Spark plugs

11/03/2003

What is a spark plug? What does it do?

Unlike some people think, the spark plug doesn't "makes" a spark. It does offer the gap necessary for the electric current produced by the coil jumps as a spark. This spark starts the ignition of the air/gas mixture compressed at the top of the piston, running the engine.

A spark plug has dimensions and features that should be taken in account when time arrives for a replacement, to prevent damage to the engine. 

a. thread diameter: diameter of the plug's threaded part, the one which goes into the cylinder;

b. thread reach: length of the threaded part;

c. heat range: the velocity which the plug is able to get rid of excess heat.

Let's talk a little about each one of those features:

Diameter: the engines used in R/C boating use spark plugs with 14 mm thread. This is the feature that causes no worry, because of the obvious reason that you  can not put a 14 mm plug on a 12 mm cylinder or v.v.

Thread reach: the spark plug with adequate reach doesn't leave any portion of the thread exposed into the cylinder, and on the other hand doesn't leave any portion of the cylinder thread not covered. A short plug allows carbon deposits on the unprotected thread, demanding a careful cleaning before installing a new plug. A long plug, however, leaves a portion of the thread exposed into the combustion chamber. On the exposed portion carbon deposits are formed, that may make it difficult to take the plug off and even damage the cylinder thread. Besides, a plug that is too long may touch the head of the piston on it's upper stroke, with disastrous consequences to the engine. As another detrimental effect, the plug with wrong length doesn't reach the correct working temperature: cooled by the incoming new mixture being compressed, heated by the ignition of the mixture, the correct plug maintain the adequate temperature. 

Heat range: To work correctly, the electrode demands  a working temperature between 752/1652 F. Below 752 non burned carbon/oil deposits will foul the plug, above 1652 the electrode tip becomes white-hot and cause pre-ignition/detonation (ignition before the correct time/uncontrolled and fast burning of the mixture.

According to their heat range, plugs are classified in "cold" or "hot". This classification has not to do with the spark temperature - a "hot" plug doesn't have a spark hotter than the one of a "cold" plug and v.v. . The heat range of a plug indicates it's capacity of getting rid of excess heat. The picture bellow shows how a plug dissipates this heat. As we can see, 58% of this heat is dissipated trough the cylinder, being this the more important way of cooling.

So, a "cold" plug dissipates heat quicker that a "hot" one. Bellow, you can see the difference between a hot and a cold plug. The last one has a shorter porcelain isolator, offering a quicker path to heat transfer to the cylinder. 

   

Accepted that the spark plug needs to work between certain limits of temperature, it should be understood that different conditions of demand from the engine have an effect on this temperature - on high RPM the engine comes hotter and the plug needs to dissipate heat more quickly. This is the reason why hoped up engines may need colder plugs than the stock one. 

Learn how to read the plug: this will show you if you are using the plug with the correct heat range. The way to do this is running your engine at full throttle for some time - the longer the best - and stopping the engine not allowing it to iddle.

  

Dark brown to black isolator: too cold plug

Light brown isolator: correct plug

White to light gray isolator: too hot plug - change immediately

Remember: in doubt, choose the colder one. The worst that can happen is an engine that easily fouls the plug and runs erratic. Nothing very dangerous..

Resistor plug: as a path for the spark, the plug is an excellent interference producer. On your car you replace you tachometer with a set of non resistor spark plugs. Tune your radio on a not to strong broadcast station and have fun with the noise of the sparks as you run your engine. With a little practice you can estimate the engine rotation by the noise on your radio.

On your boat the problem is more serious. Your receiver doesn't receive music but the signals of your transmitter. The non resistor plug creates a interference field that may even suppress the transmitter signal. If you are using a Shark Racing FailSafe, prepare to swim/paddle to recover the stalled boat in the middle of the lake. If you are not using a fail safe, pray for your boat runs off gas before hitting something or somebody.  

So, always use a resistor spark plug. It cuts the interference caused by the jumping of the spark and allows that the receiver only tunes the transmitter.

And what does that array of numbers and letters mean?

They inform all the features of the spark, the ones we already talked and others that we didn't, like the type of electrode, it's material, dimensions for the outer body and even the metal of the body.

An "R" letter on the code identifies a resistor spark plug. The reach is also indicated by a letter. The heat range has a numeric indication - on some manufactures  the higher the number, the colder the plug, on others, the opposite is true.

That's the way the manufacturers identify theirs spark plugs:

Bosh    

    NGK

  

  (click on the images to enlarge)

Besides, proper tightening the plug is important to prevent damage to the plug or, worst, to the head, made of much softer material. The manufacturers recommend those numbers:

Spark Plug:            Torque: (ft/lbs)

10 mm                     7
12 mm                   10
14 mm                   14
18 mm                   18

Anyway, always follow the indication of the manufacturer of your engine. If you need more information, use the links bellow.

Champion: http://www.edelbrock.com/automotive/sparkplugs.html

How to choose and read spark plugs: http://link.sandiego.com/scripts/wheelbase/message.idc?passin=458

How you can read spark plugs and select them - by Gordon Jennings: http://www.strappe.com/plugs.html

Interpreting your spark plugs: http://www.classictruckshop.com/clubs/earlyburbs/projects/spark/plugs.htm

Look at your plugs: http://www.xs4all.nl/~ekieboom/spit/plugs.html

Reading Spark Plugs: http://ericgorr.com/techarticles/sparkplugs.html

Resistor Spark Plugs and resistor spark plugs caps: http://www.ultralightnews.com/enginetroublshooting/resistorcapsandplugs.htm

Spark plugs: http://www.picknowl.com.au/homepages/harrals/tech/spark.htm

Spark plugs and what they say: http://www.motocross.com/motoprof/moto/mcycle/plug2/plug2.htm

Spark plugs overview by NGK: http://www.sentra.net/tech/sparkplugs.shtml