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'03-'04 Cobra (Eaton) Belt Length Recommendation Chart

Credits and Disclaimer...

The information in the Chart was obtained from Metco Motorsports. I would like to extend to them a "thank you" for allowing me to use their belt length recommendations chart. I believe it to be the simplest and most accurate method available to determine your belt size. To my knowledge, the Chart is extremely accurate when properly used. However, I cannot personally guarantee the accuracy of the information, so please use at your own risk. Lastly, I cannot offer personal size recommendations as it is beyond the scope of my ability due to the multitude of aftermarket blower configurations.

Special Notes...

  For those interested in browsing through my Original Belt Selection Guide, please Click Here. You will be directed to that page. Be aware that that Guide only provides recommendation with the factory lower crank pulley.

  Pulley diameters are all measured from the top of the tooth.

•  Pulley sizes shown are the most widely used.

•  Stock idlers are 90mm. Belts are 8-rib design.

•  Stock alternator pulley is 2.6".

  Belt manufacturers size their belts in different ways, so please veryify your specific belt part number need with your parts supply house.

  Please read the article below entitled, 'The Importance of Proper Supercharger Belt Tensioning'.

  String/yarn method to check belt size.  Here is a fool-proof way to find your supercharger belt size. Using a piece of thick string or a piece of yarn, run it around the idlers and upper pulley, apply full tension to the tensioner, and measure the string. I've had many people use this method and it works well every time.


2003-2004 Mustang Cobra Belt Length (Eaton Supercharger)...

The information in the Chart was obtained from Metco Motorsports. I would like to extend to them a "thank accurate method available to determine your belt size.

 Crankshaft Pulley Size  3.65" Pulley (Stock)  3.00" Pulley  2.90" Pulley  2.80" Pulley  2.70" Pulley
  Stock (7.375")   75.50"   74.50" 74" 73.50" 73.50"
  2 lb. (8.0")   77"   76.50" 76" 75.50" 75.50"
  4 lb. (8.60")   78"   77.50" 77" 76.50" 76.50"
  6 lb. (9.10")   79"   78.50" 78" 77.50" 77.50"
  8 lb. (9.55")   80"   79.50" 79" 78.50" 78.50"
  10 lb. (10.0")   81.50"   81" 80" 79.50" 78.50"

Add 1" with a Metco Auxiliary Idler
Add 1" with a 3.20" Alternator Pulley
Add 1.5" with a 3.50" Alternator Pulley
Add 2.0" with a 3.70" Alternator Pulley
Add .5" when changing from a 90mm Idler to a 100mm Idler

       Pulley diameters are all measured from the top of the tooth.

The Importance of Proper Supercharger Belt Tensioning...
Authored by Metco Motorsports

Proper supercharger belt tensioning is important. The serpentine accessory/supercharger drive systems on modern vehicles have little in common with the standard V-belt drives many of us are familiar with. Here are a few tips that may help minimize belt slippage, maximize pulley bearing and belt life, and diagnose problems with serpentine belt drives. Minimizing belt slippage will help to prevent a power loss at higher RPMs. The addition of a Metco auxiliary idler (stock idler setup has 3 idlers) can also help minimize belt slippage.

1.  Serpentine Belt Tension Is Critical
Serpentine belt systems are designed to operate at a very specific tension, unlike old V-belt systems that run at maximum tension to prevent slippage. Factory serpentine belt tensioners have a carefully engineered spring mechanism to maintain the designed tension on the belt, and the addition of manual belt tensioners and the use of non-stock belt lengths can lead to improper tension on the belt. Serpentine belts are very susceptible to stretch when over-tensioned, and stretch will lead to slippage and ultimately the failure of the belt. Additionally, the vast majority of bearing failures in stock and aftermarket pulleys can be traced to over-tensioned serpentine belts. Belt lengths must be chosen with care (see the Belt Length Recommendation Chart above), and manual tensioners should be adjusted with caution.

2.  Know How To Read Your Belt Tensioner
The belt tensioner used by Ford in the supercharger drive system has a convenient indicator to show when the belt is properly tensioned. The indicator is comprised of a pair of raised 1/4" long pad on the tensioner housing and a corresponding raised rib on the tensioner arm. In the photo above the tensioner indicator is highlighted in the circle. The tensioner indicator is best viewed from the passenger side of the vehicle, and when the belt is properly tensioned the raised rib will appear centered between the high and low pad on the tensioner housing. Variations of +/- 1/16" are considered acceptable.

3.  Changing Pulley Ratios And Using Aftermarket Superchargers Will Make Belt Tension More Critical And May Affect Pulley Choice
The use of overdrive (smaller) supercharger pulleys and overdrive (larger) crank pulleys will affect belt loads, belt speeds, and the speeds of any accessories that share the supercharger drive belt (like the alternator on the Cobra). Addionally, the use of an aftermarket supercharger adds significantly to the belt load because these high-performance blowers typically take more power to drive them. In these applications, extra care should be used to maintain proper belt tension, and in extreme cases, the use of heavy-duty double-bearing idler pulleys like Metco Motorsports' 90mm Double-Bearing Idler Pulley should be considered.

4.  Diagnosing Belt Noise and Belt Damage
The supercharger drive in high-performance applications is often noisy, and finding the source of the noise is not always easy. We have found that an audible squeak or chirp that is present upon engine start-up but disappears as the engine warms is often caused by the supercharger belt itself. When the engine is cold and the belt is cool and stiff, noise is created as the belt transitions from one pulley to another. As the belt warms, it becomes more pliable and the noise diminishes. In the case, the solution is to replace the belt.

A noise that is constant or becomes worse as the engine warms is more likely a pulley or an accessory on the supercharger drive. The best method to find the exact source is to simply release tension on the belt and spin each pulley in the supercharger drive by hand. A pulley or an accessory with a failed bearing should be obvious - there should be discernable drag, unevenness, or wobble evident in the faulty component.

If necessary, the bearings used in Metco pulleys can be removed and replaced (in most instances). The part number for the appropriate bearing is printed on the bearing seal, and these bearings (or equivalents) can be sourced from most parts stores. Please contact us for information about the bearing replacement procedure.

If your supercharger belt shows evidence of fraying at the front or rear edge, there is an alignment issue that should be corrected. Typically, most alignment problems involve an improperly installed supercharger pulley, or an out of alignment idler pulley. Alternator alignment can be adjusted on the Cobra by grinding the mounting points of the alternator bracket, and (if necessary) idler pulleys can be spaced forward by adding a 16mm flat washer behind the pulley. Another cause of belt damage can be a damaged/bent tensioner.

In all supercharged applications, blower belt life will be significantly shorter than the service life for a typical accessory belt. Supercharger belts should be replaced regularly with the highest quality belts available. We have found the Goodyear Gatorback and Gates Micro-V Green Stripe to be the longest lasting.