Lightweight flywheels: what kind of performance gain??

[RS]

Just wondered what kind of performance "increase" did you guys feel after install of lightened flywheel. Does it have negatives (rough idle?)? It'll hopefully be a while before I replace the clutch but when I do, I'll probably install one....thanks

[frayed]

You may want to do an archive search over at bimmer.org. I'm pretty sure I've found some interesting write ups over there.

For cost reasons, I'll probably go with UDP setup from unorthodox racing, although I'd rather have the lightened flywheel.

[Bob ///M3]

The following is a copy I had on file from discussions about this same topic:

There has been a lot of talk lately regarding engine mods and bang for your buck. One of the modifications discussed is the lightened flywheel. The mod has been used by enthusiasts almost since the car was invented and even with today's modern cars, is still in use today. Lightening a flywheel is all well and good but what must be sacrificed and are there any real advantages to bolting on a light flywheel?

There are two schools of thought concerning light flywheels. The first is that they do not contribute to power output. The second is that they do. Which thought is correct? In fact both, in a way, are correct.

If you measured the power output of an engine first with a light flywheel and then again with the standard OEM flywheel on an engine dyno, no change in power will be seen to occure. At first it appears that the light flywheel has done nothing and was a total waste of money. This is not the case! A dyno that shows maximum power at constant revs does not demonstrate what occurs to an engine's power output in real life situations - like acceleration! (Can you say "Shark" performance software?) If an engine is accelerated on a dyno (we're talking about at a rate of around 2000rpm per second) it would show a power output of around 20% to 25% less than at the constant rev state.

The reason for this is that when accelerating a vehicle the engine not only has to push the total mass of the car but the internal components of the engine need to be accelerated also. This tends to absorb more power as the extra power is used accelerating the internal mass of the engine components and is why an engine accelerating on a dyno will produce less power than at constant revs! Also it must be remembered that the rate of acceleration on the engine's internals is much greater than the rest of the car. This would then suggest that by lightening the flywheel, less power would be required to accelerate it and therefore more power would be available to push the car along.

It may seen unbelievable that by removing a few pounds from the flywheel that a noticeable difference to a 3,000+ lb car's acceleration will be made. In fact the difference is quite noticeable and the secrete behind this is hidden within the transmission! Everyone knows that cars accelerate at a greater rate in low gears, this is because a car's transmission is basically a mechanical lever and just like when using a leaver to lift a heavy object, the transmission's gear ratios reduces the mass of the car that the engine sees. For example, in first gear an engine will see the car's mass as only a small fraction (due to the gear ratio) but the engine internal mass would still remain the same as always.

It is now easier to see were the extra performance comes from when you install a light flywheel. In some cases, and not necessarily extreme ones, you could effectively "lighten" a car by 20% in first gear just by removing mass from the flywheel! As the gear used increases (higher gears) this "lightening" effect is reduced. This is why a car's acceleration reduces in the higher gears. A lighter flywheel will have almost no effect in the higher gears. A light flywheel is great for drag cars and tight track racing but will not increase a cars top speed one bit.

If having lightweight flywheels work so well, why might you ask are they so heavy from the factory? The answer to this is quite simple. When a lightweight flywheel is installed, smooth idling almost always suffers- more so if standard (factory) cam timing is used. If a longer period cam is used, the situation is not quite as bad. With such a cam the valves close much later in the cycle so it doesn't take as much flywheel energy to run the piston up on the compression stroke and the idle is by nature smoother.

Bob ///M3

[RS]

hey...good info...thanks guys

[KraZy007]

Hey Bob, so if i get schrick cams and a flywheel, my idle won't be as bad as stock cams and flywheel? thanks

[Bob ///M3]

Most likely that's true but...the VANOS may play a part in that scenario too. Others have used a lightweight flywheel on their M3 without complaining too much.

The only two things I recall hearing about was that some owners would like to be able to adjust the idle speed up a little bit and it is suggested not to add an aftermarket pulley if you already have the lightweight flywheel.

Bob ///M3