With most any
lightened flywheel, everyone sees the benefits immediately due to the
reduction of the rotational mass. However, if there is a
downside, it's the possibility of a rattle sound that may be
evident at idle/low rpm.
Note that this particular effect
is evident only
at idle with the clutch let out.
For details on what actually causes the sound, click here.
Higher viscosity (thicker)
transmission
fluid will damp the rattle effect, but may not be suitable for cold
climates or high-mileage transmissions. The goal, then, would be
to find a fluid that would have the high-temp heavy viscosity to damp
the rattle but retain the low-temp thin viscosity and synchro additives to ensure all-condition smooth operation.
After experimenting with different viscosities and types of transmission lubricants, we have come up with a formula that significantly reduces or eliminates the gear rattle in the majority of test cars.
The recipe:
1 quart bottle of Redline 75W140NS Gear Oil.
(
click
here to order ) The "NS" oil has no friction modifiers,
thus does not increase shifting effort. From the Redline
information about the gear oil: "a GL-5 which does not contain the
friction modifiers for limited-slip hypoid differentials. This makes
the transmission synchronizers come to equal speeds more quickly,
allowing faster shifting and much easier low-temperature shifting."
and...
1 quart bottle of Redline D4 Synthetic ATF. (
click
here to order ) "D4 ATF is designed to satisfy the improved
low-temperature requirements which will be in the new Dexron IV
specification, and the improved shear-stability requirements which will
be part of the new Mercon specifications. The balanced frictional
characteristics provide smooth and consistent shifts for extended drain
intervals. Provides improved shifting in cold weather."
How to use:
Drain the transmission (preferably when warm so as to allow as much of
the original fluid to drain). Put in the full bottle of 75W140NS,
then fill the remainder with the D4 ATF. Note that the majority
of BMW transmissions will not take 2 complete quarts, so you will be
left with approximately 1/5th of the bottle of D4.
HOT CLIMATE MIX
If you live in an area that is normally
hotter than average (California or any Southern or SouthWestern state)
then the ambient temps might make your transmission run hotter.
In turn, this lowers the viscosity of the transmission fluid, which
lessens it's ability to damp the gearbox rattle. Such being the
case, we suggest substituting Redline MT-90 for the D4 ATF.
So the mix is 1 bottle 75W140NS and the remainder of 1 bottle of MT-90.
MT-90: "MT-90 Manual Transmission
Lubricant; a 75W90
GL-4 gear oil designed for use in manual transmissions and transaxles.
Provides excellent protection of gears and synchronizers and its
balanced slipperiness provides a perfect coefficient of friction,
allowing easier shifting." ( click
here to order )
