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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Lightened flywheels
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Mon, 11 August 2003 11:36
 
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if it loses revs quicker when going up hills, it doesn't have enough power  heh heh.
but seriously tho, i'm looking at a lighter flywheel too, either alloy with metal insert or a steel/moly with insert.
a lighter flywheel reduces the rotational inertia. thsi alloys your motor to change it's rotational speed faster (as there is less inertial mass to accelerate/decelerate). you will notice this when accelerating in lower gears, where the inertial mass is a larger percentage of the total vehicle mass. in higher gears the proportion is lower, so it won't affect car acceleration as much.
the main issues with lighter flywheels are idle quality, and starting from stationary.
your motor needs enough stored energy (from the flywheel), when idling, to be able to get thru each compression stroke without stalling. if it's too light, the retarding force of compressing the air/fuel will be large enough to slow or stop the engine.
with low overlap cams, this can be a big problem, but with higher overlap cams (ie lower dynamic compression at idle speeds) the retarding force is lower (less compression force) and a lighter flywheel can be tolerated, as long as the long overlap doesn't cause too much misfiring (and thus lack of power strokes).
when you start from stationary, the inertial momentum stored in the flywheel is transferred in to forward motion when you slip the clutch. with a lighter flywheel, there is less inertia, so you need to put in more energy with your right foot, and be more careful with the clutch to get going without stalling.
the next thing is the danger of lightening a stock cast flywheel, or getting a new one made up. it's your legs and your choice 
another factor is the difference between the inertia stored in a big crank (like the 7M's) and the flywheel. you are trying to lighten the whole rotating mass (pistons and rods included), so 25% off the flywheel may only be a 10% gain overall....
there are plenty of good sites around that deal with lightened flywheels.
Cya, Stewart
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