Planet-9 Porsche Forum banner

21 - 40 of 51 Posts

·
Registered
Joined
·
792 Posts
I didn't say horsepower changed. Of course engine is an engine is an engine. All I said is that I agree with WV that lightweight flywheel should and will make measurable difference in dynamics of the car.

Re-reading your post, I agree with you that acceleration may be changed, disagree with your example, and WW's whole point was that there was more power. There isn't. Lots of things other than power go into acceleration.
 

·
Registered
Joined
·
1,069 Posts
As someone who has stalled his 981 more than a couple of times coming off-idle leaving traffic lights on an incline, backing into the garage, etc., I suspect I'd be the one to find the downside to a lightened flywheel -- I think I need that extra inertia sometimes.

I've enjoyed this mod on other cars, however.
 

·
Registered
Joined
·
221 Posts
It seems you're all wrong. That quicker spin up is a function of unicorn horn dust and high octane rainbow cupcakes.

Unsprung weight and rotational mass are similar to the concept of running smaller diameter wheels. I do question whether the change in weight could upset the engineered counterbalance and lead to excessive bearing wear.
 

·
Caymudgeon
Joined
·
710 Posts
I wonder why Porsche didn't install from the factory the lightest flywheel possible in a sports car like 981? If aftermarket 11lbs does great and brings no stall issues...
The downside of LWFW is increased wear and reduced engine life. With the engine able to rev more freely, more stress is placed on all of the internal components. I'm not sure how well the 9A1 engine can handle the LWFW but on M96 and M97 engines, there is a very real increased risk of engine failure. At least one prominent engine re-builder for M96 and M97 engines cautioned heavily against using a single mass LWFW in my rebuild. He essentially said he wouldn't give me any warranty if the rebuild was done with a LWFW. On the GT4, I'm sure Porsche has been very careful in their design of the lighter flywheel and matching it to other engine internal coomponents. Swapping to LWFW on a stock 2.7 or 3.4 engine will make the car lot more responsive and fun to drive, but it will also increase risk of premature failure and void your engine warranty. For me, LWFW is great in a track car but not something I will do in a street car with remaining factory warranty.
 

·
Registered
Joined
·
4,711 Posts
Re-reading your post, I agree with you that acceleration may be changed, disagree with your example, and WW's whole point was that there was more power. There isn't. Lots of things other than power go into acceleration.
I actually think that the link you posted backs up my claim. I'm not saying it adds hp. As you mentioned... that's not the right measurement. I'm saying it adds hp AT THE WHEELS.

From your link: This gear-dependent gain is also another reason why a typical 4th-gear dyno pull may not show a significant difference - the calculations show that little more than 4hp would be detected

That says that an increase (while small) WILL be detected, but in context it's saying that it will be much more noticeable in the lower gears due to the constant effect playing with the gear multiples.
 

·
Registered
Joined
·
4,711 Posts
As someone who has stalled his 981 more than a couple of times coming off-idle leaving traffic lights on an incline, backing into the garage, etc., I suspect I'd be the one to find the downside to a lightened flywheel -- I think I need that extra inertia sometimes.

I've enjoyed this mod on other cars, however.
It's worth noting that there's more to this inertia than just preventing stalling, too.

In a straight pull, I'm not sure whether the use of power to spin up the flywheel isn't offset by the transfer of that power when you drop the clutch in a gear change. When you launch from a stand still, and in every up shift after that, you've got that flywheel spinning and transferring the inertia to the transmission. The drag of the wheels through the transmission brings the engine speed down to where you need it for the next gear... but that energy in the flywheel does the opposite and serves to push the car that wee bit faster.

Just throwin' it out there as the other side of the balance the engineers need to deal with when designing these things. :)
 

·
Registered
Joined
·
792 Posts
I actually think that the link you posted backs up my claim. I'm not saying it adds hp. As you mentioned... that's not the right measurement. I'm saying it adds hp AT THE WHEELS.

