Lightening your Flywheel Author : Scott Lambie There has been a lot of talk lately regarding engine mods
and bang for your buck. One of the modifications discussed was the old
lightened flywheel. The mod has been used by motoring enthusiasts almost
since the car was invented and even with today's modern cars, is still in
use today. Now, lightening a flywheel is all well and good but, how many
people really know what are the benefits, how are they produced and what
must be sacrificed when bolting on a lightened flywheel.
Firstly, lightening a factory cast iron flywheel can have
its dangers and is not recommended. The process of casting iron produces a
metal component with a hard, fatigue resistant surface and a particularly
soft core. If this hard surface is removed during the lightening process (
which it often is ) then this will seriously weaken the flywheel and could
result in the wheel coming apart at high rpms, possibly taking the gearbox
and your legs with it.
So how does a lighten 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 occure. 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 occurs 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 kilos from the flywheel
a noticeable difference to a 1000+ kg cars acceleration will be made. In
fact the difference is quite noticeable and the secrete behind this is
hidden within the gearbox. Everyone knows that cars accelerate at a greater
rate in low gears, this is because a cars gear box basically a mechanical
leaver 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 cars mass as only around say 100kg but the engine
internal mass would still remain around 20+kg .
It is now easier to see were the extra performance comes from when you
lighten a 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 this "lightening" effect
is reduced. This is why carÉs acceleration reduces in higher gears, to
almost no effect in top gear. Great for drags and tight race tracks but will
not increase a cars top speed one bit.
Now if having Lightweight flywheels work so well, why 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 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
doesnÉt take as much flywheel energy to run the piston up on the compression
stroke and the idle is by nature smoother.
Just one final point. If contemplating installing a lightened flywheel, it
is much more important to remove mass which is close to the flywheels radius
of gyration ( the flywheels rotating center on mass ) which is mostly found
close to the flywheels outer edge rather than just all over. Due to this, it
is possible to receive greater benefit from removing 1 kg from a flywheels
outer edge than 4kg from the flywheels centre. If in doubt, seek an experts
advice.
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