# Lightweight flywheel on a turbocharged engine?



## Irish GTR (Apr 23, 2007)

Just a general question too all the engine guru,s out there.

Does a lightweight flywheel actually benifit a turbocharged engine??

Would there be much difference between,say an RB26 with a lightweight flywheel to say an Evo6 with a lightweight flywheel.

Would a 4 cylinder turbo engine benifit more than a 6 cylinder turbo engine?

Thanks.


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## jaycabs (Nov 16, 2007)

i cant answer the difference between the 4 cylinder and 6 cylinder but when i had mine changed from a standard flywheel to the the light weight one that was part of the exedy clutch kit i got , straight away you notice how much moor freely the engine revs and throttle response is instantly sharper and crisper .

i was quite suprised how noticable it was , deffinatly worth getting one i'd say :thumbsup:


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## blue34 (Jul 28, 2005)

Irish GTR said:


> Just a general question too all the engine guru,s out there.
> 
> Does a lightweight flywheel actually benifit a turbocharged engine??
> 
> ...


Hope your not thinking about lightening your standard flywheel.


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## Irish GTR (Apr 23, 2007)

blue34 said:


> Hope your not thinking about lightening your standard flywheel.


No,Im not.:runaway:

It was just something that was said to me the other evening by a mate.

So Im just wondering does a lightweight flywheel actually work well on a turbocharged engine??


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## jaycabs (Nov 16, 2007)

Irish GTR said:


> No,Im not.:runaway:
> 
> It was just something that was said to me the other evening by a mate.
> 
> So Im just wondering does a lightweight flywheel actually work well on a turbocharged engine??


your helping the engine rev easier like i said in my last post , and i would deffinatly say they do work well on a turbo charged engine .

i wouldnt even dream of switching back to standard as it just wouldnt drive as good.


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## Irish GTR (Apr 23, 2007)

Thanks for the input folks.Appreciate it.


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## Alan (Jul 1, 2001)

blue34 said:


> Hope your not thinking about lightening your standard flywheel.


I had mine done by Hewland at Silverstone big diff


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## Asim R32GTR (Sep 24, 2003)

With a light weight flywheel the only advantage is faster revs.

Downside is, less torque, use more revs to get the car running in first gear = more wear on the clutch, engine dies more easily if you dont rev it enough during engagement. 

Worst cases i have seen it makes the car lumpy on idle, but that is with superlight weight flywheels. 

My honest opinion is, use light weight flywheel on NA cars. turbo cars "need" the heavy flywheel


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## Nismosis (Apr 27, 2006)

My GTR has a lightened flywheel as part of the twin exceedy clutch that was fitted, I can't say what it was like to drive with the standard flywheel as I bought the car with a G-max clutch (had lightened flywheel). All I can say is that the car is super smooth on tickover, it revs very freely and throttle response is excellent, I love to 'blip' the throttle and I'm very impressed with how quick the revs rise and fall.. My Vauxhall Vectra's flywheel must weigh about 500kg as it takes an age for the revs to fall after blipping the throttle... (totally different car I know )

TBH I don't think it matters if it's NA or turbocharged, I'd go for a lightened flywheel all day long, so long as it's not too light..


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## jaycabs (Nov 16, 2007)

Nismosis said:


> My GTR has a lightened flywheel as part of the twin exceedy clutch that was fitted, I can't say what it was like to drive with the standard flywheel as I bought the car with a G-max clutch (had lightened flywheel). All I can say is that the car is super smooth on tickover, it revs very freely and throttle response is excellent, I love to 'blip' the throttle and I'm very impressed with how quick the revs rise and fall.. My Vauxhall Vectra's flywheel must weigh about 500kg as it takes an age for the revs to fall after blipping the throttle... (totally different car I know )
> 
> TBH I don't think it matters if it's NA or turbocharged, I'd go for a lightened flywheel all day long, so long as it's not too light..


i knew exactly what it was like with the standard flywheel then changed to the light weight flyweel and every thing you said is spot on to what its like when you change the flywheel :thumbsup:


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## jaycabs (Nov 16, 2007)

Asim R32GTR said:


> My honest opinion is, use light weight flywheel on NA cars. turbo cars "need" the heavy flywheel


asim , from reading some of your posts before i know you probably know quite a bit more in detail about GTR's more than me but to say turbo cars "NEED" a heavy fly wheel ? thats a strong word to say "need" dont you think ?

yes there can be issues with stalling easier when engaging but you learn how to get use to that as like driving any different car but every thing else about having a light weight clutch exceeds the benifits of not having one .

i run a little over 500 bhp and my tickover is smooth and i got use to my clutch and flywheel , at first i stalled a few times as my clutch is quite haevy on the leg too but after that all is fine.