From your link: This gear-dependent gain is also another reason why a typical 4th-gear dyno pull may not show a significant difference - the calculations show that little more than 4hp would be detected

That says that an increase (while small) WILL be detected, but in context it's saying that it will be much more noticeable in the lower gears due to the constant effect playing with the gear multiples.

Nice cherry-picking! You took that from the section where they are discussing virtual power gains -- not real ones. Earlier, when they are discussing real horsepower they say "If we measured the power output of an engine first with light flywheel and then again with the standard part on an engine dyno, no change in power will be seen to occur. "

You can argue that a LWF doesn't put more HP at the flywheel but does at the rear wheels. But that is totally illogical. Think about it for a minute. HP is measured at either the flywheel or rear wheels. Once you measure at the flywheel with the LWF, the only -- ONLY -- difference is because of the drivetrain (i.e., anything downstream from the flywheel), not the flywheel. You already measured at the flywheel. The effect of the flywheel has been measured! It can't be measured twice, can it?
 

·
Registered
Joined
·
706 Posts
This isn't that complicated.

Any part that reduces drivetrain losses will appear to make power on a dyno. I think that WW isn't really saying that the flywheel is actually generating wheel horse power (and certainly not BHP), rather, that it is reducing drivetrain losses resulting in a higher final output of power to the wheels. Which is true. You can throw on lightweight wheels and carbon ceramic rotors and I guarantee the dynograph will show higher output to the wheels relative to heavy wheels and steel rotors. And at the end of the day, WHP matters more than BHP.
:)

edit: to be clear, I am speaking strictly when testing on a 2wd dyno, not an engine dyno.


Nice cherry-picking! You took that from the section where they are discussing virtual power gains -- not real ones. Earlier, when they are discussing real horsepower they say "If we measured the power output of an engine first with light flywheel and then again with the standard part on an engine dyno, no change in power will be seen to occur. "

You can argue that a LWF doesn't put more HP at the flywheel but does at the rear wheels. But that is totally illogical. Think about it for a minute. HP is measured at either the flywheel or rear wheels. Once you measure at the flywheel with the LWF, the only -- ONLY -- difference is because of the drivetrain (i.e., anything downstream from the flywheel), not the flywheel. You already measured at the flywheel. The effect of the flywheel has been measured! It can't be measured twice, can it?
 

·
Registered
Joined
·
258 Posts
Guys, guys, guys! This is all really very simple. Don't make such a big deal about it.
A lighter flywheel just reduces the effective mass (static and rotational) of the vehicle. F=ma.
Less mass means faster acceleration. Horsepower has nothing to do with it.
There was a similar question about lighter wheels/tires. Same response.

However, the lighter flywheel will change the harmonics of the crank; and the harmonic damper is designed to absorb the harmonics which potentially could destroy the crank. I sincerely doubt that a small change due to the lighter flywheel will do any damage.
 

·
Registered
Joined
·
706 Posts
Guys, guys, guys! This is all really very simple. Don't make such a big deal about it.
A lighter flywheel just reduces the effective mass (static and rotational) of the vehicle. F=ma.
Less mass means faster acceleration. Horsepower has nothing to do with it.
There was a similar question about lighter wheels/tires. Same response.

However, the lighter flywheel will change the harmonics of the crank; and the harmonic damper is designed to absorb the harmonics which potentially could destroy the crank. I sincerely doubt that a small change due to the lighter flywheel will do any damage.
That's exactly what a dynomometer measures. How fast a drum of a known mass is being accelerated. So if you are able to accelerate the drum faster because the mass has been reduced, you will see an increase in power according to the dyno.

Lets just say the accelerative capabilities of your vehicle should increase with a LWFW. :p
 

·
Registered
Joined
·
1,405 Posts
I beleive you can thank the Hill Hold function for that.

As someone who has stalled his 981 more than a couple of times coming off-idle leaving traffic lights on an incline, backing into the garage, etc., I suspect I'd be the one to find the downside to a lightened flywheel -- I think I need that extra inertia sometimes.

