Light Weight Flywheels Explained

[m sprank]

Any and all light weight flywheels will "chatter". It is the nature of a LWFW, along with the change in inertia.

 

Even the OEM SMFW can have some chatter/judder.

 

All Competition Clutch kits for the LGT currently use and include a LWFW. They will chatter.

 

06+ WRX (which shares compatibility with the 05-09 LGT) uses the same kits. 02-05 WRX use a different kit (which can use the OEM FW). So, for 05-09 LGT owners all current Comp Clutch offerings include a LWFW and must be used with the provided LWFW (the pressure plate will not bolt up to an OEM FW).

 

Enthusiasts considering a lightweight flywheel are often warned of "lightweight flywheel noise", or "chatter" more correctly called "gear rattle" or "gear lash noise". In making modifications to any car, there may at times be certain compromises in other areas of the car's behavior. Some may be completely acceptable, others may not. In a luxury-minded enthusiasts car like a LGT, owners are concerned about new or excessive sounds or rattles.

 

The original dual-mass flywheel does one thing very well - exhibits inertia, the resistance to changing it's state of motion. It is more difficult to change the speed of the dual-mass flywheel, whether accelerating it or slowing it down... that means besides inhibiting engine acceleration to some degree, it also inhibits the deceleration that occurs between every power pulse. The power pulse transition is smoother, therefore no rattle.

 

A single-mass flywheel is the same sort of design as a solid aluminum lightweight flywheel: a rigid disk of metal, either aluminum or steel. In the OE applications, these flywheels are equipped with a sprung-hub clutch. The sprung-hub clutch "splits the difference" between the these situations. The friction material (the disk itself) is not "hard coupled" to the hub, but has a springy nature to the assembly. It has a suspension, the springs you see around the hub. What this accomplishes is letting the power pulse couple the transmission's internal parts softly, and then de-couple softly. ("Softly" being a relative term here, it is just a few millimeters of travel, nothing that can be felt.) The net result is the difference between banging two pieces together and gently placing them together.

 

The lightened flywheel has less mass to damp the power pulse variations. The tolerances of the gears,splines and general internals are low. They are "loose". The dual mass units and heavier singles, dampen the engine pulses/harmonics through the shafts. The lighter units, dont do such a good job of it.

 

Subaru switched over to the SMFW for LGT's in 07. So using a 06+ WRX set up is the same as a 07-09 LGT set up. 05-06 were the years that got the DMFW. Most likely to save coin, Subaru decided the change was acceptable.

 

Some users have complained of light "chatter" or "judder" when using an aftermarket clutch and the OEM SMFW. Seems to be more temp related and factored by the disc choice and pressure plate strength.

 

Misfires, CEL's etc are caused by a FW that is too light. Most LGT LWFW's are in the 14lb-15lb range and are not light enough to cause this. Going to 12lbs will definitely throw a CEL.

 

Comp Clutch LWFW's are 14.5 lbs.

 

EDIT:

As the cars are 20 years old this info has been previously posted.  For those with limited search skills/reading comprehension the OEM part numbers are:

50-06 LGT 5MT DMFW = 12345AA010

07-09 LGT 5MT SMFW = 12342AA090

07-09 LGT 6MT (SpecB) SMFW = 12310AA380

 

Exedy part numbers

5MT LGT SMFW = FJK1001FW

6MT LGT SpecB/Sti SMFW = FJK1000FW

 

Part numbers can change/become superseded.

 

Edited August 26, 2023 by m sprank

[motorbreath]

This was very useful, I have an ACT HD clutch with a 13lb LWFW, explains some of the judder I get in first.

[darkstarxi]

Can anyone provide the Subaru part numbers for the OEM SMFW & DMFW? Also, are there different part numbers for the 5MT and 6MT?

[m sprank]

15 hours ago, darkstarxi said:

Can anyone provide the Subaru part numbers for the OEM SMFW & DMFW? Also, are there different part numbers for the 5MT and 6MT?

http://opposedforces.com/parts/

Yes, 5MT and 6MT use different flywheels. 

[darkstarxi]

18 minutes ago, m sprank said:

http://opposedforces.com/parts/

Yes, 5MT and 6MT use different flywheels. 

