Ok, had the clutch and flywheel replaced on wednesday, due to both of them being totally and uttlery fubar'd

I think that the garage put a slightly lightened flywheel in, but not 100 percent sure, and not too bothered anyway.

However, question is, what would a lightened flywheel actually do? would I notice a difference? if so then what? quick to rev? holds revs better?

just curious.

 

make your car lighter

David

 

Will rev quicker dependant on how much lighter the flywheel is. Mine had 3.2Kg removed when the clutch was done and that made a noticable difference.

You will also loose engine brakeing again dependant on how light you have gone.

Duncan

 

ahhh, loss of engine braking. now that does seemed to have occured. but tis not a problem.

 

I found you have to drive it in a different way to get the best out of it,sure is fun tho like picking up a new car!

What clutch did you go for?

 

Think clutch is helix one, certainly not standard.

 

In theory, you should gain engine braking as the engine has less rotating mass to work against.

The heavier the flywheel, the more it will want to stay rotating at any current speed. A lighter flywheel can change its speed with less energy, so the extra energy goes into accelerating (or decelerating) the car.

 

Is it also best to replace the ancilliary wheels with lighter and undersized versions to gain more power too ?

 

pete i'm my car has noticably less engine braking with the lightened flywheel. I see you say "in theory" does this mean you have experienced less engine braking with a lightened flywheel?

 

No, I got more engine braking.

The pulley wheels weigh next to nothing, I wouldn't bother replacing them. Taking 5kg off the outer edge of a flywheel has a lot more effect, the pully wheels won't weigh more than 3kg combined and are much, much smaller diameters.

 

I had my flywheel lightened and balanced and the new clutch cover plate balanced separately.

The engineering shop I used managed to reduce the weight of the flywheel from 11.6kg down to 8.2kg, they said that they didn't want to take any more material off a cast flywheel.

They also fitted a third dowel pin for the cover, for some reason the standard flywheel only has two at 120 degrees to each other when the coverplate has provision for 3, any particular reason?

Pick up is far better, the car idles fine, it's happier sitting in 3rd gear around town than before, and when I'm pottering around (rarely) I can quite happily cruise in 5th at 50mph seems like a great compromise!

I heard somewhere that 6.6kg was the optimum mass for an impreza flywheel, for drivability and pick up without lumpy idle.

 

Pete,

I am confused by your theory of more engine braking.

Do we agree that the loss of flywheel weight will make an engine accelerate more quickly - with more 'sparkle' as Training WRX says?

That is because with less mass there is less resistance against the rate of change of engine speed.

Engine braking is using the engines resistance to changing speed - i.e. interia in the mass (i.e. including the flywheel) of engine. That mass is reduced - and so just like accelerating the engine will change speed more easily - therefore less enging braking.

I have a flywheel of about 8kg - certainly easier to accelerate the engine and it retains a stable idle.

Trout

PS But I could be wrong of course!

 

Trout

Engine braking is the engine trying to decelerate. The more mass connected to it, the slower the rate of change.

Imagine it had a 1 ton flywheel, it would take ages to spin the flywheel up, but once power was removed, it would continue at its speed for ages.

The lighter the flywheel, the more engine braking you get.

Group N limit for the flywheel is 7.6kg and I wouldn't want to take one lower than this, however you can remove most of the weight from the outside egde, where it has a much greater effect.

 

I wish I had taken a photo of the flywheel before it was fitted...they machined as much as they could on mine form the outside, working in towards the centre. The engineering shop also mentioned that with cast flywheels they don't like lightening them if the ring gear has been or needs replacing, apparently the design is such that the ring gear does a lot in holding the rest of the flywheel together

Re engine braking, I've found that since fitting the lighter flywheel the car is more 'snatchy' when lifting off at higher revs (over 4500) in lower gears, so to my mind the engine braking is now a little more agressive than before

 

Pete,

my head hurts

Trout

 

I'm inclined to agree with Pete in theory. Less mass attached to engine makes it basically less of a c8nt to get going and less diffucult to slow down.

Also, you should notice that hills will affect your car's speed more because the engine has less inertia. But your fuel consumption should be better I'd have thought (during accelerating).

 

I think the different views are due to different styles of driving.

If you are travelling at say 90 mph in 4th and lift off the throttle, the car with the lighter flywheel will have MORE engine braking. This is due to less rotational energy (inertia) stored in the flywheel.

If you are travelling at 90 mph in 4th, approaching a bend and you lift off and drop it into 3rd, then you will initially experience LESS engine braking with a light flywheel but just until the engine is spun up to the 3rd gear rpm. This again is caused by less energy requirement to initially accelerate the flywheel.

Andy

 

Ahh now that bit about hills seems familure, I thought I was down on power because I keep find I need to apply a bit more throtle on cruise when I go up a slight incline

 

Dredging back memorys of a physics degreed says Pete's theory is correct. I can back this up with experience - previous to the scoob, I had a Vauxhall with a 2.5 V6 (OK, we're all allowed one mistake!), which did have a 1 ton flywheel!

Acceleration in lower gears and engine braking were both a bit of a struggle. Andy's point was also true - change down and it engine braked lots until the revs matched back.

To be fair, it wasn't quite 1 ton, but it was the heaviest flywheel I've ever heard of. You could machine 11 kgs off it apparently!

 

With repect to the idea of running lightened pulleys, it's worth remebering that the crank pully (the one that drives the auxilliary drive belts) doubles as a harmonic damper for the crank, so I wouldn't suggest replacing it with anything that doesn't also service this requirement.

 

But it will help you corner faster.....

 

Andy.F,

I agree and that was what I was thinking - engine braking as a practical driving issue, especially on track, is when you are say at the end of a long straight and changing from 5th to 3rd and the car wants the revs to rise - otherwise, why heel and toe - anyway that is what I was thinking of and explained it really badly.

With a lightened flywheel - the engine will 'brake' less in this scenario.

Head hurts less now.

Trout

 

When Scoobysport replaced my clutch I got a lightened flywheel at the same time, I think it was taken down to 7.5kgs.

Anyway, I have found that the car is much more responsive, this is not just due to the new clutch.

Barge.

 

A common mod for the normally aspirated imprezas in the US is 'underdrive pulleys' Ie. smaller auxilliary pulleys to reduce inertia, and reduce the power taken off by the alternator etc. Worth about 5bhp I think?

 

Toerag

I doubt lightening the front pulley would give 0.5bhp, let alone 5 bhp. Its got a tiny diameter and doesn't weigh much. Also, as Moray said, its used as a resonance damper for the vibration in the crankshaft.

If you spin the alternator more solwly, it just works harder, putting more strain on the belt. Again, no gain.

Taking 0.5kg off at a radius of 40mm will have virtually no effect compared to removing 4.5kg at 180mm radius.

And if it did give 5 bhp, removing 4.5kg off the flywheel would give a pro rata increase of 198bhp. It would be nice, but I have my reservations

 

Lightning the flywheel on a turbo application will give you fast 0 to figures but you will loose more mid range speed.

Here is a picture of my standard lightened flywheel.

http://www.geocities.com/subman2001nz/images/flywheel1.jpg

 

Is it as simple as that? I mean to remove some weight and not every last bit.

So if I turn a swathe off the back (it is the back i can only assume) of the fly wheel that's good enough?

hmm, lightnened flywheel here I come....

Paul

 

eesh, had to delete that!



[Edited by BruceWarne - 8/8/2002 12:03:19 PM]