Not quantum physics, but

An old friend of mine named Mac Tilton makes racing clutches and his catalog had formulas for figuring all that out based on all the variables. That was before web sites. Ck their web site it might be there.

 

One other point. You need to know where the weight was removed from the flywheel , not just how much.

Removing a gram at the outer rim gives more benefit than removing 5 near the center.

The inertia of a small mass is proportional to square of the radius.

The radius being the distance from the mass to the axis of rotation, ie the crank centerline.

 

Lighten the flywheel and the engine will rev more quickly, it will also slow down more quickly when you close the throttle. You are not, however, "releasing" energy, nor adding horsepower if you reduce the weight of the flywheel, nor doing something that will give you an "inertia increase". (I think you probably meant to say inertia decrease)
You state you realize horsepower isn't gained, but only after you said you have calculated the "mythical" horsepower gained. You could theoretically calculate how much better the car will accelerate in a given gear with a lighter flywheel (if you know where the mass was reduced and do some complex calculations), and then equate that to how much horsepower you would have to add to the engine if you left the flywheel stock, but there are lots of factors. I would say just put it on and drive the car.

You also are not decreasing "reciprocating" mass, you are decreasing the "rotating" weight, and depending on where the weight removed is located relative to the center of the flywheel, you are decreasing the rotational inertia. A given amount of weight reduced at a given distance from the center of the flywheel will reduce the rotational inertia a greater amount than an equal amount of weight reduced at a lesser distance from the center, thus simply knowing the amount of weight reduction without knowing where it was taken from won't allow you to accurately calculate anything.

As in everything there are tradeoffs. Lower rotational inertia means a more responsive engine and less power used to rev the engine. It also means a generally less smooth engine, and may make the car a little harder to drive and shift smoothly. Lighter weight equals less strength, less thermal mass, and a flywheel thats easier to distort because of overheating unless you spend more money for higher quality materials and construction. Some thinner flywheels have been known to flex and effectively reduce the effectiveness of the pressure plate which can cause slippage.

If you want a reduced weight flyweel, buy an aftermarket one from a company with a good reputation. You don't want it to flex, or even come apart and destroy part of your drivetrain. I think they are a good idea in performance cars, and it makes them more fun to drive. The Mclaren F1 has a tiny flywheel and small diameter clutch, it's a big V12 and revs up an down like a motorcycle.

 

Scott has it right,
You won't increase the HP, you will only enhance the response, increase the initial ability of the engine to gain or lose revs, in other words, decrease the "moment of inertia". This effect may also require an increased idle speed and add difficulty to off the line starts.

One factor that rarely comes into common thought is that, racing flywheels/clutches get regular attention. If this is a street application that we are discussing, consider the possibility of a flywheel shattering or a clutch coming apart and what kind of damage $$$ can be done to your trans and/or engine castings.

 

A lightened flywheel will appear to increase horsepower when the car is dynoed on an inertia dyno. This is the reason behind so many extravagant claims in the tuner market about lighter flywheels giving more horsepower etc. The "extra" horsepower comes from the fact that the dyno is simply a heavy drum that the engine, via the drivetrain, is trying to accelerate. The less mass in that overall system (Engine, g'box, wheels, drum) results in an increase in apparent power.

On a brake dyno however, the power is identical, as this is measured at a constant rpm, but varying load.

What does this all mean? Simple.... in motorsport, weight is the enemy.

Dave

 

Bingo, put it on a water brake (engine dyno) or rollers where you can load them and you will see no real change in HP figures.

Have you driven a P-car with an ultra light flywheel? If you're not using it primarilly for track use i'd reconsider. The lack of inertia can make the car a pain to drive, will feel rougher, and in some cases will actually lower the top speed.

 

Super light flywheels really aren't great for street cars. Your a/c, and all belt driven accessories will not work as well if it's too light, especially at idle. You may have to raise the rpms a bit, which may have other ramifications to your car. Plus, they are expensive to buy and install. I think many people think they are racers and think they want a race car set up when in reality they do 99% street driving and would be better served by a car set up for such.

OTOH, if you track the car a lot and don't care about every day driveability, all the benefits outlined make it a good upgrade to your car.

Ken

 

Here's one...(not mine, but the same).

This is 308. I thought I had pics of the 360/550 ones I've done, but I can't find them now. Anyway, I go more extreme on those... removing large amounts from back and from around the circumference, between the clutch contact points, (as the starter ring is not in the way like 308).

Thermal efficiency is the biggest enemy. You'll find your clutch wears on the flywheel side a little more than the the pressure plate side. But not to a vast degree. (and they do that a bit anyway).
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