Alberick

Does anybody know what advantages a lighter flywheel will bring? I've been told that you can't just go ahead and swap those as the engine has to be re-checked or balanced or whatsoever, is that correct?
I found out that the original flywheel weighs 12kg, so, let's say for example, if I put a 4.6 kg one do I get better acceleration?
Thanks,
Al

Nigel Bowles

Check out this other topic.

BPM

Bear in mind the clamp of the clutch and other issues such as how the flywheel is lightened.

A too light flywheel compromises in many respects and can be bowed if the clutch clamp is too high.

A flywheel with a "holey" surface also allows oil spillage on clutch surface thus leading to clutch clip, premature failure and reduced driveability.


This is a quote from the BPM engineer in regards to the BPM offering.

"Unlike similar flywheels on the market this flywheel incorporates a unique
design in order to hold strength in essential areas. The most common problem
with lightened flywheel is they tend to distort under pressure plate force
in turn disturbing the clutch plate contact surface area. Our flywheels are
manufactured from a billet billet material which is heat treated prior to
machining , thus incorporating exceptional equilibrium quality in regards to
balance and can be remachined in the future without afecting surface
quality. When considering the force exerted on a flywheel with pressure
plate force applide, the flywheel is forced into a bowl shape, and with a
flat clutch plate on a bowl contact is only on the circumference. not only
does this distortion affect clutch face contact but also flywhhel fatigue
due to conatant flexing.
When considering weight lighter is not always better. A very light flywheel
will be an advantage down low in performance and a disadvantage up high and
on the in the other respect a heavier flywheel acts in an opposite fashion.
When designing we considered this theory in our calculations and came up
with a weight coupled with our design which is ideal at 6.67 kg's."


Hope this helps...

BPM

mike_nunan

Can Greg or anyone else explain how a very light flywheel might lead to a loss in top-end performance please? If there is less inertia in the drivetrain, I would expect acceleration to be more rapid throughout the power/rev range, not just at the bottom.

I understand that engine braking will be much more pronounced with a light flywheel. I'm wondering if this whole top-end performance thing comes from the following "logic": if you take your foot off at high revs with a lightened flywheel, the car stands on its nose. Also, if you're going up hill, you need to increase the throttle sooner, in order to keep the car on the boil. Therefore the flywheel must be "soaking up" some of the engine's power.

That is all nonsense -- the flywheel is a device for storing energy, so you get slower acceleration with a heavy flywheel (as the engine has to feed more energy to the wheel as it spins up) and less severe engine braking (because the engine's unchanged braking effect has to dissipate the previously stored flywheel energy as well as slow the car down). At a steady speed -- uphill, downhill it doesn't matter -- it doesn't make a blind bit of difference how much the flywheel weighs, the engine is still producing a certain amount of power and the flywheel is neither adding to or removing from that power output.

This can be (and has been) verified on a dyno, and is the reason why Powerstation, for example, allow the revs to rise extremely slowly when doing power runs. By keeping things as near to steady-state as possible, they eliminate most of the influence of the flywheel and other rotating masses from their readings.

AFAICS the only reason you would want to have a flywheel is to ensure stable running at low revs. At high revs, there will be enough momentum in the rest of the drivetrain to make sure the crank keeps turning at a relatively constant velocity and you won't get misfires due to timing variations. If you consider that F1 cars run ultra-light clutches and vestigial flywheels, it becomes even harder to defend the idea that light flywheels can cost top-end output.

Greg, if you're willing to reproduce the calculation that you mention, I'd be very interested to see it.

Many thanks,

-= mike =-

PS. I'm not cruising for a flame war here, I'm quite willing to be shown that I don't understand the whole story, but I would like a proper technical explanation, rather than just being told "it's true because we say it is". Thanks.

Pete Croney

The Focus WRC drivetrain has been designed so that it doesn't have a flywheel!! It uses the mass of the transfer shaft to keep it idling.

I run a 7kg flywheel. Anything less than 10kg is noticeable.

The main thing you notice is much more brutal acceleration in the lower gears. Just as you would expect.

Idling is unaffected.

With regard to distortion... I think its down to where you remove the material. We take it from the most outer areas, as these have the most influence on inertia and the least influence on the strength in the clutch area.

You also notice a big difference when "heal and toe" driving.

Cost is £138+vat exchange (must be a serviceable flywheel).

Andy Tang

iTrader: (3)

Is it true that this will have a knock on effect on engine braking?? ie - brakes need to be used more, reduced engine braking!

I assume the flywheel is balanced.

Also I've been told that lighter conrods would have a better performance increase, than the flywheel.

How much to do both??

Cheers
Andy

Pete Croney

Hi Andy

Engine braking would be improved, as there is less mass trying to continue rotating.

As for changing the rods, this is a major engine build job and would cost in excess of £2k, parts and labour.

A flywheel re-work would cost less than £400, fitted.

Andy Tang

iTrader: (3)

Can anyone answer the question above?

Considering the flywheel, but when someone (a non-scooby owner) suggests that another modification would be cheaper, and yeild better performance, I thought someone might have tried it!

Thanks
Andy

sunilp

whats a conrod?

firefox

Its a rod that cons people into buying it

J.

pat

Mmmmmmmm...

Titanium conrods and gudgeon pins.... :-) But of course the crank would need re-balancing to compesate for the reduced mass of the reciprocating parts! Well, at least when it's finished the bottom end will be fine to about 10,000 RPM, but the heads will need some attention, the valves will not follow the cams after about 8500 RPM on an STi head, never mind a "normal" head...

Overall, a lightened flywheel is a good idea, and if you need a clutch change anyway, what better excuse to go ahead and swap the flywheel...

Cheers,

Pat.

Andy Tang

iTrader: (3)

Thanks guys, I think we're bordering on ground, that shouldn't be walked by a UK engine!!!

Now if I bought a second engine......

With only 14,000 miles on the clock, I think the clutch maybe a long time in coming!!

Thanks for the advice and putting me straight!
Andy

sunilp

he he, 14k and u think a clutch is along way off?, i'm on 13k and my this will be my third one

Sounds like i should get the flywheel done too - this is not unsafe/unreliable is it gents?

Andy Tang

iTrader: (3)

Well Sunil, I think our cars have *SLIGHTLY* different specifications!!!

sunilp

My first clutch juddered within the first 3 months!, i never sorted it until the 1st annual service

Sunil

pat

Sunil,

coming to think of it, perhaps my lightened flywheel was to blame for the loss of compression in No. 4 cylinder. Then again, perhaps it had nothing to do with it, since it places a lower load on the engine and is better balanced than the original.... hmmmm.... difficult question :-) :-)

But in all seriousness, the chances of a lightened flywheel damaging an engine, assuming it doesn't disintegrate and blow away the bell housing in the process, is pretty much non existent.... in short, a well made light, balanced flywheel is about as likely to damage the engine as you eating icecream... both will bring a smile to your face, but neither is likely to break your car.

Cheers,

Pat.

sunilp

Wasnt just so much then engine but the rest of the drivetrain too that i was concerned about!

But thanks for clearing that up Pat, now then, where can i get such a well made, lightened and balanced flywheel?

CraigS

Personally I had an Exedy Flywheel , roughly 4.5 kg and went to high clamp clutch and found that the flywheel did in fact have far more negatives. When removed it had bent like a banana....

Food for thought

Craig

Mike,

I have access to the files from BPM in regards to the mathematics etc..Ill dig it up for you..

As to the cost , I think the UK is being ripped off...

I purchased the BPM billet 4140 Flywheel from and it was under 300pounds.