lighter stock flywheel weight with stock clutch combo ?

[82eye]

i'm putting my driveline together and one of the things on the list was lightening the flywheel.

so far i've got :
m20b25
rebalanced bottom end on stock bore
mild cam - 272 regrind
ie header
stromung catback

i'd like to lighten the flywheel while i've got it apart and i'm looking for a target weight that works well with a stock clutch combination. any experience is welcome. shooting for a simple street car build.
also will need to source a chip soon.

[atmh]

The stock single mass flywheel can be machined down to about 12.5lbs, which is almost half the weight. You can see the pics of mine in my build thread (third post in the thread)

https://www.r3vlimited.com/board/forum/general-forums/members-rides/the-project-forum/10021186-1984-325e-turbo-resurrection

I paired it with a 6-puck clutch and thought it was still fairly easy to drive. I'm sure with a stock clutch it would be fine. If you're going for more power, an easy and potentially cheap upgrade is to use an e30 M3 pressure plate. At least that's what was recommended to me ~15 years ago. Not sure what the prices on them are like these days, but that's my setup: lightened single mass flywheel, e30 M3 pressure plate, 6-puck clutch (I don't remember exactly what clutch I ended up going with).

That setup seemed to hold ~300hp, but to be fair I didn't run it through the paces that much before my car got put into storage for 12 years.

[Panici]

The stock single mass flywheel can be machined down to about 12.5lbs, which is almost half the weight. You can see the pics of mine in my build thread (third post in the thread)
https://www.r3vlimited.com/board/for...o-resurrection

That is some nice machining!
When I had my single-mass done I didn't give them any specific instructions.
They definitely could have taken more off as you can see in the below pic.

It doesn't matter now as I engine swapped.
Went with a RHD 7.5lb flywheel which I love. Opted to use an M20 sized clutch for less rotating mass.
He makes various combinations for clutch size and engine/starter type.

[bmwman91]

The best deal on the market, hands-down, is the RHD ultralight flywheel.

https://racehead.com.au/product/bmw-light-flywheel-m20/

I am running one on my M42 (FW is identical to the M20 one except that the starter ring tooth pitch is different). The clutch disc, PP, and TOB are all stock M20 parts. The clutch holds just fine, and my M42 makes ~200bhp (it is...not stock), so with your list of engine work I think you will have no issues.

The FW is fantastic, and wakes up the car pretty noticeably even compared to the 12lb M20 FW I was running prior to this. Flywheel weight is only part of the story, and WHERE the mass is removed from is actually more important. The stock M20 FW can only be machined down in a limited way for safety reasons, and there will necessarily be a lot of material left at the outside diameter. Since the RHD FW is purpose-built rather than modified, it can do away with a majority of the material at the outside diameter. Long story short, rotational inertia is proportional to radius to the 3rd power, so eliminating stuff at the OD makes an enormous difference.

Now, the tradeoff is that you will probably get more transmission rattle at idle when the clutch is engaged in neutral. It is pretty pronounced on the M42, but it is also a 4 cylinder so I'd expect that the M20 would be no worse. The rev matches are so smooth and effortless with this FW that I am happy to have a little more noise. I have also noticed that engine braking is more effective with the lighter flywheel.

[82eye]

The stock single mass flywheel can be machined down to about 12.5lbs, which is almost half the weight. You can see the pics of mine in my build thread (third post in the thread)

the stock m20b20 was 13lbs in the e21. i know that's been used before, but i'm not sure

https://www.r3vlimited.com/board/forum/general-forums/members-rides/the-project-forum/10021186-1984-325e-turbo-resurrection

I paired it with a 6-puck clutch and thought it was still fairly easy to drive. I'm sure with a stock clutch it would be fine. If you're going for more power, an easy and potentially cheap upgrade is to use an e30 M3 pressure plate. At least that's what was recommended to me ~15 years ago. Not sure what the prices on them are like these days, but that's my setup: lightened single mass flywheel, e30 M3 pressure plate, 6-puck clutch (I don't remember exactly what clutch I ended up going with).

That setup seemed to hold ~300hp, but to be fair I didn't run it through the paces that much before my car got put into storage for 12 years.

owow very nice. a little more involved than i am lol. i'm not gonna make near that power. i have to try keep this to what i can order through the usual suppliers on a bit of a budget. sourcing 6-pucks and m3 pressure plates is likely to spiral past getting what this build is trying for.


it's really good to know about the drivability of a 6-puck . it might be an option if it works with a milder build like mine.

[atmh]

Woah only $255 for that? When I did mine I machined it myself and then had it balanced for maybe $60, but if you're not doing the work yourself (or don't have a single mass flywheel to start with) it wouldn't take much to get up to the $200 ballpark even trying to do it on the cheap. I might have to make the switch, ha ha.

How difficult is driving with the flywheel that light?