and like nismosis said , the throttle response blipping up and how quick the revs come down is quite quick.

and in first gear i know i dont hit full power on mine but thats only because my boost doesnt hit in till 5000 rpm so obviously 1st is over and done with extremely briefly but ive still got standard cams too and in first gear it pulls more than hard enough for me not to worry about maybe losing a little torque in 1st gear as every other gear i go through feels like power increases which works realy well . ive got 400lbft so have no issues with torque loss in first gear


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## Asim R32GTR (Sep 24, 2003)

jaycabs said:


> asim , from reading some of your posts before i know you probably know quite a bit more in detail about GTR's more than me but to say turbo cars "NEED" a heavy fly wheel ? thats a strong word to say "need" dont you think ?
> 
> yes there can be issues with stalling easier when engaging but you learn how to get use to that as like driving any different car but every thing else about having a light weight clutch exceeds the benifits of not having one .
> 
> ...


No worries mate 
Thats why we have forums, to share experience. 

It is a bit difficult for me to explain in english :runaway:

Reason for me saying a turbo engine "needs" a heavy flywheel is:
Turbo engine usually dont have much VE at low rpms, and dont produce much torque at low rpms. When a heavy flywheel is used there is more mass in rotation, and helping with torque. 

Yes your engine will rev very easy when in neutral, but that is not the case when you are in 1-2 gear. The momentum is very low. 

Another problem can be for dragracing purpose, the car will bog down more easily with a light flywheel, beacuse revs do get faster up with light weight, but they also come down faster. 

Dont know if you have heard of the famous blue WRC Escort from Norway? it was my friends car, (ex carlso seins car). We have tried lots of different flywheels, and best result was with a stock weight flywheel. 

Asim


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## jaycabs (Nov 16, 2007)

Asim R32GTR said:


> No worries mate
> Thats why we have forums, to share experience.
> 
> It is a bit difficult for me to explain in english :runaway:
> ...


i understand what you mean about having that rotational weight of the flywheel as its just like those old kinder egg sweats that use to have those toy cars in that used a weighted disc for when you pushed it , the weight of the disc would have such a momentum that even if u blocked the car from traveling with your hand it would some times still be keeping the wheels spinning on the surface and obviously if the disk was realy light it would lose rotational force quite quick. if that makes sense lol ?

the thing is how long do we all spend in 1st and 2nd gear ?


when driving in gear even in the first couple of gears with the light weight flywheel it does feel rather more lively .


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## Darbo (Nov 2, 2003)

the prodrive imprezas used to use different flywheels for different surfaces
the one my mate showed me off his wrc impreza was unbelivably thin 10mm maybe with sprocket style on outer rim maybe 5mm thick
think they used a smaller multiplate not the std 7.1/4 inch

throttle response was snappy


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## jaycabs (Nov 16, 2007)

Darbo said:


> the prodrive imprezas used to use different flywheels for different surfaces
> the one my mate showed me off his wrc impreza was unbelivably thin 10mm maybe with sprocket style on outer rim maybe 5mm thick
> think they used a smaller multiplate not the std 7.1/4 inch
> 
> throttle response was snappy


on an overall basis i personaly say the the benefits with a light weight flywheel are easy to choose over a heavy one when having that experience to try with and with out .


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## WillRobDon (Jul 29, 2006)

Less rotating mass is surely a good thing?

I had a Renault 5 turbo years ago that i changed the flywheel out for a lighter one and the difference was impressive, throttle response better and seemed to rise throught the (small!) rev range much easier. 

Surely the same effect would be felt no matter what engine its on. I can't see how torque would be affected?


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## markM3 (Jan 7, 2008)

How can a flywheel affect torque?

It will keep the engine from stalling with its inertia, but the engines torque development won't vary with a different flywheel??

Plus, wouldn't the engine use mor torque accelerating the heavier flywheel?

Cheers,


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## Tarmac Attack (Oct 2, 2008)

A flywheel does not affect the torque an engine produces. It is just rotational mass that neither adds nor subtracts power. It does affect how the power is delivered, though.
A heavy stock flywheel will not rev as responsively. But because it is heavy it has a greater momentum built up so the revs don't drop as much in between gear changes.
A lightweight flywheel will rev responsively. And now that there is less mass rotating in the assembly revs will drop a bit quicker in between gear changes. But it is similar to having a lightweight driveshaft. You are reducing weight in the direct driveline, so it takes less power to turn, therefore you get a little more power to the ground.

-Justin


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## Irish GTR (Apr 23, 2007)

Thamks for the info and different pieces of advice/thought.

