Lightened flywheel ?

[Templ8e30]

Hi folks, I have a lightened flywheel in the shed ready for fitting to my 2.7 and I'm torn between fitting it and selling it.

The engine is torquey with loads of power above 4000 rpm at the moment and I'm fitting a zone BTB 6 branch at the weekend.

I expect to lose a bit of bottom end shove but gain some more high end power, however I don't want this engine to become too 'revvy' so I'm tempted not to fit the flywheel as I understand you lose low down torque with a lightened fly.

How much torque do you REALLY lose ? are the gains and losses that noticable ?

What would you do ?

Cheers,

Iain T

[stevetigger]

DFit it, Why have a 2.7 without it!

[Dan318-is]

I would be interested in the replies to this as i plan to do the same but install it on an m42 rather than an m20

out of interest, how much did you pay to get it skimmed ian?

[Morat]

I don't think you lose any torque by fitting a lightened flywheel, certainly not over any length of time. You might feel the difference in the first second when pulling up a hill but its not as if the flywheel is storing vast amounts of energy, its just there to smooth out gearchanges a bit and give a smooth idle etc.

[MikeeMiracle]

DFit it, Why have a 2.7 without it!

Ditto. On a 2.7 you should have more than enough torque for this not to be an issue.

[Templ8e30]

how much did you pay to get it skimmed ian?

I didn't, I did it myself at work (I'm an engineer) then put it on the rotary table surface grinder to get it 100% flat and even.

Cost me £40 to get it balanced though

Cheers,

Iain T

[Turbo-Brown]

It will neither lose you torque (anywhere) nor gain you power, just make it rev up quicker.

Please do your calcs on how much material you removed and where from because if it bursts at 6000rpm it will have your legs off.

[MikeeMiracle]

It will neither lose you torque (anywhere) nor gain you power, just make it rev up quicker.

Technically true.

The flywheel kinda stores momentum if you like. Say your going 40mph and lift off until you come to a stop, if you have a heavy flywheel your car will travel longer before coming to a stop than with a lighter flywheel as the heavier flywheel has more momentum.

When you depress the throttle any power/torque from the resulting combustion is "added" to the momentum the engine already has. You really need to understand/comprehend this particular point fully.

If you lighten the flywheel the engine has less mass, stores less energy so your relying more on the torque of the engine itself and relying less on the momentum the engine already has built up. This is why underpowered cars can struggle to go up a hill with a lightened flywheel, they are relying on the weeny torque output instead of the combination of both.

Not the greatest explanation I know, but I hope you get the drift.

[Turbo-Brown]

So you're saying that if we took my neighbors R5 campus and drove it up a 2 mile long 10% hill, it'd struggle less with a heavy flywheel, or more?

The mass of the fly only effects transient conditions, here's something I tapped out a few weeks back on the subject:

The mass of the rotating components makes no difference to the steady state performance of an engine i.e. up hill.

Imaging we're driving up a perfectly even grade of 1in10 and have been for 5 minutes at 30mph. We won't have to change the throttle opening at all to maintain that speed because we've reached a steady state of load on the engine which is producing enough power to overcome wind resistance and gravity etc.

Now imagine that we kill the power and coast to a halt in gear (ignoring engine braking) giving rise to a transient condition. The distance we travel without power will be a function of the vehicle mass, the rotating masses and the retarding forces on the vehicle like wind, friction, gravity etc.. If we had a massive flywheel we'd go on for quite some time. With a tiny flywheel, we'd most likely stop in much less time.

Another (more real world) example:
Say we're driving at 30mph on the flat. We'll have say 5% throttle opening to maintain our speed. Now we approach our mythical hill which means to maintain 30mph, we'll need to open the throttle to say 10%. This is where our rotating mass comes into play:

With a light fly etc, we'll need to get on the throttle pretty soon after starting up the hill to avoid losing speed, with a massive heavy fly, we've got a little more time to open the throttle. Once on the hill though, the throttle openings will be exactly the same at 10%, regardless of the mass of the rotating components.

