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All about Horsepower, Torque, speed, and acceleration.

All about Horsepower, Torque, speed, and acceleration. - Blog

If there’s one thing as old as cars themselves, it’s the craving to go faster. Manufacturers (and tuners) continually seek to achieve even greater ability to go as fast as possible. So how should you make your car faster, and what actually makes cars faster?

Torque vs Horsepower:

Usually most people only care about Horsepower (or Kilowatts for those of you that live in a place where things make sense) when it comes to making their car faster. However, torque is just as important, in some cases more important, than power. So what really is the difference between torque and power?

Simply put, torque is a rotational force coming from the engine & transmission to drive the wheels, and power is the rate at which this rotational force (aka work) is done. I could go into a whole engineering lesson about this, but I’d rather let Jason from Engineering Explained handle that. So when your car goes, it’s actually the torque that does the moving. More torque means more force you have to either accelerate your car or pull a load with, since torque is Force x distance (aka lb-ft or Newton-Meters). Power is basically how fast you can keep putting out that torque.

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A simple explanation of torque.
Power equation, notice how F times D is torque.
Power equation, notice how F times D is torque.

When it comes to acceleration, really power doesn’t matter all that much. Recall that acceleration is:

All about Horsepower, Torque, speed, and acceleration. - Blog

So if our goal is maximum acceleration, you can go about it two ways: Either increase the Force (torque) applied by the engine and transmission, or decrease the mass the engine has to move (your car). Power really only applies to speed. A car with higher power can achieve higher speeds and maintain them easier, since the engine can give more work (torque) at a faster rate.

If you want to compare how quickly two cars can accelerate, most people use power/weight ratios. However, power/weight really only matters for how easy it is for you to maintain speed. For example, cruise ships have a huge amount of Horsepower, sometimes in the hundreds of thousands, but since there’s so much weight/drag to move against they can only keep going so fast.
Toque/weight is a much more accurate way to compare car’s acceleration abilities, assuming both cars have similar structures. Whether you want high power/weight or torque/weight (why not both?) depends entirely on your goals.

If you want faster acceleration, you’re going to need more torque. So how do we get more torque?

How to increase torque:

Torque can be increased one of two ways: Getting more air to the engine or through the transmission.

Engine Air:
Your engine is basically just an air pump. The more air it gets, the more work that it can do. The only ways to increase air to the engine is either by more airflow or more displacement.
Better airflow, by allowing less restrictive flow of cold air to the engine, will allow you to have more oxygen available for combustion. It’s important that this air is “cool” too, since gas becomes denser as temperature decreases, and thus you can stuff more oxygen into the same volume.
Your Valves on the engine also have an effect since they control how much air enters. Valve size, the number of valves, valve lift, and valve duration all have an effect of the air that enters and thus the torque and the best rpm range for the torque.

Displacement:
Displacement determines how much you can compress air in the engine, and the more you can compress the more air you can fit into the engine. So higher displacements lead to more torque.
“Now hold up, there’s many low displacement cars cars that have a lot of torque since they’re turbocharged”, you might say.
Well I’m here to let you in on a little secret, forced induction (turbocharging & supercharging) is just artificial displacement. Forced induction systems compress air as your pistons compress air, but forced induction systems allow for additional compression before the pistons compress. A naturally aspirated engine has a maximum compression of 1 atmosphere (atm) of air at sea level, that’s the most it can do assuming everything is perfect. Higher elevations with less air pressure will decrease the natural compression of the engine and thus make them less torquey.
In order to get more compression with a naturally aspirated engine, you need to give it more displacement. A 4.0L engine can compress twice the amount of air as a 2.0L engine, and (in theory) will have twice the available torque.
However, the magic of boost means you don’t have to resort to a bigger engine. If a turbocharger can compress air to 1 atm (aka 14.7 psi or 1.01325 bar) you basically just doubled your displacement. 1 atm from the engine’s ability to suck in air and one 1 atm from forced induction. So a turbocharged 2.0L engine theoretically now has as much displacement and torque as a 4.0L. This is assuming the turbo works for the whole rev range at peak boost, though.

Also, don’t forget that adding a larger bore and stroke are other ways to increase displacement and thus torque.

Peaky Torque vs Torque ‘under the curve’:
Having high torque is a good thing, but if that torque is peaky (as in going from low torque to high torque rapidly in the rev-range), you might have less of a benefit. A lower yet flat torque curve which provides a greater value for the average torque over the whole rev range will lead to better acceleration. It will also be much easier to drive, since the torque to the wheels doesn’t suddenly appear.

It's not witchcraft Jeremy Clarkson.
It's not witchcraft Jeremy Clarkson.

Transmission:
Automatic or Manual, your transmission’s entire purpose is to manipulate the torque coming from your engine and what speed you can go. Calculating transmission torque to the wheels is actually quite easy if you assume a lossless (aka no friction) system. Let’s say my engine can provide 100 lb-ft of torque. In first gear I have a ratio of 3:1, and my final drive (differential) is 2.75:1. How much torque is going to the wheels? Well, it’s simple multiplication:
100 x 3 x 2.75 = 825 lb-ft of torque
In real life you’d multiply this by a frictional constant (usually 0.85 assuming 15% frictional losses) to find the torque going to the wheels and then if there are two driven wheels with an open diff (each wheel gets half the torque) you’d multiply that number by a half.

