Easy Power Plays for 4.6L 3V Mustangs (Part 2): Ford Racing Aluminum Driveshaft Install & Some Driveshaft Math

This one isn’t a sexy swap. In fact, you might say it’s pretty mundane when looking through the list of mods for Mustangs. However, it’s also routinely rated as one of the most seat-of-the-pants noticeable upgrades.

We’re going to show you how to install an aluminum driveshaft in an S197 Mustang.

But first, let’s take a quick look at why and when a driveshaft swap can help, because it might not match the reasons you’re thinking of.

This is another installment in our series of Easy Power Plays for the 4.6L 3V Mustang. Check out more here:
* Boss 302R Electric Power Assist Steering (EPAS) Conversion
* Meziere Electric Water Pump Upgrade & 2007-10 Cooling System Swap

From the factory, 2005-14 S197 Mustangs are fitted with a roughly 38 pound, two-piece steel driveshaft supported in the middle by a carrier bearing. Two-piece driveshafts are inherently more expensive to manufacture, so why did Ford opt for a heavy multi-jointed beast?

There are three main reasons for using multipiece driveshafts: chassis length, driveline angle optimization, and reduction of NVH (noise, vibration, harshness).

S197 Mustangs are reasonably short wheelbase cars with a 52 inch driveshaft, so we know that length wasn’t the issue. For Ford, it was mostly about driveline angle optimization and reducing NVH. A long, single-piece driveshaft is more prone to NVH than two shorter pieces.

The main reason most enthusiasts look to aluminum driveshafts swaps is weight savings. Typically, switching to an aluminum shaft on any vehicle will reduce the driveshaft weight by anywhere from 10 to 20 pounds. On an S197 Mustang, it’s about 16 pounds. Less weight is always good when you’re chasing performance, right? Yes, but the actual performance gain just from the loss of weight is not that high.

Let’s look at why.

The assumption is that this decrease in driveshaft weight benefits the engine’s ability to accelerate under load. That’s because weight is magnified when it needs to rotate. This is known as the moment of inertia (MOI).

In physics, MOI is the measure of the opposition that a body exhibits to having its speed of rotation about an axis altered by the application of turning force (torque). The amount of torque needed to cause angular acceleration is proportional to the MOI of the body. The heavier something is, the more torque is required to accelerate it. The faster you try to accelerate it, more torque is required. Also, the larger the diameter of the object, the higher the MOI.

So, if you’re trying to accelerate a very heavy, large diameter object very quickly, a lot of torque is required.

However, driveshafts really aren’t that heavy or large in diameter. If we spin up a very large diameter mass, or a very heavy mass, (such as a large flywheel) and we do it rapidly, it requires notable power. However, spinning up a small diameter mass, especially over a longer period, such as accelerating from stop to quarter mile costs much less power. That relatively long spin-up time combined with a small diameter and light weight, means that driveshafts don’t actually cost much horsepower.

If you do the math with an energy storage calculator, a typical three inch diameter one-piece steel driveshaft consumes less than one half horsepower. A two-piece steel driveshaft has more mass and more joints, so the consumption will be somewhat higher.

So, by losing weight and simplifying to only two joints, we are reducing the amount of power stored and consumed by the driveshaft.

Does that mean you’ll see an increase in horsepower at the tires due to less parasitic loss? Maybe, but we’re talking rather low single digits at best. We’ve seen tests show two to three horsepower on the dyno after swapping to lighter one-piece shaft and others show absolutely zero change.

But even if the potential power gain isn’t huge, there are several great reasons to swap to a lighter driveshaft.

While the small possible horsepower gains aren’t going to make much difference for quarter mile times, a lighter driveshaft does help notably in a road race or autocross car like our project, since this type of racing requires instant, repeated changes from acceleration to deceleration. Also, once the miles pile up, more joints in a driveshaft (plus a carrier bearing mounted in a rubber isolator) can mean more flex in the driveline, so a one-piece driveshaft often feels “tighter” or “more responsive” vs. a worn factory two-piece.

This is the seat-of-the-pants improvement most people feel.

When it comes to sustained high speeds, like you see on a road course or open road racing, performance driveshafts like our Ford Performance Parts unit also have an important safety edge because they’ve been designed to have higher than stock driveshaft critical speeds. Critical speed is the speed at which a spinning shaft encounters its natural frequency of vibration and will begin to bend or whip and is dependent on shaft length, weight, diameter, and rpm. This is important to keep in mind for whatever type of racing you do—and believe it or not, the answer is not always a one-piece driveshaft.

Here’s the basic formula for computing critical speed using our 3.55:1 rear gear and 26.2 inch tall rear tire to calculate engine rpm at 140 mph. Note, this is for 1:1 final drive in fourth gear, since we never use fifth on track. For true top speed, you would need to calculate for OD since it actually increases driveshaft speed while decreasing engine speed:

• Engine RPM = (MPH x Gear Ratio x 336) / Tire Diameter
• Engine RPM = (140 x 3.55 x 336) / 26.2
• Engine RPM = 144,883 / 26.2
• Engine RPM = 6,373

Since this Ford Performance Parts driveshaft has a critical speed of 7,500 rpm, we’re well within a safe range.

Ok, enough math and physics, let’s get this thing in the car!

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Click below to see details on the specific driveshaft in the article:

Ford Performance Parts 2005-10 Mustang GT One-Piece Driveshaft – FMS-M-4602-MGTA

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