Lightweight flywheels V's Laggy Turbo's

Camped on the site Joined: Jan 18, 2005 Posts: 325

Does any one have any hard facts as to the usefulness of lightweight flywheels on a Turbo engine that is running a lot of boost?

Any one got any facts and figures from the real world on the %age overall affect of flywheel lightening. Interested in 1st and 2nd gear NA and forced induction multi cylinder engines (I DON'T CARE ABOUT HOW FAST YOUR MOPED IS IN 1ST).

Interested as more and more people seem to be putting them on turbo motors and i would like to know the validity.

Posted: Wed Nov 21, 2007 6:11 pm

I'll see if I've got some articles kicking about; from what I understand, a lightened flywheel means less mass to spin= free-er revving engine. I can't validate this though (for now)- but I'm sure someone on here will (until I can find an article to prove whether my basic understanding is correct or not)

Posted: Wed Nov 21, 2007 6:11 pm

You need to do a little research into a mathematical equation for this.
Known as "Radius of gyration".

Youll also need to know the overall gear ratios you intend to run with.

If the original flywheel is very heavy as with Kv engined audis that have that huge cast iron fly with the thick ring on the periphery( i replaced mine with a wn coded fly-flat and lighter but still heavy) youll definitely gain an advantage in acceleration, but the effect is lessened as you go up the box.
Basically its possible to lose something like 10 pounds off the flywheel weight and in order to equal the new acceleration difference, youd have to lose a rather larger amount off the vehicle weight to equal it, approaching something like 100pounds-dependant on gearing.

Ive used one in the celica (4.0kg running 13psi) and have noticed no detrimental effects save a small change in rpms to pull away with, otherwise, no negatives here apart from the hassle needed to swap it out-nightmare. Smile

Other advantage is that the main bearings wont be subjected to the same levels of stress as the crank tries to wind itself out of the block against the flys resistance to acceleration.

Ill have a look through my bookwork and see if i can find anything else out regarding this. Smile

Posted: Sat Nov 24, 2007 1:44 pm

Ah a brain dump, physics i can do, think like a real gyroscope all the dense matter is right on the rotational perifery, so to keep momentum they are heavy at the edge but require more effort to get them to run. So a lighter one would spin with less effort but also not maintain its momentum. But flywheels are designed to store energy short term, or to even out the torque through the transmission which drives it. Think like a pull start on an engine or such like... So i would guess on a car a lightened flywheel would allow for a bit quicker throttle response as the engine would require less effort to generate the momentum, but it would also loose it as quickly.

My only experience with flywheels is on the the A6 1.8T, it needed a new dual mass effort. Heavy material on outside lighter on the middle so its a happy compromise, but using two different materials means they can seperate as such, odd feeling its like a slipping clutch only when the car is under heavy torque, i.e. 5th 45-50mph accelerating. I did look at stronger flywheels for the car (considering remap at the time) which where basically single material mass lightened wheels but the idle can be lumpy and they were too much. Dual mass alone was near £300! On a turbo the torque delivery isnt as linear as an NA engine so i guess a lighter flywheel will allow the flywheel to spin quicker without bputting more strain on the turbo as it puts a huge load ont eh transmission/drive as the turbo spools, esp on tuned turbos the torque climb between a few 100 rpms can double. that puts a lot of torque on the drive train, i guess you would want the flywheel to rotate quickly, ie be lighter. .

Camped on the site Joined: Jan 18, 2005 Posts: 325

Sorry , but i mustn't of explained my self well enough. Radius of gyration (american term??) is relative to an entity at rest we need moment and transfere across various entities. The crux is the percentage of acceleration due to engine reciprocating mass as an entity V's gearing. i.e in what gear and at what speed does a lightweight flywheel become a benefit as this must surely be at odd's with a large high boost turbo. Does anyone have any BTDT with facts ???
Bottom line its money that could be spent elsewhere for greater benefit (or not???)

Posted: Sat Nov 24, 2007 3:19 pm

General term (drilled into my head in education) radius of gyration is just momentum on a circular path, the maths are no different in mechanical applications.

I don't have experience in the effects of doing it. As with most of my contributions they are academic, but i do believe the latter half of my gibberings are key. ie a turbo spooling the torque quickly will benefit from a lightened flywheel, how much? haven't a clue... benefit is probably more key as you know modifications tend to work together, giving benefits in some areas and reductions in others.

4v6 is probably better placed to give you a real world answer. I know aspro ran a lightened flywheel in his throttle bodied 16v coupe.

Posted: Sat Nov 24, 2007 3:24 pm

Knew i had this someplace.
Acknowledgement to whoever as i have no idea where i got it.

Effects of lightening rotating engine components: (0.5 x n2 x r2 + R2) / R2 n = total gear ratio (gear ratio x diff ratio) r = radius of gyration R = Radius of wheel/tyre This formula gives what accelerative weight the engine sees of the car per lb. Radius of gyration of a transverse engine's flywheel is approx. 3.75" So to determine "weight loss" for a flywheel weight of say 18 lb to 10 lb, the engine would see an overall weight loss of the car to accelerate in first gear of: 3.33 = 1st gear of 4 synchro 'S' box 3.44 = diff ratio (0.5 x (3.33 x 3.34)2 x 3.752 + 9.52) / 9.52 = (0.5 x 131.2 x 14.06 + 90.25) / 90.25 = 1012.59 / 90.25 = 11.22 lbs So for every 1 lb removed from the flywheel, the engine sees 11.22 lbs less to accelerate off of the total car. Therefore by lightening the flywheel by 8 lb, the engine sees a total reduction of the cars accelerative weight of 89.68 lb.

Any use Gib?

Camped on the site Joined: Jan 18, 2005 Posts: 325

Head Nail Hit

Supply the missing?

Thanks now just need some real world proof, time to hunt in the libary me thinks.