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+ car's 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 car's 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
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 car's
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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(c) 2003-2011 UUC Motorwerks * http://www.uucmotorwerks.com *
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