I've spent an embarrassingly long amount of time on various Subaru parts websites, including that one, trying to figure out the different part numbers to compile a list for the site and am having a tough time coming up with clear answers. I have an '07 spec.B which is the year that things changed somewhere in the middle of the year AND the 6MT, which is obviously less common. It would probably be very helpful for the masses if a list of flywheel part numbers were compiled here, but unfortunately, I'm not the man for the job and I don't want to share incorrect information.

I figure it would look something like below if anyone with a better clue than me would care to contribute..

05-06 Legacy NA 5MT: (Single/Dual Mass) Subaru Part #
05-06 Legacy GT 5MT: (Single/Dual Mass) Subaru Part #

07-09 Legacy NA 5MT: (Single/Dual Mass) Subaru Part #
07-09 Legacy GT 5MT: (Single/Dual Mass) Subaru Part #
07-09 Legacy spec.B 6MT: (Single/Dual Mass) Subaru Part #

06+ WRX 5MT: (Single/Dual Mass) Subaru Part #
06+ STI 6MT: (Single/Dual Mass) Subaru Part #

[m sprank]

The cars are 20 years old.  This info had been posted ad nauseam.  This is NOT the place for posting part numbers.  This is basic information about flywheels in general.

 

Honestly, If you cant figure out the two different part numbers for 2 LGT 5MT and 1 SpecB 6MT flywheel..... Ever heard of Google?

 

Just for you, OP (which is a copy and paste from a previous thread I posted in 2013) has been edited. 

Edited August 26, 2023 by m sprank

[darkstarxi]

You're right - this thread may not be the best place for this kind of information - or maybe it is. I happen to think it would be great info to add here, but what do I know? I'm inferior to m sprank and deserve to be talked to like I'm a moron.

With that said, I sure have heard of Google and I spent a bunch of time trying to get to the bottom of it there too and am still uncertain. I'm not one of the Facebook idiots or the kind of person who makes a post every time I have a question without spending lots of time looking for the answer(s) first - I promise. I typically find the info I'm looking for on here or on other forums via Google searches - or I figure it out on my own if I can't and share what I learned with the community. In this case, my efforts haven't yielded what I'm looking for and so I posted on the forum looking for information for myself and for others. Shame on me, m sprank...shame on me.

[darkstarxi]

Thank you for the edit. Would it have been so hard to point me in the direction of the thread you created in 2013 in the first place without being a prick about it?

Edit:

Here's what I've learned relevant to my '07 spec.B with build date 12/2006:

- Current Subaru part number for the OE single mass flywheel is 12310AA410
- This part number supersedes part numbers 12310AA280, 12310AA340, & 12310AA370
- Subaru part number 12310AA410 is for the OE single mass flywheel for the following vehicles:

- The part number that m sprank lists above as "07-09 LGT 6MT (SpecB) SMFW = 12310AA380" is for the OE single mass flywheel for the following vehicles:

- For whatever reason, the 2008 spec.B is the only vehicle listed as compatible with both 12310AA410 and 12310AA380.

Hope this helps someone.

Edited August 26, 2023 by darkstarxi

[m sprank]

Original thread:

They  are all the same. 

07-09 Spec B 6MT and 04-2021 (and probably 2023) STi all use the EXACT SAME FLYWHEEL.  Long ago (in 2008) Subaru had a different part number for the SpecB and it had a slightly different backing than the STi, but that is no longer the case.  It did not matter than either, everyone used an STi FW. 

05-09 LGT 5MT all use the exact same FW.  It is shared with sooooo many other Subies.  Chances are at this point Subaru has one part number for all of them. 

This info is all over every Subaru board.  This is 15+ year old information. 

 

Whats next?  20 threads on what up pipe fits an 4th gen LGT?  What resistor to use to turn off the CEL for the EGT sensor when you replace the up pipe in an 05 LGT?  What downpipe fits a 4th gen LGT?  That was the forum 16-17 years ago. 

 

You get jaded to the same questions for 15 years.  I thought we were passed all that.  Cars were built up, torn down, built up again and are now falling apart due to age and being restored again.  Yet here we are back at "what flywheel fits my car" like you cant call a dealership and ask. 

