Hey, with an aluminum flywheel and underdrive pulleys I shaved about 25 lbs. off my engine and it makes 5 times the difference in performance my fiberglass hood and aluminum driveshaft (shortened Mark 8 with M5OD yoke) did. Why, how?
Here are some reasons and or things to consider when thinking about it:
Crankshaft weight:
You gain 2.5 HP per pound removed off the crank at the wheels. Stock AOD converters even without fluid are much heavier than clutch & flywheel assemblies. I put a 15 lb. aluminum flywheel in my SC and use the factory 35 lb. unit for a grinder stand base. I gained 1/2 a second on this mod alone. This is with a wide ratio 5-speed, which is not good for the drags; the most gear I can run with this tranny is 3.27’s. 20 pounds less times 2.5 HP per pound = 50 HP that was used for spinning a big hunk of iron through every gear now getting to the rear wheels.
Rotating weight absorbs more power when accelerating than static weight on a car. I can't remember the exact figure but it seem to recall the SAE says that every pound of rotating weight on a car is equivalent to 1.7 pounds of static weight. This is why we always reduce rotating weight first: wheels, pulleys, drive shaft, etc.
But consider for a moment that the crank assembly is not only a rotating weight but also the only one that is accelerated multiple times in the timed quarter mile. This weight must be accelerated through each gear while the wheels and drive shaft only achieve max speed once, at the finish line. All static weight is the same start to finish. So the most significant place to remove weight to increase performance is the crankshaft assembly. Easily 3 to 4 times more than the drive shaft yet more people think of the drive shaft first. Easily 10 times more than equivalent static weight removed from the car yet most remove static weight second.
Ask any under drive pulley manufacturer where the power comes from, rotating weight reduction or slower accessory speeds. The vast majority of the gain is from the weight reduction.
Lower crank weights make it easier to modulate wheel spin, especially with performance gearing and or street tires. Comes in handy in the rain also.
With light crank weights drive train life is improved due to lower peak torque loads at shifts. This also improves drivability when cruising.
With a lighter converter higher stall speeds can be used without putting the tires up in smoke.
By the way aluminum driveshafts don't usually save more than 5 lbs. due to their larger diameter. What would really flip you out is the new Dodge Ram and I think GM suv shafts, they are some kind of poly matrix. these big long shafts weigh next to nothing.