[Demlotcrew]

Please do your calcs on how much material you removed and where from because if it bursts at 6000rpm it will have your legs off.

I would be worried about this and not how much torque you have lost.

Andrew

[dark_sounds]

Tim_s wrote up a very good reply to my quewstion which was practically the same, in his for sale ad for a M42 fly wheel... someone search it and read what he said, made good sense and logical..

bascially makes no difference to torque.

[Simon13]

if it's cammed up it won't idle as smooth and will be more effort to drive around town so i understand.

early 318iS 89 ones came with an M40 flywheel

[dark_sounds]

if it's cammed up it won't idle as smooth and will be more effort to drive around town so i understand.

early 318iS 89 ones came with an M40 flywheel

this would be a G plate right? mines a G plate and drives so much differently to my other is i had which was a H plate.

the H plate one drove much better, had more lower grunt. and smoother.

[Simon13]

that could be it!

[Demlotcrew]

No! the difference between a good 318is and a bad one is the cam Timing not fly wheel.

Andrew

[Templ8e30]

Please do your calcs on how much material you removed and where from because if it bursts at 6000rpm it will have your legs off.

I would be worried about this and not how much torque you have lost.

Andrew

I took about 3kg's off the fly all at the back at the outer edge behind where the ring gear is fitted so there is no structural material removed from the thinner centre.

It was balanced to 10,000 rpm so it's more than capable of holding up under my 6500 rpm limiter

Cheers,

Iain T

[DanThe]

Please do your calcs on how much material you removed and where from because if it bursts at 6000rpm it will have your legs off.

I would be worried about this and not how much torque you have lost.

Andrew

Take no notice Iain, fit it!!

[Dan318-is]

No! the difference between a good 318is and a bad one is the cam Timing not fly wheel.

Andrew

Expand please?

are you saying BMW put the power band in the wrong place like?

[DRIFTBOY]

I have had many flywheels lightened (and not even bothered to balance some of them when I was a poor student!) and have never noticed any detrimental effect to climbing hills or to the idle, even with a hairy camshaft in the engine - when set up properly or with fuel injection. But to be quite honest I found it difficult on some engines to notice much of a useful gain either. However on a few engines I have noticed a crisper feel to throttle response and a more 'racier' noise when you blip the throttle. WHAP WHAAAP!!! The effects are more noticeable the lower the gear that you are in, it's a little complicated to explain why though so won't bore you with it!

Just fit it! Nothing to loose, plus it lowers the overall weight of the car!

[SHAZ325iSPORT]

if you dont fit it sell it to me ,anyone know where i can get mine done?

[MikeeMiracle]

So you're saying that if we took my neighbors R5 campus and drove it up a 2 mile long 10% hill, it'd struggle less with a heavy flywheel, or more?

From the way it was explained to me then yes it you would struggle less. I must admit your version sound perfectly feasible also

[Turbo-Brown]

Please do your calcs on how much material you removed and where from because if it bursts at 6000rpm it will have your legs off.

I would be worried about this and not how much torque you have lost.

Andrew

I took about 3kg's off the fly all at the back at the outer edge behind where the ring gear is fitted so there is no structural material removed from the thinner centre.

It was balanced to 10,000 rpm so it's more than capable of holding up under my 6500 rpm limiter

Cheers,

Iain T

Fair enough if you're sure, although going to 10000rpm once or twice isn't quite the same as the constant cycling of speeds it'll go through on the car.

Bit puzzled as to why they span it up so far too! If it's balanced at 10rpm, it'll also be balanced at 10000rpm!

Only time I've heard of things being balanced to different rpm points is where the huge mass of component being balanced causes it to deform by a measurable amount between the bearings due to gravity, but that doesn't apply to the flywheel on a typical car, and that still wouldn't explain it!

Thing is (just to be alarmist) taking metal from the thick part is also removing it's ability to resist going pop! Not saying it is gonna happen, just saying be sure it isn't!

