Thursday, March 24, 2011

Modification: Lightweight Flywheel

A lot of people ask if they should buy a lightweight flywheel for their car. My 2 cents are: If you can get it cheap, when you've already got the car apart for a clutch job, do it. It's been a big plus in every car I've ever done it on, noticeably accelerating faster, but I've always gone from factory, where all the weight is on the outside, to aftermarket aluminum with NO weight on the outside ring. Why does that make such a big difference?

First, let's go over why the factory flywheel is so heavy. Manufacturers put weight on the outside (perimeter) of the flywheel to increase the moment of inertia. Check out a picture of a factory flywheel here. The higher moment "stores" more energy, so it feels to the driver as if you have more torque when starting from a stop. Another big plus for manufacturers to have heavy flywheels: smoother idle, as it takes more energy to speed up or slow down the wheel, so it "smooths" the dips and peaks of the four-stroke cycle of your engine. Also, shifting becomes a bit easier, with the revs changing more slowly.

Those sound like things you wouldn't want to sacrifice, until you realize that you are sacrificing performance for a perception of performance.

A lighter wheel (really the perimeter around the wheel, the weight inside the wheel matters much less) allows you to speed up and slow down the engine much faster - that means quicker acceleration (more effective in lower gears where the engine is changing speeds faster) - and it's less overall weight in the car.

The downsides are that your idle can be rougher the lighter you go, and you have to rev the engine a bit more on every launch. Lighter flywheels also transmit changes in speed (both from the engine or the tires) with a lot less "smoothing" so it makes for a bit of a rougher ride.

The intensity of these factors varies from car to car and flywheel to flywheel. In some cars it is barely noticeable, in others it will amaze you.

The effect on MPG is probably very small, you'll drop some MPG if you have to raise your idle, you'll gain some if it causes your driving habits or if the weight makes a big impact.

Now that you've learned about heavy or light flywheels, don't forget about dual mass flywheels. They use springs to absorb the shocks of driving, giving you a smoother ride then even a very heavy flywheel.

The downside being added complexity, price, and they can fail.

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