Its not for a skyline,but its 3.7 kilos in weight,against the normal 8 kilo flywheel.

Anyway,thanks again.


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## R.I.P.S NZ (May 7, 2004)

I'm with Asim on this one although my experience has been more with street/strip type combinations.
I know of several guys who have put in light flywheels and gone alot slower at the drags, they stall, have trouble getting off the line, no real "grunt" behind the launch.

An incredible amount of energy can be stored in a quickly spinning flywheel and the heavier it is, and depending on the design/shape etc it can make a massive difference to how a car performs.

A light flywheel may allow you to blip your throttle in neutral and get the quick response etc but unless your making more power and actually accellerating your car faster, you simply do not get your revs rising quicker through the gears on the road with a light flywheel, its impossible (unless your clutch is slipping).

If you went to an extreeme with a suitably designed heavy flywheel spinning at high speed, with a suitable coupling and clutch/transmission arrangement you could even turn your motor off at the startline and still run extreemly quick times at the drag strip (as in quicker than most of will ever run in a normal style car) all in total silence. :thumbsup:

Rob


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## Asim R32GTR (Sep 24, 2003)

R.I.P.S NZ said:


> I'm with Asim on this one although my experience has been more with street/strip type combinations.
> I know of several guys who have put in light flywheels and gone alot slower at the drags, they stall, have trouble getting off the line, no real "grunt" behind the launch.
> 
> An incredible amount of energy can be stored in a quickly spinning flywheel and the heavier it is, and depending on the design/shape etc it can make a massive difference to how a car performs.
> ...


Rob

Thanks for the backup mate  
I had a hard time explaining this. :chairshot

Asim


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## markM3 (Jan 7, 2008)

A heavy inertia flywheel will be good for a drag launch, but for normal driving it will slow down your pick up, as the engine has to accelerate more driveline mass rather than launching with 7K revs.


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## Irish GTR (Apr 23, 2007)

This billet flywheel and clutch WONT be doing drag launches.Its for a 4 cylinder turbo engine and will be for a fast road/weekend fun car.

But deffinately no drag launches.

Thanks for the different views on this,makes for interesting reading.


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## jaycabs (Nov 16, 2007)

ASIM AND ROB -

it does make sense what your saying completely to me but all im saying is that on the road when i changed from the standard flywheel to a light weight flywheel the GTR just feels much more better to drive , engine feels more lively even in gears , and overall just more responsive.


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## ExScoobyT (Jan 6, 2004)

Thats not quite right.

A heavy flywheel benefits engines with low torque at low RPM only during launch. 

It also reduces effective torque available to accelerate the vehicle.

A heavy flywheel, once rotating at a given rpm has a given inertia. A lighter flywheel at same RPM has less inertia.

The energy required to accelerate the mass of the flywheel will be greater with a higher mass.

So, for the examples given above, i.e launching a heavy car with minimal torque at 1300rpm, a heavy flywheel helps because the energy stored in the rotating mass is transfered to the drivetrain during clutch slip. This helps to accelerate the mass of the vehicle (overcome the total inertia).

Once launched, a lighter flywheel will always increase the rate of acceleration of a given vehicle / engine. 

The mass of the rotating assembly (this includes engine pistons, crankshaft, flywheel, clutch, gearbox input and output shaft, propshaft, driveshafts, wheels, brake disks, diffs etc etc etc) requires energy to accelerate it. 

It gets this energy from the flywheel (obviously).

For example, in a typical 1400kg FWD car 25-30% of the available flywheel torque is used to accelerate the inertia of the drivetrain. In 2nd gear its around 8-10%.

This is why carbon propshafts have such a noticeable effect in high torque GTRs.

Of course there is a balance in all things. Too light on a turbo engine and you will get crap idle, crap clutch burning launches etc.

You also need to account for the clutch inertia. So most aftermarket kits advertise a nominal drop in flywheel weight but fail to mention the increased total mass of the 4 clutch plates, friction disks and basket!

In the case of a drag launch, you are in fact feeling this effect in action. The `bogging down` often discribed is the point at which all the inertia of the flywheel has been transfered to the drivetrain and the clutch has locked, if at this point you don`t have revs or boost, you bog.

The best launches are balancing this energy trasfer with clutch slip, so that at the point the clutch locks you have maximum beans available from the engine.

Not that I know about drag launches, but I imagine its pretty blood hard in a 900hp, 4wd, effectively slick shod nutter car! Put a lighter prop / flywheel in and the balancing act becomes even harder. However once the clutch is locked the same applies, your rate of vehicle accel for a given torque output will be higher.


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## R.I.P.S NZ (May 7, 2004)

Sure, I can see what your saying is correct in theory, but I'm still on the fence as to "how much quicker" the lightler flywheeled car would accellerate once at a reasonable rolling speed.