[polwutna]

I've got a lightened flywheel in my car (not an E30, but principles apply).

Basically revs rise and fall more quickly, around town in stop start traffic, this can be a bit of a pain in the arse just because of the lack of enertia in the flywheel. You have to adapt your driving slightly to deal with it and it just seems like a bit more effort. However, its not what I would call a problem, just different.

That said, when nailing it or on normal A roads, its fine, and during a sprited drive, it makes you feel like the engine is reacting quicker to throttle response.

So it depends on how you drive, if you want a lazy drive using the torque of the engine, don't do it. If you like driving fast and trackdays, do it.

[Adammcf]

Fit it.

[stuartgallafant]

if you dont fit it sell it to me ,anyone know where i can get mine done?

sutton rebore. dunno the number, but you can look it up...

[cliffybabe]

go on fit it, u know u want to

[stuartgallafant]

if i was you, id fit. its been said, its not gonna reduce the amount of torque. it purely reduces rotational mass, allowing the engine speeds to accelerate and decelerate at faster.

i know what i'd do if it was engine

[Jhonno]

as has been said, lightening it neither loses nor gains torque

it just lets the engine rev up quicker.. bad points.. harder to drive in traffic, need a little extra gas goin up steeper hills and harder to make smooth gear changes

Fit it, long as its safe that is

[Andreas]

I've had two different aluminium flywheels in my Alpina B3. The first one had a bonded friction surface (the guy from PAECO IMPORT PARTS said he would never sell a bolt-on friction surface since they come off) ; well his bonded friction surface disintegrated on the race track and my buddy had to tow me home for 300km at night.

After driving for a while with the standard flywheel, I had to get another one. This time I got one with a bolt-on friction surface from www.jbracing.com and after much track abuse it is still going strong, nothing has ever come loose.

I think a light flywheel definately helps with acceleration, my Alpina is faster than my previous e30 325 was with nitrous. I think uphill acceleration and accelerating at speeds over 200km/h suffers from the light flywheel, I seem to remember that it used to perform better with the standard flywheel for those two conditions. On a race track you will however benefit in my opinion.

With my first alu flywheel I had the 3.91 Alpina diff and you had to change into 1st gear for 90 degree turns into roads to avoid jerking and the car became much more fuel efficient.

With my second alu flywheel I had a 4.44 diff and this eliminated all jerkiness at low rpm - I could pull off smoothly in 2cd gear even though I had a 292 degree cam and light flywheel. Fuel consumption went through the roof though.

Here are two pics of my current flywheel which weighs 3.54 kg (my standard flywheel weighs 8.50 kg)

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Alpina B3 2.7 Racecar under construction.

[stuartgallafant]

well i think you have too much money!!!

[WillG]

so if it breaks it will go throught the bell housing and floorpan/carpet and still be going fast enuff to cut your legs off , not disagreeing I have no idea, has this happened before?

[billgatese30]

is it justme or does 3kg sound liek a lot to take off, what was its original weight Iain ?

[Turbo-Brown]

so if it breaks it will go throught the bell housing and floorpan/carpet and still be going fast enuff to cut your legs off , not disagreeing I have no idea, has this happened before?

Have heard of them going pop on a couple of FWD cars after an over enthusiastic bout of lightening and the resulting bits getting well and truely into the passenger compartment

It's right by your feet too on the E30!

Exercise caution is all I'm saying and either do some calcs on when it should burst, build in a factor of safety and also look at the fatigue life of the thing!

[stuartgallafant]

Have heard of them going pop on a couple of FWD cars after an over enthusiastic bout of lightening and the resulting bits getting well and truely into the passenger compartment

It's right by your feet too on the E30!

sod that!!!

[MikeeMiracle]

is it justme or does 3kg sound liek a lot to take off, what was its original weight Iain ?

Sounds about right. My mate takes 4kg of a 12kg Alfa flywheel and still pushes over 300bhp through it without problems.