Jay_

Was speaking to David at API today (very helpful!!) and have bought his Exedy uprated organic clutch kit for my JDM STI 7. I was going to buy a lightened flywheel but have heard my stock one is approx 8kg so it's not worth the massive expense of buying a new one just to save 1 or 2kg.

Does anyone know what the stock flywheel weight is on this model? Have done a bit of searching on here and can't seem to find the answer. Thanks

Mark A

iTrader: (1)

I remember David spending hours trying to save weight off a 6sp flywheel and in the end he felt it was not worth the hassle.

He also (and might want correct me) reckoned that the cheap alloy ones are not worth the money or effort to fit.

Mark A

Jay_

Thanks Mark.

Anyone know the weight of OEM STI flywheel? cheers.

SPEN555

Hasn't David already told you it is approx 8kg?

I always thought the flywheel must be lighter comparing JDM MY03 vs EU MY00 as I seemed much more likely to stall it lol!

APIDavid

iTrader: (4)

Mark,

My opinion is that for normal road use a 4.2 Kg alloy flywheel is too light. The counterbalance ratio to the front crank pulley is being stretched a bit far and if you do go for an alloy flywheel for whatever reason, it is more imperative to fit lightweight front pulleys.

I know a number of people on here will counter that comment with their own experiences. In strict engineering terms it is too light fitted as a stand alone piece.

David APi

PS, Thanks for the business Jay, I have spoken to your fitters and we are getting it all together in good time for you.

Jay_

Cheers David. Do you think it worthwhile sending down that 5.3kg billett flywheel with the order? Just in case my existing flywheel is knacked or isn't 8kg!!!??

Neilo

Jay, if you are going to fit a lighter one, then i would recommend getting a light crank pulley at least, they arent that expensive either....certainly <£100 if you shop around. I know that is what i will be doing, as i will be going for the 4.2 as the car is now mainly not for road use.

Andy.F

David, "in strict engineering terms" can you expand on the reasoning behind your above statements ?
In particular the requirement to retain the flywheel-pulley counterbalance ratio ?
Not a trick question by the way, just curious

Andy

APIDavid

iTrader: (4)

Andy, The front and rear pulleys on any engine are designed to work with each other to counterbalance the harmonic vibrations of the crankshaft.

There are much better people out there that can explain this than me. There has to be a counterbalance if you think about it, as you cannot just think up a flywheel weight and stick it on there, it's weight is carefully worked out to do what is necessary for the inertia of the engine.

It thus follows that there must be a weight on the opposite end of the shaft to counteract the flywheel. If you disturb those harmonics by removing flywheel weight there must [ or should be ] a sympathetic reduction in the weight that works with it.

Cranks flex and l reason that the flywheel and crank pulley are there to even out that flexing.

Quite whether the pulleys that we all supply and fit have the same attention to weight and detail that a manufacturer pays, is open to discussion.

I suspect not. But something should be done to the front crank pulley, rather than not.

In my opinion.

David APi

APIDavid

iTrader: (4)

Jay, I don't have one in stock this very second. They are slow sellers and the one we keep was sold just before Xmas and we haven't caught up yet with a re-order. I can/ will send down a standard one as we discussed just to have the options covered though.

Kind regards David

p1mark

David, you are kind of correct, but i don't fully agree with your engineering principles.

Cranks can suffer from harmonic vibration, and a central mass at either end of the crank can help stop this.

Harmonics at certain RPM's can be noted by FE analysis at the design stage, but it is a secondary consideration compared to the inertia and hence the driveability/road manners of the car. this is what determines flywheel mass.

Harmonics does not necessarily mean flex, and these can be designed out by close attention to the crank web wieghts/sizes etc. and looking at main/pin overlap (i.e the CSA betwen a main/pin determined by journal diameters, stroke, and cylinder pitch etc). For example a 2mm increase in pin journal size might make a massive difference at the FE design stage to the harmonics during running..

What can cause a crank to whip and flex is if on a lightened crank to much material is taken off the webs.

Centralised mass at either end can help, but it is central so will not induce any vibrations into the crank (as long as it is in reasonable balance)

You will find many motorcycle flywheels that wiegh a large %age of the crank, yet they have absolutely nothing on the other end (maybe a little ignition pick up), and do not require any secondary balance shafts.

These cranks have a relatively large overlap, are long, thin and rev at 14K +

Another way of demonstrating it in a practical sense is this- Put a bit of long thin bar in a lathe. Turn it and unless you have correct speeds/feeds/tip geometry you are quite likely to set harmonics into the bar and get chatter. Stop the machine, stick a blob of plasticine around the shaft somewhere and then turn it again. You will find the harmonics either minimised or gone completely. Not the same as i agree as a crank but its the same idea.

