Tony Quinn

Has anybody bought or does anyone know of a place that sells a lighter flywheel than that fitted as standard? All info and leads will be gratefully received.

Benny Boy

Hi Tony

The consensus of opinion on lighter flywheels is that you lose some driveability due to the reduction in inertia. A heavy flywheel, having more inertia than a lighter one, will hold its energy for longer.

This means that the engine revs will be dropping faster with the lighter flywheel and therefore necessitate faster changes and a change in driving style.

Those that use a lighter flywheel have said that whilst you change the way you drive it isn't something that takes a long time to develop.

With a lighter wheel you will be relieving load from your brakes and whilst the mass difference in the flywheel is not that much, the effect on the energy required to slow is something along the lines of 3 times less.

You probably knew all that Tony and unfortunately I do not know of a supplier in the UK but could probably find you one abroad.

Kind regards

Ben

Tony Quinn

Thanks for the background Benny. I'm not entirely sure what you mean by the "three times less" comment though.

Quicker gear changes is my reason for looking at making this change. I have always found that I have had to wait for the gearbox when changing, so it will be more of a case of reverting to type rather than changing my style.

AlexM

Hi,

I had the flywheel on my R5 GT Turbo lightened by 30% by machining the center (i.e. non-friction surface) out. I found that this didn't compromise the idle and drivability at all. In fact, the engine was much more blippable and gear changes were much quicker and smoother both up and down the box.

I wondered if the slowness with which the revs fall off between gears is partly a function of the ECU program, rather than simply a massive flywheel. There is already considerable rotational inertia in the drivetrain, so I would be suprised if a heavy flywheel would needed too.

Changing 1st/2nd at high revs seems to mean waiting for ages to avoid putting a shock load into the transmission.

Cheers,

A.

P.S. I think JUN and HKS do modified flywheels for the Impreza. I think I remember seeing them on TDIs website.

Benny Boy

MMmmmmm. I was typing too fast I think!!

What I was attempting to say is that if you save 4kg say in flywheel weight then it equates to approximately 12kg in overall weight.

This is information I have *******ised and using somebody else's theory. To me it makes sense so have used the answer here.

Since this is not a Physics class I shall not bore people!!

Ben

Benny Boy

I have just noticed that the BBS language censor has cut up my perfectly good english!!

The word involves not having a Mum & Dad!

Mike Tuckwood

Tony, I don't see how a lighter flywheel will speed up your gearchanging process (unless you have a straight cut, non-synchro gearbox).

The flywheel effectively smooths out the 4 sequential explosions as the cylinders fire in turn, allowing the mass of the flywheel to keep things going.

Lightening it would make the engine seem as though it was running rougher, at the cost of quicker pick-up of revs, due to the lower mass.

Or am I talking bo!!ocks again?

Mike.

Tony Quinn

Thanks Benny, now I understand.

Mike, at the moment I do have a semi-non-synchro box (see the Gearbox trouble 'crunch' thread). I thought that having the box removed to fix that problem was a good excuse to change the flywheel at the same time and enhance performance somewhat. All upward gear changes on my 'box feel pretty awful as the synchro struggles with the higher engine speed, which causes me to change more slowly than I would like. Therefore I'm pretty keen to do whatever I can to help out the 'box.

Tony Quinn

Alex,

I did wonder about the ECU because initially I thought that I couldn't have lifted right off the throttle, mind you I never used to when pushing on (different car) and never had a problem then. You may be correct about this but I could never figure out why it would only be a fourth gear thing in that case. I suppose the same could be said about the flywheel theory too. Hmm..! Better stop typing, I think that I'm getting further away from understanding what might be happening.

P.S. Surely the weight of the drivetrain is not relevant to the rate at which an engine slows when the clutch is disengaged.

AlexM

Tony,

You are right of course - when the clutch isn't engaged, only the mass of the flywheel would matter.

Maybe Subaru need to fit a heavy flywheel to make the idle acceptably smooth and make pulling away easier. The engine doesn't feel very 'blipable' when you are at rest.

When you say that you're waiting for the gearbox, do you mean waiting for the revs to fall to match those required in the next gear up?. Do you also have problems with 4th gear syncro?

Cheers,

A.

LEE.R

your right !hks do two if memory serves me a lighten and a lighter still(technical name)and also bpm do one£485 pete at c.k manchester will sort you out 01612241431.
lee

firefox

Hi guys...

You are all correct (I think all of you are). The lighten wheel will show up the more rough idle on the scoobs.... and will (marginally) drop the revs quicker....

Lightened wheels cost about 500 quid upwards ... several sources do them..Jun...HKS...even BPM... mainly Jap companies... They are made from a material called chromoly and usually forged.

Fitting a lighter wheel will also increase throttle response...(less inertia).

If I had the gearbox out, then I'd change the flywheel and clutch mechanism too

Hope this helps

J.

Tony Quinn

Alex,

You're right, 4th gear synchro is shot. I am having to wait a couple of seconds between depressing the clutch and engaging fourth gear so that the revs fall away to 4th gear level. I'm going to the dealer on saturday so that they can test the car and give an opinion.

It is a bit worrying to hear of so many g/box failures though. Any ideas on how best to protect the box from a future failure. Presumably synthetic oil and a lightened flywheel will help protect the synchro. Is the STi box different from UK cars? It does have more power to cope with.

AlexM

Tony,

I don't think fitting a lighter flywheel will help out your fourth gear syncro - The flywheel is bolted to the end of the crank, and so won't affect the load on the syncro unit.

Your engine revs should definately not take a couple of seconds to drop by 1500rpm, or whatever the gap is between 3rd and 4th. If this is the case, you could have a slightly sticking throttle. I'm suprised that so many people are having problems with syncro in the higher gears - the gap between gear ratios is smaller 3-4-5 than 1-2-3, so the syncro should have an easier time of it. I think this is a design problem.

As far as I know, the STi transmission is quite a lot stronger than the UK transmission - A different, stronger casing and higher spec stronger internals.

I don't think there is any alternative but to have the box taken out and the syncro unit replaced. You would have to drop the gearbox to replace the flywheel anyway.

Hope you get it sorted out.

Cheers,

A.

Doc

Take a look at Gryphonic technology. (There is a link to them on the SIDC site) They have a picture of a lightweight flywheel but no details.

The address is