Originally Posted by article

Flywheels - Lightweight vs. Heavyweight
To answer the much heated and most debated question. Should i get a light, or should i stay with stock? Let me try to explain this as easily as possible and let you decide.

Rotating mass takes energy to spin it from one RPM to another. Therefore, it takes power from the engine that could otherwise be used to accelerate the vehicle.

The significant measure of rotating mass is called the mass moment of inertia. To keep it simple, weight is bad, but weight farther from the center-of-rotation is much worse. The mass moment of inertia is measured by the mass (weight) multiplied by the distance between the weight and center of rotation squared. For instance if you had a weight of 10 pounds mass, 5 inches from the center of rotation, its' mass moment of inertia would be 10 lb x 5 in x 5 in = 250 lb in^2. That same 10 pounds only one inch from the center of rotation would only have a mass moment of inertia of 10 lb in^2 (96% less). This is why lower diameter flywheels are an issue and heavy larger wheels can have an effect.

When you were a child you may remember playing on hand pushed marry-go-rounds. Kids would stand on them and other children push to get them spinning. You may also remember that it was much harder to push when there were more kids on the marry-go-round and they stood near the edges.

Now for the stock flywheel. I am told the stock flywheel has a mass moment of inertia of 280 lb in^2 and I used this value in these calculations. Let me warn, the effect of rotating mass is not constant for RPM or road speed. In other words, the effect in 1st gear is different than second, and in any gear the effect changes with speed. This is why, if anybody quotes a given horsepower savings measured on a dyno, it is not accurate because chassis dynos DO NOT simulate accurate transients. They measure horsepower at the wheels just fine, but they can not measure the effect of a lightened flywheel, tires, or wheels. They will measure a difference, it just isn't accurate. But it is easy to calculate the difference.

From simple calculations the stock flywheel (280 lb in^2) takes 10-20 HP to spin it while accelerating in 1st gear. In second gear it takes about 5 HP. In 3rd gear it takes 2-3 HP. Therefore, if your lightweight flywheel had half the stock flywheel mass moment of inertia, you could save half the above values. To me, this would be more significant in a 1/4 mile run where the launch and 1st gear is very important. On a road course, not as important.

You might wonder why 1st gear is so much larger? The most stock engines (B or D series) spins from idle to REV LIMIT in less than 4 seconds (give or take) in 1st gear. It takes a lot of power to spin this mass to high RPM very quickly. In 4th gear, the stock flywheel takes 10-20 seconds to go from MID to REV LIMIT RPM, therefore, much less power required.

A transmission can be thought of as a fulcrum and lever in a car. First gear has a really long lever; second gear has a shorter lever, etc. The lever represents the mechanical advantage that gears give your vehicle. When your car is moving, you have two factors that are present during acceleration, one is driveline losses, which are constant and the variable, which is vehicle weight and the mechanical advantage supplied by each gear. We know that within reason, vehicle mass is a constant. Now imagine if you reduced the driveline loss from 45 to 35 with the use of a lightweight flywheel. Since the engine has less drivetrain losses to compensate for, this means the "gained" horsepower can be applied to moving the vehicle mass. Using mathematics, one can realize that the higher you go up in gears, the less effect that a lightened flywheel will have to the overall equation.

While the performance characteristics of a lightweight flywheel seem to be the perfect solution, there are compromises. Low end performance is affected. This usually means that higher revs are necessary for smooth starts due to the reduced rotational mass. For drag racers, this can be a BIG issue.

-xproductionz.com

Originally Posted by deschlong

Wow, math in an HT tech post. That is a most welcome rarity.

Originally Posted by crazyhouse2011

I didn't notice any difference in any of the videos lol. I'm guessing it just feels different.