has anyone reduced the weight of the flywheel (i think its about 14lbs.) down to 7? an old trick but the tq and hp numbers im sure don't change but shouldn't the crank spin up alot faster?

thanks
jf

 

i'm not sure what mine went down to, i think it's 7 or 8. it was a few years ago, but i seem to recall it spinning up quite a bit quicker. not night and day, but noticeable. i also put in a kep 2 pp at the time, which supposedly has an aluminium outer (i forget what it's called), so that may have contributed some of the difference as well.

 

I'm not clear on the benefit a lighter flywheel gives.

I see how less mass means it spins up faster, but doesn't it also mean less moment of inertia to do its job? Seems as if it would be a neutral trade...

 

Especially when a turbo needs reciprocating mass to load properly.

 

Motorsports teams dont throw on a super heavy flywheel when boosted, or add mass to their cranks,rods, and pistons, etc. .... lighter is better. Your load for the turbo is traction.

 

Motorsport is one thing, road use is another one.

 

agreed.

 

The track car is down to 9.4 and my street car is 10.8 on factory flywheels.
I would guess you'd need a dyno and full data log runs to see how much of an actual difference it makes. However, in terms of getting things light, get 800# or more out of 944T with the same engine hp/torque and see how much fun you have.

 

It does also take some power to accelerate a flywheel. Digging into the depths of my brain here, but I think it works like this: Moment of inertia of a spinning disc (I)= mass X radius^2 Moment of inertia for a 6kg flywheel of .15m radius is .0675 kgm^2. Torque = I X angular acceleration. I guess angular acceleration is about 6000 RPM/S or 628 rad/s^2.

Torque = .0675 X 628 = 42 Nm to accelerate a 6kg flywheel at a rate of 6000RPM/s. Or 31 pound feet. If the engine isn't free revving, it probably isn't accelerating at 6000RPM/s so it takes less torque. With half the moment of inertia, it takes half the torque to accelerate the flywheel.

I'm not an engineer, so the guru's can pick my calculation apart now

 

Spent some time screwing around with theoretical acceleration calculations about a year ago, changing only the mass of the flywheel. This was done when PCA Club Racing allowed cars with dual mass flywheels to replace them with single mass flywheels w/o class penalties. Those of us with single mass flywheels thought we should then be allowed to use lightweight flywheels w/o class penalties - to no avail.

In the end, it does rob/take torque to accelerate a spinning mass/disc. So by reducing your flywheel mass, you are reducing your driveline inertial losses. The faster the acceleration, the more torque it takes to accelerate the flywheel, so the losses are greater in the lower gears, and become insignificant in the higher gears.

The purpose of the flywheel is to store energy to help the engine to maintain motion inbetween cylinder firing cycles. This is critical at low engine speeds, but not very at higher engine speeds. So, if you lighten the flywheel on a street car, you will notice that you have to be a little more carefull with engaging the clutch in 1st gear - especially with the A/C and/or on a hill; the engine will be much easier to kill.

For track cars, that really isnt a concern - so there is no real draw back to using a lighter flywheel, to get a little quicker reving, and save a couple hp not wasted on accelerating a heavier flywheel.

 

as stated above for the track lighten everything possible.
For the street I don't touch a lightened flywheel on a turbo car. We do have really teeny turbos but still. On my E36 M3 however it was a great improvment in the car.

 

horses for courses. it is definitely harder to manage hill starts, and it tends to bog down a lot in general (partly driving style). but it does rip through the rev range very quickly in 1st and 2nd - by 3rd i think the difference is pretty negligible.

i like it, and i don't mind the compromises. ymmv.

 

Hmmm, I had my stock flywheel lightened and was told 9 lb. was a good compromise for the street. I lightened a flywheel on a 4-banger Datsun some time ago and liked the result. Now I'm wondering if I went too light ...

 

have you driven it yet?

 

Light fly to about 9lbs, knife crank, lighter rods, (pistons, not sure?), and now 6 puk clutch. Drive this on the street and yes there is a technique or an adjustment to how you drive, especially in the lower gears as stated, but no problems getting used to it. Like anything, you adjust to what you drive very quickly. The benefits when driving quickly are way more than the negatives though. Especially with the S2 cwp. The car just absolutely ZINGS through the gears. If you just want a nice cruiser these are not the mods for you though.

The tq thing with the stock fly has always done my head in a bit. The heavier the mass is, the more tq is needed to get it to move, but once it's up and running then wouldn't it generate it's own torque through the heavier recirculating movement? My logic is patchy at best, granted...