i read somewhere that a lightweight flywheel can be too light? i bought a 9 pound flywheel. is that too light of a flywheel?

 

Not for drag racing, but for the street, yeah probably.

There is a practical limit to what weight (rotating mass) can be eliminated.
9lbs sounds a bit much for a street car to me... especially if its your daily driver.

When you reduce the rotating mass, it will require more rpms and clutch slipping to get the car moving from a dead stop.
Since the mass helps the engine continue turning, once reduced so will fuel mileage,
Depending how radical you get it can also cause normal driveablility issues, such as abrupt changes of the throttle pedal will cause the car to learch and wain more than normal.

Some reduction can be a good thing too. It allows the engine to rev more freely, freeing up hp lost trying to keep that mass spinning, and especially to get it spinning.
A lighter flywheel is said to be easier on the clutch as well, so possibly a longer clurtch life. I have nothing to back this claim up though. Just passing along what I had heard.

For a 3 or 6 series car, I would stay more to the 12-14lb range for every day driving. The Mazdaspeed "Accessory" flywheel is right around 14lbs, IIRC.

 

i understand what you are saying. but if you have a lightweight flywheel, then your cars turns alot easier so i would think it would take less gas, not much at all, to engage the clutch. im putting a stage 1 clutch in it too. i just dont want to put something on my car that could make it worse. do you have any website info to back up any info above? not calling you a liar. i just want to read up on it. thanks for some advice though

 

The point you seem to be missing is that 4-Cylinder 4-Stroke engines like yours operate through a principle called "Torque Reversal" in that during the power stroke the engine drives the flywheel, however, during the gap between the end of one power stroke and the beginning of the next, the flywheel drives the engine. If you reduce the mass of the flywheel by as much as you're suggesting, then the drivability and of your car on the street will suffer and vibration will increase rather dramatically.

 

yeah. i understand all of that. but keep in mind there is always a power stroke going on. one goes off. then half a stroke later the next goes off then half a stroke later the next goes off the half a stroke later the last one goes off. so in just 2 strokes total all 4 cylinders have been on a power stroke. i know the flywheel keeps the engine rotating but if it was lighter wouldnt that mean its a little easier to keep it rotating because its less weight to turn? i could see the engine vibrating more. that makes sense.

 

Completely incorrect. A power stroke lasts for only about 140 degrees of crank travel; a four cylinder four stroke engine only has two power strokes per revolution of the crank which means that during any one rotation you will have the following four periods:

1. Power for 140 degrees
2. Negative power for 40 degrees
3. Power for 140 degrees
4. Negative power for 40 degrees

For more reading on the subject, here is a link to a site that shows what I wrote above graphically:Here is a snippet from a dissertation I wrote several years ago on the same subject:

 

ok. im not sure on that whole degree thing. but ill rephrase what i just said because it sounded dumb when i just read it. on a 4-cylinder engine. you have running mates or paired cylinders whichever you want to call it. that means two cylinders are at the same point of travel but on the complete opposite stroke. i know that cylinder numbers 1 and 4 are running mates and cylinder numbers 2 and 3 are running mates. so that would mean that if number 1 cylinder is at TDC then number 4 cylinder is at TDC. and no two cylinders are on the same exact stroke. so if number 1 was on compression stroke then number 4 is on exhaust and numbers 2 and 3 are on either power or intake stroke. it takes two complete revolutions of the crankshaft for one cylinder to complete all four strokes (intake, compression, power, exhaust) so that means that in two complete revolutions of the crankshaft all 4 cylinders complete all 4 strokes. they just dont start on the same stroke. so in every 180 degrees there is a power stroke. i do know what i am talking about but my only question is would a lightweight flywheel hurt my car in any way. i know the flywheel keeps the momentum going in a engine. so would it actually hurt my engine?

 

Your engine requires a heavy flywheel to operate properly in an "on street" environment (as opposed to a high-rpm track application). Why? Because there are two forty degree periods of each rotation of the engine where most of the forces inside the engine compartment (i.e. friction, pumping oil, running accessories, drawing in mixture, pushing out exhaust, compressing mixture, and reversing the direction of travel of the pistons) that are doing everything in their power to stop the rotation of the engine; virtually the only thing keeping the well choreographed ballet under the hood operating properly is the energy stored in the flywheel (energy stored in the crank and harmonic balancer do provide a very small amount of energy for this purpose as well). Now add to all of the forces bent on stopping the engine an off-idle clutch engagement (even a gradual one), and guess what? The engine either chugs and vibrates or it stalls. The only way to operate a car on the street with a light flywheel is to give the engine much more "gas" than you otherwise would, and that can lead to either tricky controllability or rapid clutch wear.

Like it or not, believe it or not, the Mazda engineers knew what they’re doing when they designed your car; the weight of the flywheel is perfect for this application. Now if you want to turn your car into a dedicated track rat, then knock yourself out and change the flywheel.

 

now that all makes sense. and i see what you mean. but i have one last question. since the flywheel is lighter does that mean it would hold less energy to keep the engine moving compared to the stock flywheel?

 

Yes, the lighter the flywheel (assuming the diameter and thickness profile of the flywheel remains constant) the lower the rotational energy (also known as angular kinetic energy), and the rougher the engine will run at lower RPMs. The only way to imbue a lighter flywheel with the ability to store the same amount of rotational energy as a heavier flywheel is to either gear it so that it runs significantly faster than the RPM of the engine, or to locate what mass there is further from the axis of the flywheel; neither of which are practical for this type of an application.