downside to running a lightened flywheel?

[sark]

Are there any disadvantages /downsides to running a lightened flywheel as opposed to a stock one?

Thanks!

[permit]

I believe they get louder as they get lighter.

[blunttech]

not any downside worth talking about. some clatter a little at idle but its no big deal. the benefits outway any noise. do it

[e30Matt]

Easier to stall if you're a noob at driving stick.

[uofom3]

no downside i can see... i love mine.. do it

[bimmer8604]

i love mine, nothing to complain about. Took like a day of driving to get used to, but no downsides that i can think of.

Kyle

[bala_eos]

A lighter flywheel would require more effort by the starter.
And possibly a slightly rougher running engine at lower rpms.

Ofcourse, how bad things are affected on depend on how much "lightened" the flywheel is.

[blunttech]

A lighter flywheel would require more effort by the starter.
And possibly a slightly rougher running engine at lower rpms.

Ofcourse, how bad things are affected on depend on how much "lightened" the flywheel is.

no it wouldnt. how is it going to put more stain on the starter? it will put less but its so miniscule its not worth mentioning

[schnitzer teil-jäger]

A lighter flywheel would require more effort by the starter.
And possibly a slightly rougher running engine at lower rpms.

Ofcourse, how bad things are affected on depend on how much "lightened" the flywheel is.

.....and pushing a minivan up a hill is easier than pushing a yugo.
.....and throwing a bowling ball is easier than a golf ball
.....and having a fat lady on our face is better than sophie moone
.....and ......

w

[sark]

Great thanks guys. I don't suppose anyone knows the weight of the stock flywheel on my '87 325is? I guess i could just bring it to a machine shop and ask them to grind it down? Thanks again?

[Tangent]

A lighter flywheel would require more effort by the starter.
And possibly a slightly rougher running engine at lower rpms.

Ofcourse, how bad things are affected on depend on how much "lightened" the flywheel is.

.....and pushing a minivan up a hill is easier than pushing a yugo.
.....and throwing a bowling ball is easier than a golf ball
.....and having a fat lady on our face is better than sophie moone
.....and ......

w

Just a theory: The added strain required to overcome the inertia of the flywheel once < the added strain of not having that same mass there to resist the engine slowing down on every compression stroke. Think of it as the difference between keeping a minivan rolling at a slow walking pace over a dozen speedbumps vs doing that with a light car that lacks the inertia to easily go over the speedbumps without losing most of it's speed.

In any case the difference there is probably irrelevent, as has already been stated. He does have a point about a rougher running engine if it's severely lightened though...

[bala_eos]

The starter cannot do its job without a flywheel. It spins up the flywheel and uses the inertia of the flywheel to crank past the compression. Without a flywheel, you would need a monster of a starter to crank the engine. So, if you reduce the mass of the flywheel , you will reduce its ability to store the inertia energy. A lightened flywheel will be able to give you lesser assistance at starting. The additional torque will have to be supplied by the starter motor. If the flywheel is lightened just enough that the starter motor is still operating within its design limits, it will not be an issue, ofcourse.

The same "lesser inertia energy from a lightened flywheel" will cause the rough running at low/idle speed. The flywheel absorbs/stores a bit of the peak energy at combustion and supplies it for the next compression stroke. A lightened flywheel will not be storing/supplying as much as a stock flywheel resulting in rougher running.


For rougher running engines, check out the following reference :

http://e30m3performance.com/now_showing/e2_flywheel/

From http://www.bmwcca.org/Roundel/2002/0...ureStory.shtml :

This time around, I pay close attention to how quickly the motor revs. Sometimes one of the most challenging aspects of driving a true highperformance race car is getting it going from a stop; the flywheel is so light it is very easy to stall the engine, and it can be very frustrating. But just as a light flywheel doesn?t have the inertia to keep a car running at low revs, it also lets you over-rev in an instant. Adriver really needs to be aware of this at all times. Other teams just put a warning shift light when you are getting close to the rev limiter. PTG put a series of six small LEDs on the very top of the dash, giving information back to the driver using peripheral vision so you don?t have to take your eyes off the track just more of that attention to detail.

A good read on the merits of running lighter flywheel topic :

http://www.nissanperformancemag.com/march03/ask_sarah

Q: Many people say that lightweight flywheels can save power and increase performances (eg. on 300ZXTT: "Each lb. Of mass taken off the crank/eccentric shaft is worth approximately 2.7 hp"). OK, I know that the lower the weight of the car, the best the ratio weight/power. But with lightweight flywheels, you can save only a few pounds and the car is around 3000 pounds so the driving performances should be very low... I read that rotating mass has more important impact than others. Is that right? And why?

