I agree, no way you shave .5 sec by changing to aluminum drive shaft.
From discussion on the subject on a drag racing site.
Drive Shaft Example
Now let's think about a drive shaft. The driveshaft is a fairly thin hollow tube. Nearly all drive shaft weight is at the outside, since it is (of course) hollow. The shaft also turns at the same RPM no matter what the driveshaft diameter, because the RPM is set by the rear end ratio, tire diameter, and vehicle speed. If we make a driveshaft lighter and keep everything else the same, the vehicle acceleration change is often insignificant.
Why would it be insignificant in most cases?
In the first place, the drive shaft is small in diameter. With a small diameter, less energy is stored for a given weight. In the second place, a driveshaft is really not that heavy. A steel Mustang driveshaft weighs somewhere around 30 pounds, so we just can't take that much weight out.
Also, the driveshaft spins up gradually and smoothly over a long period of time. It accelerates fastest at slowest speeds, and that is when it needs the least energy to spin up. Because it has a long time to spin up, is a small diameter, and because it does not weigh much, the driveshaft does not remove very much horsepower at any instant of time. Despite what we are told, a change in driveshaft weight has, at best, a very small effect on acceleration. Likely any change is immeasurable in a street/strip car.
Now a lighter shaft certainly can help in a very light vehicle. It can also help in a road race car (as will a light crank and flywheel), because road racing requires instantly changing from acceleration to deceleration.
A light driveshaft won't change anything significant or measureable in a 3000-pound 11-second car, except how fast dollars leave your wallet!
Another worry is driveshaft diameter. If we go from a 30-pound 3-inch steel driveshaft to a 30-pound 3.5-inch aluminum shaft, we move the weight out 3.5/3 = 1.167 times. That increases stored energy 1.167^2 times, or 1.36 times. If we store 0.3 horsepower in the shaft, changing the diameter will increase that to 0.4 horsepower. We would have to reduce weight 14.3% to 25.7 pounds just to break even with the diameter increase.
The worst thing about a driveshaft is the diameter is so small, and the acceleration time is so long, there just isn't much horsepower being sapped from the system. A typical steel driveshaft in a typical 12 or 13 second car only stores an average of about 1/4 horsepower. If we got 100% of that back with a zero weight shaft, we would never notice it.
Good reasons to change a driveshaft are to get rid of vibration and harmonic resonances in the shaft, to make it stronger, or to simplify a two-piece driveshaft system. The silliest reason is to speed the car up. Even if we only pay $100 for a shaft, it would typically be much less than 1/4-horsepower average gain. That would be paying much more than 100/.25 = $400 per horsepower. Paying a lot more than $400 per horsepower is not a good investment.