Cars are heavy, and getting them going from a dead stop is a challenge. That’s why we have transmissions, to multiply the torque from the engine enough that we can move our vehicles. However, gears can only move so fast and your engine can only rev so high, so we need multiple gears to allow greater speeds. The lower gears have underdrive ratios, like 3:1, meaning you’ll have greater torque (and thus greater acceleration), but a lower top speed since the engine revs higher than the wheels. Direct drive, with a ratio of 1:1, means your engine will provide the torque sent to the rear differential and your wheels will rotate at the same rpm as the engine. The higher gears have overdrive ratios, like 0.8:1, which means you have less torque going to the wheels but you can go faster since the wheels can rev higher than the engine. If you were to try and start in 4th-gear (usually 1:1) you wouldn’t have the mechanical advantage provided by the torque manipulation, and thus would stall your engine since it can’t push hard enough. We downshift when slowing down so that we can limit the speed the car can move (engine braking) and also have a great torque potential to accelerate with.

Some cars, like the new MX-5 Miata, have VERY large first gear ratios (5.087:1) to make up for their torque lacking engines, which is part of the reason the new Miata accelerates so briskly (other than its lightweight). V8 American muscle cars generally have smaller first gear ratios (My Camaro is 2.667:1) because the engine is already providing enough torque and tires are expensive. So your transmission and final drive ratio can matter just as much for acceleration as your engine torque output.

These do something.
These do something.

Power

This is where power comes into play. Whether in Horsepower, Kilowatts, or that weird German unit for Horsepower, Power is what matters for pure speed. Torque gets you up to speed, power keeps you there. Higher power means higher speed, and that’s what we want on our racecar. Your car’s top speed is limited by both its weight (and your weight, fatty) and the drag forces caused by both the air and tires. Eventually you’ll reach a point where the power your engine provides can no longer overcome the forces of drag, and you won’t be able to go any faster. Remember that drag by air increases exponentially with speed, so going 200 km/h results in 4 times as much drag than going 100 km/h, and thus you’ll need 4 times as much horsepower (and then some for tire drag). As I said earlier with the cruise ships, they may have thousands of Horsepower but the forces of their own weight and the friction of the water they pass through means they can only chug along at residential speeds for cars.

So how do we get MOAR power?
(1.) By having greater torque, which I explained already above.
(2.) By allowing the engine to rev higher.

Engine revs:
Have you ever noticed that Honda’s engines not only tend to rev high, but also make decent power for their small displacement? That’s no mistake, engine revs have a lot to do with how much power you can make. And without getting to much into angular velocity conversions and such, the equations for power are:

In Imperial units
In Imperial units
In useful units.
In useful units.

Notice some things about these equations:
(1.) Power is directly proportional to both Torque and engine rpm. The greater the torque or the greater the rpm, the more power you will have.
(2.) Whether metric or imperial, both equations have a constant divided at the bottom. You will find this constant by doing some conversions, but the important thing is at the point where the rpms equal the constant (5252 rpms imperial, 9549 rpms metric), Power will exactly equal torque, and the lines for torque and power on a dyno graph will intersect there. We normally will only see this happen on Imperial graph, since most engines don’t rev to 9549 rpms.

Stock LS1 Engine Dyno graph. Notice at 5252 rpm that torque and power meet.
Stock LS1 Engine Dyno graph. Notice at 5252 rpm that torque and power meet.

When is power important? How do I get a higher revving engine?
Us car guys obviously love more power, since we can go faster. However, when is power really needed? If you have a commuter car that will almost never see high speeds, there’s really no need for power. High Power is only useful at high speeds when there is a lot of drag against the car.

Track cars and race cars are known for their high revving engines and loads of power. In Formula 1, the engines have low displacement and despite having forced induction, make very little torque, only about 215 lb-ft ( 290 N-m). However, these engines rev to monstrously high levels, sometimes 20,000 rpm, and thus can make over 800 HP (600 kW). Why don’t Formula 1 cars have torque? Well truth be told, since they weigh so little they can still accelerate quickly, and the goal of Formula 1 is to not accelerate. You want to be at your highest top speed possible all of the time, so Formula 1 cars are designed to maintain high speeds and thus stay in the high rpms where all of the power is. You do need torque to accelerate quickly after slowing down for a corner, but since Formula 1 cars take corners at most cars’ top speeds they don’t need a lot.

Track performance sports cars, like the Honda S2000, new Shelby GT350R, and E92 M3 are all considered “lacking” in torque for their class. But like Formula 1 cars, they don’t really need torque, they are designed to remain at very high rpms and very high speeds, like you’d find on a track.

In order to get a higher revving engine, you’d want several things:
(1.) Lightweight piston, valve, crankshaft, etc. construction, so that there is less reciprocating mass to impede on higher piston velocities. Motorcycle engines especially have small and thus lightweight engine components, allowing them to rev very high.
(2.) Good oil lubrication, since higher engine speeds demand more lubrication.
(3.) OHC design, since pushrod designs use heavier springs and thus limit the max rpm before valve float happens. Also, OHC designs allow for faster camshaft speeds since there’s multiple camshafts rather than one.
(4.) Good cooling and airflow abilities, since the engine will need air quick and will run very hot.
(5.) High bore/stroke ratio. Deep strokes allow for greater compression and thus more torque, but shallow strokes mean the piston has to move less far and thus can obtain higher piston speeds. High revving engines need fast piston speeds. Notice in the old Formula 1 engine crank I posted below how short the strokes are.

Well, that’s all from me! Feel free to post any comments, questions, suggestions, etc. down below!