Sarah: I should have never ditched those physics lectures back in college. Although a lightened flywheel does not increase horsepower, you've got the right idea- decreasing mass and rotational inertia will free up power quicker. Inertia is the property of matter resisting a change in motion. The flywheel resists a change in the speed of rotation of the driveshaft, consuming power during acceleration. A flywheel has mass and inertia. It takes torque, the rotational equivalent of force, to start and stop its turning. The rotational equivalent of mass is the moment of inertia. The more mass an object is, the harder it is to get it to start or stop moving. Moment of inertia depends on the mass of an object and where it is concentrated close to the axis of rotation. Think of the flywheel as a disc. This oversimplifies the properties of a flywheel since a flywheel is not a homogenous mass but thicker in the center than the edges. However, it will make calculations easier to understand and still offers a good estimate. The formula for the moment of inertia, I, is: I=0.5mr^2. Holding r constant since the size of the flywheel is not changing you'll notice mass has a large effect on the moment of inertia. Substitute rotational equivalents for straight-line motion variables in Newton's second law of rotation to determine the amount of force exerted on an object. Use torque for force, moment of inertia for mass, and angular acceleration for acceleration.



If moment of moment of inertia (mass) decreases, less torque is needed to keep the acceleration variable the same. If the same torque is applied, the angular acceleration increases. Now that you've gotten the Physics 101 lecture, let's take this to a more advanced level course. Let's apply this to the 1993 Sentra which weighs about 2,500 pounds and makes about 140 horsepower at 6400 rpm in 3rd gear. Using 3rd gear and final drive ratios, the 3rd gear wheel rpm is 1192 (6400 rpm x 1.286 3rd gear ratio x 4.176 final gear ratio). Calculating for 3rd gear torque and assuming a stock tire radius of 175mm (0.574 feet), this gives a forward rate of acceleration of 0.43 g, or about 13.8 ft/sec^2. Angular acceleration, or how fast the rate of rotation of the flywheel is changing, is 24.1 rad/sec^2 (linear acceleration/tire radius). Multiplying this by 3rd gear ratio and by the final drive ratio, the flywheel is accelerating at 129.4 rads/sec^2. The stock flywheel weighs 18 pounds and is about 0.365 feet in radius, giving a moment of inertia of 0.0434. Flywheel torque is the flywheel angular acceleration times the moment of inertia, or 4.8 ft/lbs and the power consumed by the flywheel is torque times angular velocity, or 5.86 hp. Changing the flywheel weight to a 9 pound lightened flywheel from Unorthodox Racing while holding all other variables constant, the flywheel only consumes 3 hp, freeing up 3 hp in 3rd gear. A lightened flywheel frees up more power in the lower gears since it is harder to begin momentum that to continue it. Solving for 1st and 2nd gears, the lightened flywheel frees up 16 hp in 1st and 6 hp in 2nd. Gains in the higher gears are negligible, 1.7 hp in 4th and 1 hp in 5th.

A car with a heavier stock flywheel will obviously benefit more from a lightened one. The Spec V flywheel weighs in at almost 29 pounds, a heavyweight in the Nissan lineup. Switching to the 12 pound Jim Wolf Technologies flywheel will free up 16 hp in 2nd, 7 hp in 3rd, 4 hp in 4th, 2.5 hp in 5th, and 2 hp in 6th. Amazing, the stock QR flywheel consumes 50 hp more than the lightenend one in 1st gear! It revs happier and has more pickup than a Playboy bunny at a Wall Street bar during happy hour! This engine is already a torque monster so invest in sticky tires to make the most traction of that power.

Since a flywheel absorbs some of the energy generated by the engine during acceleration, lightening the flywheel increases the torque on the driveshaft since less energy is used to accelerate it and more is used to turn the drivewheels. Keep in mind it's not just how much mass is removed from the flywheel but where it is removed. The concentration of mass affects how much resistance the flywheel offers to changing rpm. Rotational inertia increases with the square of the distance of the axis or rotation. Removing weight on the edge of the flywheel is 4 times as good as from the midpoint between the center and the edge. Finally, a flywheel will not show much of a gain in horsepower on a dyno because it doesn't increase fuel or air and can't increase horsepower. Remember, you're not actually improving your car's power, just how quickly it can get to that power. An inertial dyno will show gains by the engine reaching a predetermined rpm sooner with a lightened flywheel.

- Bala

[aaron_silva]

I dont think the mass of the flywheel is what makes it easier to turn the enigne over. Just thinking conceptually, I would think the the radius of the flywheel makes it easier. The larger the radius, the more torque (moment). It is not like the starter gives the flywheel a "nudge" and the inertia takes over. Reguardless of mass, the amount of torque required to spin the flywheel will be the same.

Aaron

[Stu]

My starter spins over very quickly with the aluminum flywheel, compared to my old dual-mass. I don't have much experience with a lightweight flywheel in a stock-motored E30, but the difficulty of driving the car smoothly becomes a factor. Load changes become much more abrupt and it takes getting used to.

[gstuning]

I helped out with a flywheel swap in a M52B23 engine last weekend

it´s a 4kg UUC one and with a M5 clutch,
idle was perfect, owner wasn´t used to a good clutch or a light flywheel so stalling was happening,

It is proving to be very popular and is very effective