lot of times (but not always) a car with a higher peak torque will also have a better torque curve, which
is important for acceleration (see #3).
that was amazing :worship: 
