So the flywheel "stores" HP?? Maybe for use at a later time or date?

I don't think so.

There are 2 measures of HP. At the engine, and at the wheels. The measurement at the wheels is always less because some HP is being used to move the flywheel, transmission, driveshaft, rear end, rear axles, wheels and tires. I remember somewhere seeing this number quoted as around 13 percent loss on a FD. Don't quote me on that.

 

Its a nice mod,
i'd wait till you need a clutch, then do the whole deal, because there is a fair amount of labor involved.

 

Crap, I can't believe I didn't get one...I cheapened out :/

 

This site talks about "storing" hp:
http://www.dim.com/~mackys/z/mods/flywheel.html
This site talks about the hp or no hp debate:
http://www.geocities.com/MotorCity/7177/flywheel.html

 

The flywheel does store energy, and it is probably more than you would expect. I calculated that it took 50 HP less to accelerate a light flywheel than a stock flywheel through first gear at full throttle. Of course, energy and HP are two different things, but 75 HP for two seconds is a good bit of energy, I would imagine. You use the stored energy when you shift -- it is converted to heat in the clutch and some forward motion for the car.

The amount of power required depends on how much rotational inertia the flywheel has (closely related to its weight) and how fast you need to accelerate it from one RPM to another RPM. The power lost to flywheel inertia is greatest in first gear and quickly drops off in higher gears because it takes a lot more time to rev in higher gears.

It probably takes less than 0.1 HP to keep a flywheel spinning at 8000 RPM; all you are fighting there is a little friction. It takes around a thousand times as much to accelerate it from 3000 to 8000 RPM in 2 seconds because you are up against the considerable inertia of the flywheel.

The drivetrain loss percentages are just a rough estimate. In reality, some of it is inertial, some frictional, etc. Because of the inertia part, the real losses will be quite different from gear to gear. But 13% or so seems like a reasonable estimate for a third or fourth gear dyno run, as evidenced by the dyno plots of cars with known flywheel HP quantities. But again, it is just an estimate, so be careful not to mistake the estimate for what is really going on.

There have been some eco-friendly car designs based on the principle of using a (big, heavy, fast-spinning) flywheel as an energy store. You "charge" the car by getting the flywheel spinning really fast, and then you use the energy stored in the spinning flywheel to drive the car around. As you convert the flywheel's energy to forward motion for the car, the flywheel slows down. At some point, the flywheel will need to be "recharged" by using an outside power source to accelerate it up to a higher speed.

-Max

 

It doesn't store HP. The weight of a heavy flywheel acts to keep the engine from moving slower or faster at a rapid rate. The inertia (not HP) of a heavy flywheel helps keep the engine turning when you let out the clutch, making it easier to move the car when it is stopped.

 

yes, lighter wheel will give you more HP at the weel, i have a seen a dyno sheet of a car in a magazine (can't remeber which one) and there was 4 hp at te wheels of difference for the same tires, same size, only one set had wheels 6 pounds lighter each.

i'll check if i can find it back

 

Let me play engineer for a minute (if you can stand it)

A flywheel is a kinetic energy storage device.

Kinetic Energy = 1/2*I*w2 , where
I = moment of inertia (ability of an obeject to resist changes in its rotational velocity)
w = rotational velocity (rpm)
I = k*M*R2 (M=mass; R=radius); k = inertial constant (depends on shape)

Inertial constants for different shapes:
Wheel loaded at rim (bike tire); k = 1
solid disk of uniform thickness; k = 1/2

A lighter flywheel has much less inertial. Racing Beat lists their aluminum flywheel as being 55% lighter and 80% less inertia.

The moment of inertia of an object is a measure of how difficult it is to start it spinning, or to alter an object's spinning motion. The moment of inertia depends on the mass of an object, but it also depends on how that mass is distributed relative to the axis of rotation: an object where the mass is concentrated close to the axis of rotation is easier to spin than an object of identical mass with the mass concentrated far from the axis of rotation.

You can figure out the rotational equivalent of any straight-line motion equation by substituting the corresponding rotational variables for the straight-line motion variables (angular displacement for displacement, angular velocity for velocity, angular acceleration for acceleration, torque for force, and moment of inertia for mass). Try this for Newton's second law:

F=mA

Replace force by torque, m by I, and acceleration by angular acceleration and you get:

T=I a

So I would say that a lower inertia flywheel has less torque for a given angular velocity (rpm)

Horsepower is:

Hp = rpm * T (ft-lb) / 5252

so a lower torque at the same rpm will give you less horsepower. So a lower inertia flywheel has less stored kinetic energy as Hp at a given rpm.

I think we need to rewrite the torque equation to look at this another way:

a=T / I

so a lower inertia flywheel has a higher angular velocity at the same torque. Given that the motor makes a certain amount(constant) of torque at a given rpm, a lighter flywheel will have a higher angular velocity.

If you go and substitute this back into the first equation for straight line motion, you get more force for the same mass (the car weight).

A lower intertial flywheel is easier to change the angular velocity. This is what makes it easier to bog off the line (easier to drop rpm). There is less torque available from the flywheel and more is needed from the motor. Once the motor no longer needs (doesn't bog - rpm, hp, torque drop) the stored flywheel inertia, the motor must use it's torque to increase the angular velocity. This is easier to do with a lower inertia flywheel so you have greater acceleration.

