How a lightweight flywheel works

How does a lightweight flywheel work?  Amongst the majority, 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 we measured the power output of an engine first with light flywheel and then again with the standard part on an engine dyno, no change in power will be seen to occur. At first it appears that the light flywheel has done nothing and was a total waste of cash. This is not the case. A dyno that shows max power at constant revs does not demonstrate what happens to an engine's power output in real life situations - like acceleration. If an engine is accelerated on a dyno (we are talking about a rate of around 2000rpm a second ) it would show a power output of around 20%-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 a motor 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 internals is much greater that 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.

Now, it may seen unbelievable that by removing a few pounds from the flywheel a noticeable difference to a 3000lb+ carís acceleration will be made. In fact the difference is quite noticeable and the secret behind this is hidden within the gearbox. Everyone knows that cars accelerate at a greater rate in low gears, this is because a carís gear box basically a mechanical lever and just like when using a leaver to lift a heavy object, the gearbox 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 around say 250lbs but the engine internal mass would still remain around 45lbs.  

As for the "virtual" weight loss of a typical lightweight flywheel in the 3-series or M3, we've prepared the full mathematical analysis:

Calculations for UUC flywheels showing exact "virtual" weight loss in each gear -

M3 / 3-series (E36 and E46, 1992-2004) click here to download Acrobat .pdf file

M5 / Z8 / 540i (1996-2003) click here to download Acrobat .pdf file

It is now easier to see were the extra performance comes from when you lighten a flywheel. You effectively "lighten" a car by more than 10% in first gear just by removing mass from the flywheel. As the gear used increases this "lightening" effect is reduced. This is why carís acceleration improvement reduces in higher gears, to very effect in top gear. Great for drags and tight race tracks but will not increase a car's top speed.

You will see the calculations include the diameter of the flywheel, weight lost (same overall rotating mass difference in UUC Stage1 or Stage2 due to pressure plate weight differences), gear ratios including 6-speed application, and typical diff ratio.

The effective "virtual" weight losses are:

GEAR M3 and 3-series "virtual"
weight lost:
M5/Z8/540i "virtual"
weight lost:
1st gear 346.5 lbs. 394.4 lbs.
2nd gear 133.15 lbs. 151.7 lbs.
3rd gear 68.9 lbs. 75.4 lbs.
4th gear  46.18 lbs. 48.5 lbs.
5th gear 36.15 lbs. 37.6 lbs.
6th gear 30.04 lbs. 31.0 lbs.

A general rule of thumb for weight loss equivalence to "gained" power is approximately 10lbs/hp. That is for every 10lbs lost, the car gains the effective performance increase of 1hp.

With that in mind, the effective performance increase expressed in gained power can be expected to be the same as the "virtual" weight lost due to the flywheel in each gear divided by 10:

GEAR M3 and 3-series "virtual"
performance gain:
M5/Z8/540i "virtual"
performance gain:
1st gear 34.6 hp 39.4 hp
2nd gear 13.3 hp 15.2 hp
3rd gear 6.9 hp 7.5 hp
4th gear  4.6 hp 4.9 hp
5th gear 3.6 hp 3.8 hp
6th gear 3 hp. 3.1 hp

This gear-dependent gain is also another reason why a typical 4th-gear dyno pull may not show a significant difference - the calculations show that little more than 4hp would be detected, yet a 4th-gear dyno run shows nothing of real-world acceleration through the gears.  Improvements in rev-matching and upgraded clutch clamping power remain regardless of gear.

Due to the nature of the "virtual" weight loss, typical 4th-gear dyno runs may show miniscule differences.  Real-world acceleration runs will show improvement equivalent to the "virtual" weight loss.

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