Lets say we have 2 identicle cars, one with a flywheel/clutch combo that weighs 15kg and another with a flywheel/clutch combo that weighs 25kg, everything else about the cars is the same.

They both drive along the road at 60mph in 4th gear (say around 4000rpm) and both nail it, what sort of difference in accelleration are you saying the car with 10kg less flywheel/clutch weight would have? 

I'm sure in theory it would be measurable, and some people might even say they can "feel" its quicker but your engine is still making the same power, the second car weighs 10kg less so thats a very minute advantage, but I can't see the lighter car pulling away enough to be able to say "holy shyte" that light flywheel sure makes a big difference!!" 

I think of it sort of like guys who come in here and say they have or have been in REAL FAST GTRs that have made 800hp on the dyno with some hot cammy 26 with a big turbo, sure it might feel cool and you might get a big hit of power etc but then I take them for a ride in a 650hp RB30 GTR with a smaller turbo, very mild manners and a glovebox full of timeslips the other GTR owner could only dream of getting.

Then I ask them how much power do you think the car you just went in has, most say around 900-1000, I tell them its 650 and they walk away with alot to think about.

As with anything, its the total combination that gets you from A to B the fastest and what might be great (or slightly faster in a dedicated track car with data logging etc) might in fact be quite a bit slower in real world, on the road or at the dragstrip/trackday conditions.

Rob


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## ExScoobyT (Jan 6, 2004)

In your example not a great difference!

The difference becomes larger the greater the rate of acceleration and the higher the moment of inertia of the rotating mass. Its not just about the mass, its the distribution of the mass relative to the centre of rotation. 

So the biggest difference is when rate of change of rpm is at its highest e.g 1st and second gear.

So take a street GTR at say 550/600hp, one with carbon prop and decent low inertia clutch and flywheel and one with std bits. The `lower inertia` car would definately pull away from the other in 1st and 2nd. Small advantage in 3rd and almost nothing in higher gears (higher roadload and lower rate of change of rpm).

And as I`m sure you know, how the car feels subjectively can make a massive difference to the driver experience. A car that is a little more responsive to heel and toe downshifts, picks up better between shifts etc cant be a bad thing!


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## bigkev (Mar 2, 2008)

ooh physics and cars my 2 favourite things.

just to reitterate, as some have already said, a lightweight flywheel is a massive dissadvantage in the drag strip. the turning inertia (torque) is stored in the flywheel during 'prelaunch' and dissipated through the clutch to the drivetrain during launch, the flywheel is actually not part of the drivetrain, it is permenantly connected to the engine. a higher mass fly has more momentum storage ability and therefore helps the engine to not lose rpm by keeping it turning even after there is no input to the engine, eg when you take your foot off the accelerator to change gear. i always understand things better when i go to extremes. imagine a massive 6 foot diameter 500kg flywheel, in a normal weight car, get that up to 8000rpm lift up the clutch (mental strong clutch) and your car will rocket towards the 1/4 mile finish line without even slowing down, and you wont need any accelerator once launched. sure it wont rev 'responsivly' but it will win. 

food for thought with regards to weight that some people may not know. if a car is capably of a top speed of 200mph, it is also capably of 200mph filled with water, (dissregarding an increase in tyre friction but thats getting deep) it will just get to 200 with a slower acceleration. 

try dropping 2 ping pong balls at the same time, one filled with water, they will both hit the ground at the same time.

However a lightweight flywheel may be an advantage in a very very light track car. remember every action has and equal and opposite reaction. an engine will rev faster with a lightweight flywheel but opposing forces (eg reallife 'hills', 'wind' 'friction' will have a greater effect on the flywheel and thus the engine (they are connected). a rally car on gravel would require a lighter flywheel than on tarmac because its wheels will be spinning between gear changes so that road surface will not be opposing the momentum of the engine as much.


my advise is to do the maths and pick the right weight flywheel for the car.

in most normal cases for most normal cars i would recommend saving weight somewhere else other than the fly.

kev :thumbsup:


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## ExScoobyT (Jan 6, 2004)

Good man!

But the car would no longer be capable of 200mph due to the increase in the roadload (can't neglect that!!!!)


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## bigkev (Mar 2, 2008)

Did that all make sense? im fine with science but my english sucks, i struggle to get my point across. :flame: LOL yeah it wouldnt be capable of 200mph but the difference wouldnt be that much, so you see the point i was trying to make though.

kev :thumbsup:


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## Irish GTR (Apr 23, 2007)

Lads,forget 200mph,I will be doing very very well if this engine/car gets to 155-160mph MAX.


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