Lightweight Flywheel and Crunchy Gears, A nice upgrade when changing the clutch Options

[cobas]

After a month or more of driving with my new lightweight flywheel (and new stock clutch)... I like it.
I got a Chro-moly flywheel off eBay for about $200. That weighs 9.6 lbs, compared with 21 lbs for the stock one, according to the seller, and the cost is similar.

Anyway, since there are several (old) threads discussing lightweight flywheels and one or two say it makes the car hard to drive smoothly, I wanted to disagree. Maybe with a stronger clutch it would be an issue, but with the OEM clutch, a 10lb flywheel is subtle. I can drive smoothly in stop-and-go traffic, upshifts are smoother, downshifts are rev-matched as well as ever and acceleration feels a bit better.

Honestly it's hard to compare stock-to-new directly because the clutch changed too, and the even the clutch master cylinder was replaced. After years of complaining about the 2nd and 3rd gear synchros and having to shift very slowly into 2nd... it's better now. I haven't crunched a gear since, not even the 1-2 change in a hurry. So maybe the clutch hydraulics were the problem more than the synchros.

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For newbies, the flywheel is a significant portion of the engine's rotational mass. Heavy flywheels give you more inertia, which is good if you're trying to start off without stalling the engine, but pretty much otherwise bad. The difference matters whenever RPMs have to change quickly, like accelerating in 1st and 2nd gears and when changing gears. In terms of acceleration, the gearing multiplies the importance of flywheel weight, so in 1st gear, weight on the flywheel and other engine internals matter 10 times more than weight in the trunk.

I found this document doing the math of how a flywheel affects performance (looks, good, I didn't check it) on an M3:
http://www.uucmotorwerks.com/flywheel/flyw..._mass_calcs.pdf

The numbers change a little for my little Elantra but: I calculated 307.5 mm tire radius, gearing of 13.216, 7.483, 5.074, 3.869, and 3.066, a flywheel radius eyeballed at 100mm and stock and lightweight flywheels of 21lbs and 9.6 lbs.

1st gear: Difference x 10.2 = 116 lbs
2nd gear: Difference x 4.0 = 46 lbs
3rd gear: Difference x 2.4 = 27 lbs
4th gear: Difference x 1.8 = 21 lbs
5th gear: Difference x 1.5 = 17 lbs

It's a lot easier to cut 10lbs from the flywheel than 116 lbs anywhere else. But just as important linear acceleration is the way the shifts feel. When you upshift quickly, the revs have to drop suddenly when you let out the clutch again - that feels smoother and quicker with the new flywheel. Fast upshifts don't shake the whole car as much and downshifts can be rev-matched a little faster and are also a bit smoother if you don't match quite right.

So, IMO, a totally worthwhile upgrade. You're spending $800 on clutch parts and labor, or your whole weekend on the floor, the cost of upgrading the flywheel while you're at it is neglegible.

[only1db]

agreed!! i wish i had the money when i did my clutch to get the flywheel. I installed a new tranny on my buddies tib and while we were there we did a clutchmasters stage II and a fidenza...IT WAS A HUGE DIFFERENCE!!!

there are alot of neigh sayers out there....but i too want to get rid of that heavy freaking flywheel...it will be interesting to see how the MPG's go...

[Bobzilla]

HAving both, and going back to the "heavy" one, there was no mpg difference. My fastest 1/4-mile run is still with the stock flywheel.

Glad you're happy, I will never recommend one to anyone again.

[cobas]

Glad you're happy, I will never recommend one to anyone again.

Why not? I don't remember what your experience was but I know you're not a fan.

[silet]

Bob, what stage clutch did you use with the flywheel the time you didn't like the results?

[Bobzilla]

"Stage 2" which was a joke. 11lb flywheel. Still had hanging revs, so that didn't help. Chattered like a mother. the disc hub was almost seperated from the disc likely because of the chattering in 30k miles. Starting from a stop required a lot more clutch slippage and more RPM's to get a smooth launch, so you used just as much (if not more) fuel, so it did not help mileage. It did not help in the 1/4 mile. It DID make daily driving in stop-n-go traffic much more... tiring.

[cobas]

Ok. Well, I'd blame the clutch; I have no problem driving in traffic.

[silet]

^^^ I would to tend to think so. I will consider a lightweight flywheel when I'm due for a clutch.

[Bobzilla]

the chatering came later on.... not immediately. Likely from the super thin metal warping because of the extra heat from the required extra slippage to make it move. Once the chattering started, it was a fast downhill slide into a painful driving experience.

[silet]

Then I'll wait and see how cobas' flywheel fares

[cobas]

It may last a long time now that I have another flywheel... with two clutches on it.

[silet]

You got the GTI????