Before we can answer the questions above have to explain what turns a flywheel and what a flywheel does. If you already know this then skip the next two paragraphs.
We must first start with the engine. Without going into too much detail, the explosion inside an engine caused by the ignition of the petrol and air mix pushes a piston down a cylinder. This piston is attached to a connecting rod the other end of which is attched to the crankshaft via a bearing. The straight force of the piston/conrod is changed to a rotational force by the crankshaft.
The flywheel is bolted to and is turned by the crankshaft. The flywheel has two main jobs. The first is to transfer the rotational force of the engine to the gearbox via the clutch. The second job is to keep the engine turning and 'smooth out' the rotation of the engine when it is running. Now this may sound strange but an engine would run very eratically (lumpy), particularly at low revs, if it did not have a flywheel. Imagine a unicycle turned upside down. A unicycle does not 'freewheel' like a bicycle, the pedals are fixed to the wheel. Now turn the pedals faster and faster, the wheel and the pedals are all turning together. Now try to stop the wheel by stopping the pedals from moving - the wheel will try to keep the pedals moving. The same thing happens in an engine, The flywheel will try to keep the engine turning.
The flywheel, which is turning with the engine will try to keep the engine turning at a constant speed. The heavier (higher mass of) the flywheel the more able it is to keep the speed constant because it 'stores' the rotational engery and the higher the mass of the flywheel the more energy it can store. Due to the mass of the flywheel it resists change to this speed (see unicycle example above). The higher the mass more energy is required to increase or decrease this speed so more throttle is required to accelerate. Conversly, when lifting off the throttle to slow down the flywheel will try to keep the engine turning. It is this effect that helps the car maintain its speed more smoothly. The undulations and bumps in the road have small effect on the cars speed due mainly to the flywheel. So, the flywheel in a car maintains rotational inertia in your engine as well as helping to balance the crankshaft.
With a lightened (lightweight) flywheel there is less mass to the flywheel so it takes less energy to increase or decrease the speed. Therefore when you accelerate it will take less poweer to make the flywheel go round faster so you will accelerate more quickly - better and faster response. You will noticably have more power because you can rev more freely. Theoretically, a lightened flywheel can save you fuel because it takes less throttle to accelerate.
Conversly, when you lift off the throttle the wheels of the car will need to use less force to keep the engine and flywheel turning so you will get less 'engine braking' (engine braking is where you use the engine to slow the car instead of the brakes). This will cause you to use your brakes a bit more than you otherwise would have.
Another disadvantage of a lighter flywheel is gear changing. Because there is less mass to the flywheel it is unable to store as much rotational energy and so when you remove the acceleration and push in the clutch to change gear the engine revs will drop faster than with a normal flywheel. Rally cars that have ultra light flywheels will be driven by keeping the acceleration on while changing gear.
So, does a lightened flywheel increase or decrease torque?
Firstly you have to understand horsepower/torque relationships. Both are measuring the energy output of an engine. Torque is the amount of rotational force it puts out, usually measured at its crankshaft or its car's wheels, quantified by distance (ft) and mass (lb). Imagine tightening a bolt to 74lb ft of torque with a two-foot-long torque wrench. If you're holding the wrench at its end when it clicks, you're pushing with 37 nm of force (74lbft/2ft). If you're holding it halfway toward the head, you're pushing with 74lbft of force (74lbft/1 ft), Either way, 100nm of torque is still exerted on the bolt.
Now, imagine tightening those bolts in one second and you're on your way to understanding horsepower. Horsepower adds time and a constant multiplier to the equation. One horsepower is defined as the energy needed to lift 550 pounds one foot in one second, and an engine's horsepower rating is simply it's torque multiplied by rpm, divided by 5,252*.
[*For the tecnically minded, here is how we arrive at the 5,252 in the calculation:- You can think of a revolution as a measurement of an angle. One revolution is 360 degrees of a circle. Since the circumference of a circle is (2 x pi x radius), there are 2-pi radians in a revolution. To convert revolutions per minute to radians per second, you multiply RPM by (2-pi/60), which equals 0.10472 radians per second. This gives us the "per second" we need to calculate horsepower. Let's put this all together. We need to get to horsepower, which is 550 foot-pounds per second, using torque (pound-feet) and engine speed (RPM). If we divide the 550 foot-pounds by the 0.10472 radians per second (engine speed), we get 550/0.10472, which equals 5,252. Source: Howstuffworks]
Therefore, any change in torque results in a proportionate change in hp. No matter how fast you tighten that bolt, it's still experiencing the same amount of force.
A Lightened flywheel reduces the amount of energy needed by an engine to move its components (parasitic loss). Since flywheels are solid, unsprung (dual mass an exception) components of a vehicle's driveline, a lightweight flywheel will decrease parasitic loss at a constant rate, improving a vehicle's horsepower and torque output (measured after the flywheel) throughout it's entire rev range.
By removing a few pounds from the flywheel a noticeable difference to a 3000lb+ car’s acceleration can be made. The difference is quite noticeable and the reason behind this is hidden within the gearbox. Cars accelerate at a greater rate in low gears. This is because a car’s gear box basically a mechanical lever and just like when using a leaver to lift a heavy object, the gearbox reduces the mass of the car that the engine sees. For example, in first gear an engine may see the car's mass as only around say 300lbs but the engine internal mass (crankshaft, conrods, pistons, flywheel) could still remain around 60lbs.
By removing say, 9lbs (15%) from the weight of the flywheel this could effectively reduce the weight of the car (as seen by the engine in first gear) by 450lbs.
It is now easier to see were the extra performance comes from when you lighten a flywheel. You effectively "lighten" a car just by removing mass from the flywheel. As the gear used increases, this "lightening" effect is reduced. This is why car’s acceleration improvement reduces in higher gears, to very little effect in top gear. Great for fast pull away and tight race tracks but will not increase the car's top speed, only the acceleration.
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