Flywheel Question, Is lighter better? Options

[ScottD914]

(IMG:style_emoticons/default/idea.gif) Gentlemen and Ladies;
I've spent the winter rebuilding my 1.8 up to 2142 (or there about). The
question I would pose is about the weight of the flywheel I need to purchase.
My new Eagle Rods and KB pistons are much lighter than the stock equipment
from 1974, does the sum of the mass in the case need to correspond with the
mass of the flywheel somehow? I've seen two choices: 17lbs and 12 lbs. Your
thoughts? Thanks. ScottD

[bandjoey]

Copied from the Nissan site: se-r.net. Experts?? Check the math. Is it good math or fuzzy math?
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As far as perceived acceleration goes, your engine sees the mass of your car as a point stuck way out on some lever arm that it has to twist. If your engine is direct drive (i.e. no gear reduction), and you have an M3, you'd need quite a bit of torque to get that 3175 lbs moving faster. So somebody invented gears, which has the effect of changing the length of the lever, as far as the engine is concerned. In an M3, for example, first gear is 4.20:1 and final drive is 3.23:1 so what looked like 3175 lbs to the engine out at the end of that lever without gear reduction now looks more like 234 lbs (3175/13.57), assuming your rear tire has a radius of one foot give or take a few inches.

Suppose you had a magic flywheel with all the mass concentrated at the outer edge. Now the flywheel is stuck directly to the engine, so you can't reduce its effective moment via gearing. The only way you can reduce the moment is by lightening it and/or changing its mass distribution. If you could somehow remove 10 lbs from the rim of the flywheel, and the flywheel's radius was also one foot, then that would have the same effect on acceleration in first gear as reducing the mass of the car by 10x13.57 or 135.7 pounds. Now I am guessing the flywheel's radius is more like six to eight inches or so, so 10 pounds off it's outer edge would have the same effect as reducing the car's mass by more like 70-100 pounds (in first gear). Only you can't take that much weight off the edge, and moments of disks look more like 1/2mr^2, etc. etc. Point is that in first gear, the mass of your car appears to be only 20-30 times that of 10 pounds out at the edge of your flywheel, as far as the engine can tell. So the reduction of weight of the flywheel begins to be pretty significant. Expect bigtime effective acceleration improvements in first gear for proper flywheel lightening, similar to what you'd expect from reducing the weight of the car by anywhere from 70 to 100 lbs or more. The benefits decrease in higher gears in proportion to whatever the ratio is.

Obviously, the lighter your car is to begin with, the bigger an acceleration improvement you'll see since the flywheel mass represents a larger portion of the perceived mass of the car.

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