Motor Hp is usually measured at the flywheel. It looks from the equations above that you have less torque when you decrease inertia at the same angular velocity. So it would seem at a given rpm, the motor\flywheel combo has less torque.

This type of torque at the same rpm is only good for a motor which is only used this way - maybe a generator or turbine. Cars need to change rpm and I would think that the ability to change angular velocity would cancel out the need for more torque at the same rpm.

Does seem like a circular argument at times - anyone else want to take a shot?

 

I'm sorry, my terminology was not correct. flywheels store energy, not power, my mistake.

but yes, it does store ENERGY for use at a later date.


ok, for the magazine that tested the hp gain of a flywheel. What kind of gearing/power did they have?
I could see that a high power engine in a lower gear would make it appear that there is more hp gain over a heavy flywheel. so in a tall gear, where acceleration is not as great, the flywheel would have less of an effect on the "measured" hp. Therefore a more accurate reading.

 

I would like to respond to that, but I do not have my interpreter with me.

 

A flywheel acts to maintain the present inertia, or rpm of the engine. A heavier flywheel will make it more difficult for the engine to slow down or speed up. An engine with no flywheel might rev uncontrollably, or die instantly with no throttle.

 

I think I screwed up a little.

a - angular acceleration, not rpm

w - angular velocity, rpm

This changes some of what I said...

The same motor torque for lower inertia would have a greater angular acceleration...etc

 

This is some of the best stuff I have read. Keep that information comming.

Thanks

 

Ok, aside from the philosophical debate .... what are some of the side effects of having a lightened flywheel? I heard a complaint about idle .... I know I've been told the clutch may slip when cold. Anything else?

 

"what are some of the side effects of having a lightened flywheel?"

May almost stall when air comes on.

Idle a little lumpier

Need to slip clutch more, esp when starting from a stop on a hill.

Revs come down slower (when air not on)

Uncontrollable accel/spinning in first gear going into second

No issues with slipping if you have a good clutch and pressure plate.

Glad you guys liked my analysis...so do we agree that the flywheel adds HP since it increase the rate of angular accel? Power is the ability to do work over time. The motor can "work" more quickly with a light flywheel...

 

I have a 9.5 lbs SR flywheel. Truthfully, it didnt make much of a difference in how fast the car revved. I didnt even really feel a difference in acceleration. I'm sure the numbers showed on the dyno, but it just wasnt worth $400+ or whatever i paid for it. Anyway, just my opinion.

 

What about re-surfacing the stock flywheel, or is there any way to lighten that? Or would the only big difference be felt with drastically getting lighter by the $$$ ones?

 

Certainly there are ways to lighten the stock flywheel, but I have read several times over the years that it can be a dangerous thing to play with. You don't want to unbalance it (balance has to match the rotors and front counterweigt) or weaken it to the point that it will fly apart and chop your legs off or fill them with schrapnel (sp?) as you rev it out to high RPM and slam a shift. It isn't worth the risk.

You can resurface the flywheel to give it a new friction surface, but that doesn't significantly change the rotational inertia.

The best thing to do if you are on the fence about the change is to drive someone's car that has a light flywheel. I love the way it feels with the light flywheel. And if drag racing is your thing, it is probably best to stay with the stock flywheel.

-Max

 

I say, get the damn 9.5lb flywheel. Philosophies and engineering aside, I am glad I didn't listen to the nay Sayers, even my friend who is a RX-7 mechanic.

I am a relative layman and saw improved revving, better response and an overall better connection to my car. The idling thing is so insignificant it really didn't even register in my grievances. In my mind it made my car feel more like a modified car rather than feeling like it was made for production.

I almost bought lightened steel racing beat flywheel but bit the bullet and got a SR package clutch flywheel package. Although I wouldn't tout it as the best bang for the buck (close to 1000 dollars), I really don't know how I got around without it. It brings your car to better level of performance. I think it has something to do with all of these conjectures of our fellow forum members.

Needless to say, my mechanic friend now wants one (he drove my car around after he installed it and loved it). I really don't understand also, how if you race a 1/4-mile you get a disadvantage of having a 9.5lb flywheel. If you do it is only slight and probably equals out since it is transferring power more efficiently. I even called SR about this and talked to a tech. He said they use them on their drag cars. That is good enough for me.

The wheel doesn't even equate to 9.5 pounds with the counterweight so it really acts as if it weighs 13lbs (or so I was told by an authority more than myself). So that doesn't mean much other than if you are worried about being in the single digits as far as weight is concerned, you're not.

Garrett

 

Kewl. I was having second thoughts about the flywheel that's occupying space on my living room floor.

Any auto-xer impressions on the lightened flywheel? Better first gear response? Easier to keep in the power band?

 

A light flywheel is better for EVERYTHING except launching the car from a stop. The heavy flywheel has a slight advantage there.

 

I have a 11lbs flywheel from RB.

Had it for 5 years and love it.

No regrets.

Right off the bat I noticed that the car seem to have freed up HP and that has helped in 0-60 numbers.

 

adamc is right.

the lightweight feels better once the car is rolling. up and down shifting is awesome.

But off the line traffic light racing......... not the best idea.

 

Ah, just what I was looking for...a simple explanation.

 

I got a 9.5 lb flywheel made, and was told best to balance the rotors and the counter weights to have the best result and for the safety reasons too.