[82eye]

The best deal on the market, hands-down, is the RHD ultralight flywheel.

https://racehead.com.au/product/bmw-light-flywheel-m20/

I am running one on my M42 (FW is identical to the M20 one except that the starter ring tooth pitch is different). The clutch disc, PP, and TOB are all stock M20 parts. The clutch holds just fine, and my M42 makes ~200bhp (it is...not stock), so with your list of engine work I think you will have no issues.

so much drool. how's chatter running something that light on a stock clutch ? chatter is an issue i see popping up on this topic in relation to stock lightened m20 fw's.

The FW is fantastic, and wakes up the car pretty noticeably even compared to the 12lb M20 FW I was running prior to this. Flywheel weight is only part of the story, and WHERE the mass is removed from is actually more important. The stock M20 FW can only be machined down in a limited way for safety reasons, and there will necessarily be a lot of material left at the outside diameter. Since the RHD FW is purpose-built rather than modified, it can do away with a majority of the material at the outside diameter. Long story short, rotational inertia is proportional to radius to the 3rd power, so eliminating stuff at the OD makes an enormous difference.

the stock m20 flywheel is stepped. it's only the step that can really be brought down. and only so far.

Now, the tradeoff is that you will probably get more transmission rattle at idle when the clutch is engaged in neutral. It is pretty pronounced on the M42, but it is also a 4 cylinder so I'd expect that the M20 would be no worse. The rev matches are so smooth and effortless with this FW that I am happy to have a little more noise. I have also noticed that engine braking is more effective with the lighter flywheel.

oops that answered above now that i read proper ...

thanks for the input guys.

[82eye]

Woah only $255 for that? When I did mine I machined it myself and then had it balanced for maybe $60, but if you're not doing the work yourself (or don't have a single mass flywheel to start with) it wouldn't take much to get up to the $200 ballpark even trying to do it on the cheap. I might have to make the switch, ha ha.

what weight did you go with ? duh. nvm.

[82eye]

the machinist is suggesting something in the 16 - 18 lb range as a ballpark compromise for drivability / fun.

[bmwman91]

Woah only $255 for that? When I did mine I machined it myself and then had it balanced for maybe $60, but if you're not doing the work yourself (or don't have a single mass flywheel to start with) it wouldn't take much to get up to the $200 ballpark even trying to do it on the cheap. I might have to make the switch, ha ha.

How difficult is driving with the flywheel that light?

Difficulty is zero IMO. It takes about 3 minutes to become used to the additional throttle required when getting going from a stop. Aside from that, the improvement in overall drivability is awesome.

[bmwman91]

the machinist is suggesting something in the 16 - 18 lb range as a ballpark compromise for drivability / fun.

IMO, the PP and crank are heavy enough that it doesn't matter if you go super light with the FW. If my 4 cylinder is good to drive on a 7.4lb FW (for some reason RHD increased the mass by 1lb since 2019), a 6 cylinder will be great. It's all personal preference though, so you gotta decide what is right for you. Having run a stock M20 FW, a 12lb M20 FW, and now the RHD unit, I find that the lightest one is also the best in every regard (other than transmission rattle lol...but all of the M20 ones were worse than the stock M42 DMF in that respect).

[atmh]

82eye - I didn't notice a huge difference, to be honest. That's part of why I went with a 6-puck clutch: it's a blend between driveability and grabbiness. I would say losing the weight off the flywheel made less of a difference than I was expecting. 6-cylinder cars are going to be harder to stall than 4-cylinder, and the e30 is pretty light. The e30 is one of the easiest cars to start from a stop, in my experience. My 2015 VW TDI on the other hand - I stall that regularly.

bmwman91 - Thanks! I might just go that route. My car is long past the days of daily-driven, so even a little bit of a sacrifice here is not a big deal.

There's currently a RHD flywheel for sale in the parts forum... I'm tempted, but it will be a long time before I need a flywheel, so I'm going to direct my hard earned cash towards more pressing matters. But it's relevant to the thread.

[82eye]

Difficulty is zero IMO. It takes about 3 minutes to become used to the additional throttle required when getting going from a stop. Aside from that, the improvement in overall drivability is awesome.

m42 is a bit of a different animal. they're revvier than an m20 at every gear. it's closer in architecture and philosophy to the s13. the m20 is a bit more linear power and older in approach.

[82eye]

IMO, the PP and crank are heavy enough that it doesn't matter if you go super light with the FW. If my 4 cylinder is good to drive on a 7.4lb FW (for some reason RHD increased the mass by 1lb since 2019), a 6 cylinder will be great. It's all personal preference though, so you gotta decide what is right for you. Having run a stock M20 FW, a 12lb M20 FW, and now the RHD unit, I find that the lightest one is also the best in every regard (other than transmission rattle lol...but all of the M20 ones were worse than the stock M42 DMF in that respect).

i'm going in blind with nothing to compare against except stock. no one close can really help. where i am an e30 is a total unicorn. you can't park a decent one at 7-11 without creating a crowd. i only know of 1 other person who has one, and he's 250km away lol.

[82eye]

so guys i gotta try keep this to stock or stock modified parts ... i can maybe step outside for a clutch.

under those provisions, where would you head ?