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The capacitor discharge ignition uses capacitor discharge current to the coil to fire the spark plugs.  Engines with high lift cams (several steps above stock) can benefit greatly, as a CDI system will burn fuel in the low rpm range very efficiently. You will not see any serious street rod without this type setup. 


CDI was originally developed to overcome the long charging times associated with high inductance coils used in inductive discharge ignition (IDI) systems, making the ignition system more suitable for high engine speeds (for small engines, racing engines and rotary engines). 

The capacitor discharge ignition system can be traced back to the 1890s when it is believed that Nikola Tesla was the first to propose such an ignition system. In U.S. patent #609250 first filed February 17, 1897, Tesla writes 'Any suitable moving portion of the apparatus is caused to mechanically control the charging of a condenser and its discharge through a circuit in inductive relation to a secondary circuit leading to the terminals between which the discharge is to occur, so that at the desired intervals the condenser may be discharged through its circuit and induce in the other circuit a current of high potential which produces the desired discharge.'

This was put into practice starting in 1906 on the Ford Model K. The Model K had dual ignition systems, one of which was the Holley-Huff Magneto, or Huff System, manufactured by the Holley Brothers Company. It was designed by Edward S. Huff with US patent #882003 filed July 1, 1905 and assigned to Henry Ford.

You can read more about the history of CDI on Wikipedia.

For those needing technical info, please give us a call at 423-722-5152.

Why your engine needs CDI

Engines with medium to aggressive cams cannot burn fuel efficiently in the low rpm range. This is due to the aggressive cam profile, large cylinder head and intake runners, and high CFM carbs in most all medium to high performance engines. Engines with a so-called lopey idle that doesn't smooth out till around 2000 or more rpm are responsible for much of the carbon build up in the combustion chambers, and occasional plug fouling when cruising for any length of time. This also worsens the drive-ability, performance and fuel mileage. 

CDI systems improve combustion to such a degree that you will notice the difference in many ways. The intense multi spark function is the key to it all. If you have a medium to high performance engine, and are running an HEI distributor, then all the above issues mentioned will most likely become a reality to some degree. If you do have an above stock street rod engine with a conventional ignition system, then you are holding it back from its full potential, not to mention the other issues listed in this article by not running a capacitor discharge system.

Even if your engine is not in the high performance range, this system will still offer improvements in many ways over an HEI distributor:

  • faster start ups
  • better low speed drive-ability
  • improved fuel mileage

There are no drawbacks! The MSD unit comes with very clear cut installation instructions for use with many different type distributors. 

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