In short, having a really light flywheel and keeping the front pulleys o.e will not do anything detrimental at all as regards to vibrations etc. How light you go with a flywheel and the obvious effects it has (some good, some bad) on the driveability and behaviour of the car is a personal thing. I have a 4.6KG one on mine and i think any lighter would make the car more obnoxious than i would care for.

Sorry for the long post!

APIDavid

iTrader: (4)

See!, That's sort of what l was trying to say

So, there is no relationship between the weight of the front crank pulley and the flywheel.?? In terms of one being a percentage of the other because.............................

Thanks for the help with that. There is always someone on here who knows best. Much appreciated for the assistance.

David APi

harvey

iTrader: (48)

Very interesting post. Thanks Mark.

p1mark

Thank you Harvey!

Sorry David, i admit if i did not waffle as much it might be easier to understand

The crank as an item alone is designed to combat any harmonics that may be produced in an engine throughout its rev range. A poorly designed crank, or one that is to light will produce harmonics even if it is in perfect primary balance.

The crank alone will be way too light to give any useful inertia so a mass has to be added (and obviously you need a starter ring, a workable clutch, and something to drive ancilliaries at the other end)

A calculated mass is then added in the shape of a flywheel and a crank pulley. The ratio is not important. obviously packaging comes into it - for example the manufacturer may not be able to package the mass required easily into the flwheel end (bell housing may have to be bigger, may set the gearbox back 20mm or so etc) so he will add that mass at the front on the pulley.

A secondary affect of this joint mass is that it can help dampen any unforseen harmonics or vibrations, but it does not mater where it is.

APIDavid

iTrader: (4)

You da Man !!

Thanks again.

One day I hope to be clever and know everything, but I suspect that day may well be a loooooong way off.

I'm more of a: saw it, file it, spanner it, type of engineer.................

I can do waffle too though !!

David

p1mark

Crank/cam design and manufacture is my job - CNC m'c tools etc are my tools. I have forgot what a file looks like i am afraid

We are both engineers though - and part of the job description is to waffle and bore people

Andy.F

Yes that was more my understanding Mark. There is no need to maintain a counterbalance ratio as such.
In fact the primary limitation on flywheel (or pulley) minimum weight is simply to make it big enough to carry the clutch and strong enough not to fly apart at high rpm !
If in any doubt as to how light a flywheel can be used on a Subaru, check out an auto box flywheel 0.5kg ?

From a driveability point of view however I agree with David. A 4.2 kg can be too light for a road car. Not only does it make pulling away from a standstill more of a chore but on a 'normal' upshift, the engine rpm can drop rapidly and when you let the clutch up there is a momentary deceleration 'shunt' through the drivetrain before power is applied again. The heavier flywheel maintains engine rpm better during a shift and the process is much smoother.

For circuit race use then I would use as light a flywheel (and pulley set) as you can find, this will ease the shock loadings on fast racing shifts and make the car less likely to lock up under downshifts during hard braking.

For drag racing I would go for a mid weight circa 8kg to help with launch consistancy.


Andy.

p1mark

Yep the 2 factors you mention Andy, along with mass for inertia are all a flywheel/crank pulley combo is required to do.

As said, the 4.6KG one on mine (along with lightwieght pulleys) i feel is marginal. Pulling away is fine (only a couple of hundred RPM more required) but you are not as smooth on high RPM gearchanges.

Where the regs allow it, on certain bike engines i tune i completely remove the flywheel rotor and everything associated (only used for charging on some bikes) and machine the crank to in some cases 75% of its wieght (which takes some doing)

Gives fantastic response, good top end power, but a loss in torque (not so important on the track unless its wet or a tight nadgy track) The other downsides are a 3.5K+ tickover is required and you need to reshell it frequently as the crank does whip a bit and cause scuffing.

With a 'crash' type bike gearbox, change speed and clutchless shifts means you cannot feel the relative lack of inertia.

Not really practical on the way to Netto in the scoob

APIDavid

iTrader: (4)

Ah ! BUT, the flex plate for the auto box is just that, the torque converter usually has an annulus weight ring that represents the flywheel, or its weight is built into the torque converter.
Now you are on ground that I am certain about.......

David APi

Andy.F

Thats the T/C though David so it is more like the clutch cover if you want to split hairs
Every rotating element such as the water pump and camshaft have a flywheel effect, thats not to say they can be called a flywheel.

Mark, how does the lightweighting of the crank reduce torque output ? I would expect it to increase the torque transferred to the rear wheel during acceleration as less torque is consumed in the storing of kinetic energy in the form of rotational inertia.

I used to have a 6.0 small block chevy V8 that had a small diameter flywheel that was under 4kg and had a very lightweight, low moment of inertia 7" triple plate clutch fitted.
On that particular set up the inertia to keep running at idle came mainly from the crank which was some 24 kg IIRC !


Andy