Super Light Flywheel Experiences?

[bmwman91]

I will finally be getting my rebuilt-rebuilt 2.1L M42 back this week, and will be ordering all sorts of parts to do as fresh of a reinstall as possible. Since the engine and transmission are already out, I am toying with the idea of going with an even lighter flywheel. Before the head gasket went, the engine was already running an 11lb M20 flywheel and stock clutch. The idle was fine, and I am pretty sure I have been in stock M42's with rougher idles from vacuum leaks. However, I am not sure how much lighter I can or should go. The RHD flywheel is ~7lbs, with even more mass removed from the outermost sections of the diameter, so I imagine that it would be a significant change from the 11lb one (which was a machined down stock M20 FW).

The car is not a daily driver anymore (got a used Jetta TDI for that), and I doubt I will be doing any road trips in it. It will be running stock engine & transmission mounts as well, and has a full custom tune from Sssquid (including idle maps). Anyway, what have other people's experiences been with really light flywheels, ideally with the RHD ones specifically? My only concern is that the idle will be a lot rougher and be annoying as shit, and I do not want to increase the idle speed since CA SMOG testing will fail me if it is off much from the stock spec (although I could just have 2 tunes for it, one with a higher idle speed). I am well aware of how noisy the transmission will be since it already sounds like a coffee can full of rocks with the single mass flywheel, and I don't care if it gets a little noisier. Making other people at stop lights wonder WTF is going on with the car is part of its allure lol.

Also, the clutch disc, PP and TOB only have ~30K miles on them and look to be in great shape. I don't mind replacing all of it, but could those be reused with a new flywheel? I could see there being a break-in period where the the clutch disc wears to the new flywheel, but it seems like a new one would do that too. It is a few hundred dollars that I would not mind saving.

[jeffnhiscars]

I installed the RHD fw in my cabrio when I did the 5spd swap. I do have an M20 so YEMV but I love it and have zero issues. I took Bradnic for a ride when he was down here and he said it was the best running e30 he had ever been in (this from a guy w a fresh MM engine sitting in his garage).

I get what I'd call a bit of a turbine whine but don't really notice it anymore. No rattle or other issues I can think of.

[TobyB]

On an M10, I removed the flywheel completely, and ran a 7 1/4" Quartermaster clutch.

It was a blast to drive, and ran just fine. BMW cranks are so heavy...

t
because raceycar

[bmwman91]

Thanks. I did more searching on forums and people seem pretty happy with them elsewhere. A couple of people sold them because they did not like the new sounds during starting (apparently these "ring" a lot more than the cast iron ones), but I can like with that. Crazy noises in parking lots will just add more to the mystique of the vehicle lol.

And yeah, the crank is certainly no lighter than stock (M47 88mm stroke, forged & fully counter-weighted), plus the stock M20 PP is not exactly a lightweight item either.

[digger]

is that M20 11lb one a MM machined one? i'm not going with a lighter than 12 lb as i don't like the rocks in a can sound but that's the only reason. driveability will be fine. On a stock m20 with motronic with big cam trying to run A/C would not be recommended without faster idle as it would be harder to catch the idle so to speak but with full ECU control its not an issue

[bmwman91]

Yeah the 11lb machined one is from MM. I am likely going to sell it with the clutch disc & PP (relatively low miles) after I am up and running with the 7.4lb one and assuming I like it. The 11lb one felt pretty good on the M42, despite the coffee can full of rocks sounds.

[bmwman91]

Just to close on this, I am running the 7.4lb RHD flywheel and LOVE it. While some people reported "odd" noises while cranking, I have not noticed anything like that. While the FW is "only" 3.6lbs lighter than the 11lb MM one, the RHD flywheel removed almost all of the additional mass from the outside diameter. Without getting too into engineering stuff, the moment of inertia ("resistance to spinning") for a mass is related to (radius)^2, so taking material out at the outside diameter makes for a much bigger difference than indicated just by the total mass of the part.

The engine revs like a beast (granted, it was also just totally rebuilt, and some lingering compression loss issues were eliminated at the same time, which definitely helps) and is effortless to rev-match with little throttle blips. The feel of the car is fantastic.

Downside: Idle speed had to be increased a tiny bit (~20RPM), along with a fully custom tune of a whole bunch of parameters related to filter times of various engine parameters. Sssquid tuning did a great job. I found that the engine would stall coming off of the throttle, and very rarely when just sitting at idle. There just isn't enough inertia in this flywheel to keep a strung-out 4 cylinder going (recalling that I have a 2.1L M42 with 11.5:1 CR and moderately hotter cams, along with significantly lightened internals). I'd guess that a stock'ish M42 would have less of a problem here. Anyway, it is MORE than worth it, even with occasional stalls (! per week so far) since there's only so much the tune can do unless I increase the idle to 900RPM or more (which I prefer not to do).

Anyone with a Getrag transmission & a light single mass flywheel knows that transmission rattle is a thing. I do not find it to be any worse with the RHD flywheel than it was with the 11lb M20 FW, or even with a stock M20 FW (which I ran years ago).