 

Edited August 26, 2023 by m sprank

[Boltonator]

I think some information that is missing here is that there is a difference between the Single Mass Flywheel that comes stock on Subaru cars and a solid flywheel that you would likely buy as an aftermarket lightweight flywheel. 

The stock Subaru flywheel is what is called a 'Flexible' flywheel which has a hard rubber coupling in a sandwich between where it bolts to the rear of the crank, and the friction surface that the clutch cover is bolted to. These are manufactured to a patent by Valeo and the principle is that when the power strokes are at the max through the crank there is a certain amount of flex that can affect the flywheels geometry. The coupling is tuned to minimise that distortion and smooth engagement. 

An aftermarket solid flywheel doesn't have any of that comfort built in. 

[Max Capacity]

Those "flexible" ones were the Dual Mass FW that came in a 05-06 Legacy GT with the 5mt. They also used the Torx T-50+ bolts. 

 

Here's a picture of the first Spec LWFW that was installed in my 05 GT wagon back about June 2007 I think it was. Photo is from April 2012 some 65,000 miles later.

 

 

[m sprank]

Thank you "Max".

 

See how easy it is for misinformation to spread.  Again. 

[Boltonator]

The 07-09 5MT single mass flywheel with the code 12342AA090 that is touted as the upgrade for the 05-06 sprung dual mass flywheel is a flexible type flywheel and not a 'solid' flywheel. I have a 2008 GT and its what I have.

I might have gotten the explanation a bit wrong it seems to be about vibration management and shifting it outside of operating speeds. Its not fully solid like the 6 speeds seem to be

[Max Capacity]

10 hours ago, Boltonator said:

The 07-09 5MT single mass flywheel with the code 12342AA090 that is touted as the upgrade for the 05-06 sprung dual mass flywheel is a flexible type flywheel and not a 'solid' flywheel. I have a 2008 GT and its what I have.

I might have gotten the explanation a bit wrong it seems to be about vibration management and shifting it outside of operating speeds. Its not fully solid like the 6 speeds seem to be

Are you sure, do you have a picture of that ?

 

I'm pretty sure the 07-09 OEM FW I bought was in fact a SMFW like any other. I sold it to a member here back about late 2020 after using it for 5 years. Sorry, I can't find any picture of it. But I think member jaylaw bought it. 

[motorbreath]

Not sure if this has been answered before, but if my car was tuned with a lightweight flywheel, should I replace with the same flywheel (weight or weight and design) when I replace the clutch?

[Max Capacity]

14 hours ago, motorbreath said:

Not sure if this has been answered before, but if my car was tuned with a lightweight flywheel, should I replace with the same flywheel (weight or weight and design) when I replace the clutch?

When I changed from the SPec LWFW to the stock 07-09 smfw I did not change the tune.  Can you just get a replaceable friction surface for your FW, like the one on my Spec LWFW ?

[motorbreath]

21 hours ago, Max Capacity said:

When I changed from the SPec LWFW to the stock 07-09 smfw I did not change the tune.  Can you just get a replaceable friction surface for your FW, like the one on my Spec LWFW ?

I currently have an ACT HD kit installed which I am not too happy with. I am pretty sure that its a single piece and the friction surface cannot be replaced, plus I prefer spending the extra $$$ on a new replacement versus resurfacing. I realize now that I probably should just ask my tuner, so I'll do that and post the reply on this thread since I think it is relevant.

[Max Capacity]

If you want a great setup, that Spec LWFW and 2+ clutch is a very easy set up to drive. Light paddle pressure but great hold. Call them and get exactly what you need. Great customer service. 

[motorbreath]

On 10/19/2023 at 7:33 AM, motorbreath said:

I currently have an ACT HD kit installed which I am not too happy with. I am pretty sure that its a single piece and the friction surface cannot be replaced, plus I prefer spending the extra $$$ on a new replacement versus resurfacing. I realize now that I probably should just ask my tuner, so I'll do that and post the reply on this thread since I think it is relevant.

Probably an obvious answer to most of you, but he confirmed that I could use any similar mass flywheel without tune issues.

[m sprank]

Should not need to tune for a clutch/fw. 

 

There is a backing plate on a OEM 5MT SMFW.  It is removable (always pulled the plate after marking orientation and bolt location when resurfacing).  It is not the dual mass.