Lightweight flywheels..how light is too light?

[bflan2001]

Please post up any feedback you have from your lightweight flywheel setup.

I'm worried about the idle issues I've been reading about when people go to a lightweight flywheel. The car is my DD so I want it to be a comfortable drive, but I also want to squeeze as much HP out of this little engine as possible.

[KenC]

I have a 13lb 323 flywheel with the M20 clutch/PP.  Rattles a little at idle, but the idle is still silky smooth (helps that I have zero vacuum leaks).

[91 e30m42]

i have the JB racing 9lb flywheel for the m42 did spend about 5-6 hundred but it was well worth it no issues at all and acceleration is incredible and deceleration is just as good... i recommend this flywheel if you have the money

[harvey2]

I also have the 13lb flywheel from a 323 with the M20 clutch and I have no issues at idle at all.  The idle seems unchanged from stock.

[M42_Jester]

i'm lookin at doin the same thing...is an 8.5lb flywheel too light? can the M42 powerful enough to move this flywheel properly??

[romkasponka]

You only can have issues with emmision, but other things should be better than stock, except tire burning

[91 e30m42]

you can also remove your ac as well... thats what i did.. no ac, 9lb flywheel, still have power steering and no issues, problems nothing.

[bflan2001]

This engine is actually going in my E21, so no AC & no PS. I ended up getting the 13lb flywheel from an E21 323i. I'm going to try swapping the ring gears so I don't have to worry about starters. I read that they can be removed with a dead blow so I'll just bring in the new FW with the M42 ring gear to get it resurfaced & the ring gear pressed on.

How can this effect emissions? I'm not too worried about it, just curious

[wazzu70]

Excellent call on switching the ring gear. I hate knowing if I have a failure somewhere I cant use an off the shelf part.

My machine shop said it would be about $75 to switch ring gears so its not bad at all. Im getting it done so I can use an OE starter as Im not a fan of a 'rigged one.

[91 e30m42]

it wont effect emissions at all your just have quicker acceleration and deceleration thats all...either way you go on the weight you will love they way it responds...

[romkasponka]

it wont effect emissions at all your just have quicker acceleration and deceleration thats all...either way you go on the weight you will love they way it responds...

It will, I know it from my experience. Flywheel was lightened to 5,4kg, balanced and fitted to the car. After that idle was good, but smoke test was not passed.

Could be because off compression ratio 1:11, but with stock flywheel it was within limits. BMW dealer did not found any problem during diagnostic.

IMHO, lighter flywheel has less energy stored in it to pass TDC so engine rpm decrease and ECU adds some fuel to keep the same idle rpm.

Mass is not everything that should be noted, moment of inertia is much more important here. M42 flywheel big part off it's mass is stored in outside flywheel part and such design increase moment of inertia.

[91 e30m42]

It will, I know it from my experience. Flywheel was lightened to 5,4kg, balanced and fitted to the car. After that idle was good, but smoke test was not passed.

Could be because off compression ratio 1:11, but with stock flywheel it was within limits. BMW dealer did not found any problem during diagnostic.

IMHO, lighter flywheel has less energy stored in it to pass TDC so engine rpm decrease and ECU adds some fuel to keep the same idle rpm.

Mass is not everything that should be noted, moment of inertia is much more important here. M42 flywheel big part off it's mass is stored in outside flywheel part and such design increase moment of inertia.

mine passed no problem way below the limits and thats with my other mods as well, so it may be your compression ratio but hey every car is different

[wazzu70]

My car passed emissions easily as well too in Seattle which has pretty strict standards. I wasnt even close to failing.