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Old 07-04-2009, 01:04 AM   #1
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Default Spark Plugs for SC Motors

There are certainly many different types of spark plugs available for your SC engine. Many of the plugs you'll find on the market also have a great deal of hype around them. Like they will give you performance gains or improve fuel economy. For the most part, these claims are hard to prove and the performance gain or increased in MPG is actually because you replaced plugs that needed replacing anyway.

We get lot'* of questions about what plugs to run in the SC 3800, and though this should not stand as the forums endorsement for any plug manufacturer, this guide should stand on proven field trials by forum members.

Solid copper core plugs are the preferred plug, because they dissipate heat better and can help prevent predestination (knock) on SC motors. Most of us are running Autolite'*, because they are good solid core plugs and inexpensive. Another solid copper core plug is the NGK TR55'*. Some of these plugs have screw on tops and when they come loose (not a matter of if...) that can cause KR from a poor spark.

Plug'* are designed based on heat range. A hotter plug is usually used on engines that run at lower RPM or put around town and the hotter plug will help burn off deposit and keep the plugs efficiency up. A colder plug is preferred when running at higher RPM'*, huffing on Nitrous or adding boost.

As a general guide, here'* some suggestions for selecting a good Autolite plug for your modified SC motor :
  • If your car is similarly modded like this: has a 3.40” supercharger pulley, CAI (cold air induction) setup, and a lower-temp thermostat. Use 605'* (one heat range colder).
  • If your car is similarly modded like this: has a 3.25” supercharger pulley, 1.80:1 or 1.90:1 rocker arms or very mild cam, CAI setup, lower-temp thermostat, exhaust headers, cat-back exhaust system, and reprogrammed PCM. Use 104'* (two heat range colder).
  • If your car is similarly modded like this: has a 3.0” supercharger pulley, 1.90:1 rocker arms or aggressive cam, CAI setup, lower-temp thermostat, exhaust headers, cat-back exhaust system, port and polished supercharger housing and/or throttle body, and reprogrammed PCM. Use 103'* (three heat range colder).
(Thanks to 3800 Performance for the recommended plugs vs mod list above - it'* the best I've been able to find that explains the relationship between mods and heat range)

It is recommended that you set the gap on these to .055”-.057”.

There has been some discussion on the benefit of opening the plug gap wider to gain better burn on the air/fuel. However; this does and does not really work as you would explanation...

On weaker, or stock ignitions, opening the gap puts added strain on the other ignition components in the system. So, opening the gap beyond stock gap size is not a good idea.

On race ignition systems you are already taking advantage of the output of the ignition box, extended spark duration, and more efficient energy transfer. So, opening the gap beyond recommended plug gap size does little if any good. The ICM/system on the SC 3800 is one of the better stock systems and is more related to a performance type system.

The only honest way to discuss modifying the plug gap is to experiment on your individual application. You'll either see a benefit, or you won't.

If you really want to see where a plug gap is at its optimal size for your application, you need to do a little trial and error testing. This of course will burn the better part of the day up, but if you increase the gap at 0.001" - 0.002" steps and you get to a point where you lose power, you will need to close the gap up 0.002"-0.003" to be at the optimum gap.

Here'* some things to think about:
  • The ignition coil may not have enough stored energy to fire, or in the least case, not enough energy by the time it gets to the plug to be able to jump the gap.
  • Plug wire leads will break down at a faster rate due to the added resistance as the spark tries to reach ground.
All of this occurs because the greater the gap, the higher the voltage requirement necessary to get through the components to jump the gap. The harder these components have to work, the shorter their life span. So...stay within the .055 to .057 and you will be right where you need to be.

There'* a great deal of more information available on the web and several different systems approach to selecting a plug that will work the best for your application. Good luck!'* everything you could ever want to know about plugs
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