Planet-9 Porsche Forum banner

41 - 51 of 51 Posts

·
Registered
Joined
·
128 Posts
PDK also comes with a dual-mass (ie 2-piece) flywheel.

In case of the GT4, which is manual only, it comes with a 2kg (4.4 lb) lighter dual-mass flywheel. It is still pretty heavy when compared with a single-piece flywheel. Not sure if this can be fitted to the PDK.

In case of the 991 GT3-RS, which is PDK only, it also comes with a lighter dual-mass flywheel. But I have not seen the exact weight saving on this. Not sure if this can be fitted to the PDK of 981.

Most after-market light-weight flywheel is 1-piece, which is much, much lighter than the OEM dual-mass flywheel (at about 27 lb if I remember correctly).

Of course an 1-piece flywheel will give much faster acceleration (due to much lower inertia), but it also will be much rougher. The engine is prone to stall, not just because the flywheel is now much lighter, but the mapping of the ECU is NOT designed for a flywheel so much lighter.

I am also dying to lighten up the massive dual-mass flywheel. It seems the best bet would be:

- for manual transmission, see if the GT4 dual-mass flywheel can be fitted; and

- for PDK, see if the GT3-RS dual-mass flywheel can be fitted.

Even if the lighter dual-mass flywheel can be fitted, the difference (a 2 kg weight reduction) will probably NOT be dramatic. The difference would be much, much bigger, if a single-mass light-weight flywheel is fitted, but then there will be many more other potential problems and roughness.

Does anyone know if the GT3-RS dual mas flywheel can be fitted to my 981 with PDK?
 

·
Registered
Joined
·
190 Posts
I have an Aasco Aluminum LWF in my 981. There is no harshness. It as smooth as the stock one. I have zero drivability issues. I love it.
 

·
Registered
Joined
·
324 Posts
I think you guys get the big picture but if you want to quantify it, it's in the torque. Torque is a measure of acceleration. Once the engine is spun up and it crosses the TQ/HP threshold it's already moving which is why no one thinks that they increase power. On a dyno, since torque is used to measure power, the benefit can be seen below 5K RPM.
What's the TQ/HP threshold? (Asks the guy who uses metric/SI units)
And what does 5k rpm have to do with anything?

Does the gear ratio make a difference? (He asks knowing the answer is a very strong yes)
 

·
Registered
Joined
·
324 Posts
A lightened flywheel does not increase power or torque, it reduces rotational mass.
yes this makes acceleration easier, on the engine side of the gearbox.

Depending on how a dyno works it may or may not show up.
if the dyno measures time to accelerate, then yes it will look like an increase in both torque and power. But so would changing weight of wheels. How much would it show up? Probably very little since the drum it's spinning up should have a lot more inertia than the drivetrain components.
if the dyno can do a static measurement i.e. At steady revs and speed, then it wouldn't show up at all.

Rotating parts like wheels are often said to "count their weight twice" since in accelerating you need to add energy to incense their rotation and then also energy to increase their speed as part of the car. So their mass counts to the vehicle mass and to the rotating mass.
the higher the speed, the harder it it to accelerate.
Kinetic energy is 1/2mv^2
so more energy is needed to accelerate as your speed increases.
the same principle applies to rotational kinetic energy.
so wheels and tyres are harder to speed up rotation the faster they spin.
Reducing overall weight, or better, rotating weight, will help acceleration and make the engine feel more responsive.

A fly wheel is similar to wheels and tyres in adding to rotational inertia.
But the big difference is that it's rotation is proportional to engine revs, not vehicle speed.
When you accelerate but change to a higher gear, engine revs drop. This means that it's now easier to spin up the flywheel compared to the wheels. As the vehicle speeds up the wheels are suffering v^2 effects, but the flywheel is only suffering rpm^2
So a lighter flywheel makes the vehicle accelerate better. Similar to reducing overall weight or lighter wheel.
But since it's at engine revs, not road speed, it's mostly noticeable in 1st gear. In higher gears it's much less effective since v^2 road speed effects start to dominate.
It does also have some other possibly useful properties during gear shifts (of manuals mostly) in that it's easier to the engine to change revs while clutch is disengaged. So revs drop quicker, but should also increase quicker/less throttle for a downshift blip.

Overall you might get a better improvement in performance by lighter wheels.
Not just lighter, but less rotational mass (lighter further out from the centre)
But if you have light wheels and taken excess weight out of the car, then the flywheel can be another place to reduce inertia.
As long as you understand that it will be most effective in lower gears and have very little effect by 5th or 6th gear.
 

·
Registered
Joined
·
61 Posts
Any tips or links to replacing the 981 flywheel? Can you pull just the transmission?
 

·
Registered
Joined
·
61 Posts
I finally got around to installing a lightweight flywheel. I'm a bit disappointed. Even though it shaved 11 lbs off the flywheel and clutch setup (from 39 to 28 lbs total), the engine does not rev that much quicker. Perhaps an engine tune would take more advantage of it. There's also more mechanical noise which seems odd considering it just spins. The dual mass flywheel has a lot more give in it than I would have thought so I guess it's doing a lot of work to quell the vibrations back there. I guess if I had an exhaust and other performance mods, I'd be happier with the change, but being pretty stock I'm not impressed. Perhaps going with just the slightly lighter than stock GT4 flywheel would be better on an otherwise stock car.
 

·
Registered
Joined
·
994 Posts
I wished I had read this thread last month. I had my clutch kit replaced when I had a Wavetrac ATB LSD installed. Cantrell Motorsports mentioned rattling/chatter noise from a LWFW and clutch plate, so I passed as 80% of my driving in daily.

David713 could you give us a long term after market review on your experience with your new lightweight flywheel/clutch package. Would you do it again or stick with a dual mass flywheel if the clutch disk needed to be changed. Thanks,
 

·
Registered
Joined
·
479 Posts
Take a look on Jake Raby's site about crankshafts exploding. He concluded it was from LWFW and NON-sprung clutch setups. IIRC when I did mine I looked up how the GT3 was. LWFW with a sprung clutch. That still allows for driveline vibration absorption. That is my understanding of all of this.
 
41 - 51 of 51 Posts
Top