I've enjoyed this mod on other cars, however.
 

·
Registered
Joined
·
221 Posts
Guys, guys, guys! This is all really very simple. Don't make such a big deal about it.
A lighter flywheel just reduces the effective mass (static and rotational) of the vehicle. F=ma.
Less mass means faster acceleration. Horsepower has nothing to do with it.
There was a similar question about lighter wheels/tires. Same response.

However, the lighter flywheel will change the harmonics of the crank; and the harmonic damper is designed to absorb the harmonics which potentially could destroy the crank. I sincerely doubt that a small change due to the lighter flywheel will do any damage.
You said the same thing I did, but you went like this *waves hands* (see old boardroom FedEx commercial for context)
 

·
Registered
Joined
·
4,711 Posts
That's exactly what a dynomometer measures. How fast a drum of a known mass is being accelerated. So if you are able to accelerate the drum faster because the mass has been reduced, you will see an increase in power according to the dyno.
This is the point of contention here. I believe this is true. Others believe it is not true. None of us has come up with any substantive evidence one way or the other. :D
 

·
Vendor
Joined
·
699 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. The factory doesn't use them because they fear noises and harmonic vibrations will annoy owners like it did with the GT3RS. Mark's car with the super lightweight single mass aluminum wheel made more torque than any other 981 conversion to date.

All 9A1 DFI motors have a harmonic balancer / crank pulley as one so the harmonics of the new motor are more equipped to handle this upgrade than anything from the M97 era.

I think this is one of the best bang's for the buck and it will be one of the first upgrades I put on our GT4. We recommend them for every track car we build.


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
44 Posts
Thanks for the insight, BGB. From your standpoint, are the NVH effects too significant to justify use on a daily driver?
 

·
Vendor
Joined
·
699 Posts
Thanks for the insight, BGB. From your standpoint, are the NVH effects too significant to justify use on a daily driver?

I don't think so. The problem is that the only way to increase torque with these cars is via headers, tuning and intake. After a point there is no way to get more torque UNLESS you start dropping weight from rotational items. You are just trying to speed up the time in which it takes things to get spooled up and up to speed. I don't think it's too much for daily driving which is why i want to do it by itself first to quantify the noise, drivability, etc. on a stock car. Mark W. chimed in above though and as he mentioned, we did his with the stock clutch disc because i was hesitant to have him invest in a sprung hub disc design not knowing whether or not it would get rid of the noise that the car makes at idle. He has a mild chatter like a Mini Cooper that i used to own...in neutral at idle, it would chatter. Once you push the clutch in, it would go away. The reason for this is that the dual mass OE flywheel dampens this vibration that comes via the gears in the gearbox. Since then we have tried a sprung hub clutch disc design and were successful in creating something for the street that did not have the chatter because the dampening is done in the clutch disc via the springs in the hub. Mark didn't mind the chatter because his 3.8L 981 with its short gearing roasts the tires under acceleration. He was all about the torque and didn't mind the accompanying noise. We have a solution now though so it's not a concern any longer.
 

·
Premium Member
Joined
·
1,847 Posts
BGB: So the factory LWFW in the GT4 is not light enough, or you don't like the design?
It reminds me of the GT3RS, but not my 987 with the Aasco, which made a LOT of noise (people would sometimes ask if I had something wrong with the gearbox!)
Thanks
 

·
Vendor
Joined
·
699 Posts
It's a few lbs lighter than the 22lb 991 3.8L flywheel but I don't have any concerns with the design. I have a target power output in my mind for a GT4 of somewhere between 450 and 470 and I think the flywheel is added cushion.


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
1,966 Posts
The M9X engine's crankshafts were reportedly made from powdered metal construction and the dual mass flywheel was important to them to keep them from breaking due to harmonic vibration. Looking at the GT4 SIT, it calls the GT4 crankshaft 'drop-forged', which should tolerate a lightened flywheel better, assuming that 'drop-forged' is not a Porsche euphemism for powdered metal.
 
21 - 40 of 51 Posts
Top