Spark Plugs for Turbo FTO

[jonowong]

i just found this on a website...

Too sharp a tip will melt the electrode.

If the temperature of the (spark plug electrode) tip reaches the melting point of the metal that it's made of - you can kiss it goodbye.

Here are the melting points of some commonly used metals (Celsius):

Zinc == 420
Aluminum == 660
Copper == 1083
Steel == 1400-1500
Platinum == 1772
Iridium == 2410

This problem is mainly concerned with the volume of metal at the spark tip - if the temperature even instantaneously reaches the melting point, some of that metal will disappear. You can see that Platinum and Iridium coated plugs can withstand significantly higher temperatures, and thus can have sharper tips than their steel or copper counterparts. To add insult to injury, if some of the metal does disappear from a very sharp tip, then you've actually opened up the spark gap some. To prevent that from happening, we have to start with a wider tip, such that any small amount that is eroded will not change the size or geometry of the tip by too much.

Caveat 2: Too sharp an electrode tip will create a "hot spot" in the combustion chamber.


Even if you don't reach the melting point of the metal, you can still get it glowing hot. If that tip is still glowing red hot when the next compression stroke comes about (two full engine revolutions since the last spark) that residual heat can actually ignite the air/fuel mixture before the spark is supposed to occur. This is pre-ignition. It generally creates even more heat - leaving the spark plug even hotter than the last time thus repeating the cycle until you melt a piston. Ouch!

To avoid this, we want a wide area near the tip to conduct as much heat away from the tip as possible.

Here are some of the thermal conductivities of some commonly used metals (Watts / centimeter*Kelvin) :


Zinc == 1.16
Aluminum == 2.37
Copper == 4.01
Steel == 0.70 - 0.82
Platinum == 0.716
Iridium == 1.47


You can easily see why Copper is the metal of choice for the core of the spark plug. It's just about the best thermal conductor on earth. Occasionally, you still find plugs with an aluminum core - stay away!


So, what we want is the sharpest tip possible such that it does not melt the electrode nor does it stay so hot as to cause pre-ignition. Let's break it down:

Bare Copper

They have a low melting temperature and the tips will vaporize away - they have a very wide tip so each little bit that disappears will not change the gap size greatly, but they still must be inspected often to make sure there is sufficient electrode material left. They are great for very hot running engines which must avoid pre-ignition at all costs, since the wide tip will not stay hot(high boost forced induction and nitrous engines come to mind).

Platinum

Platinum plugs are usually constructed similar to copper plugs except that they have a thin coating of Platinum sputtered onto the electrode tips, about 0.010" thick (a human hair is about 0.005" thick). Because of the high melting point of Platinum, the tips can be made significantly sharper without fear of the gap changing shape. But the copper core is still sufficient to whisk the heat away fairly quickly. These are great all-around plugs, particularly for use on NA engines, and they should last a very long time. Very high heat engines should probably not use them because the sharper tips may not conduct enough heat away to prevent pre-ignition under adverse conditions.

Iridium


This is the new guy on the block. They are much like platinum plugs just with iridium in place of the platinum. Because of the extremely high melting point of iridium, they can have very sharp tips without risk of melting and they should last a very long time. These would be best for high-rpm NA engines where the sharpest tip is needed for the best spark, but there is little danger of pre-ignition....

so am i right to conclude that i should use copper on rear and front banks? to minimise detonation on a turbo set up?

[MarkFTO]

Iridium would be the plug of choice for a turbo setup. I ran iridium plugs one grade colder than normal and never had any problems.

Copper plugs will tend to foul on rich running engines, and as the boost increases are more prone to spark blow out.

Iridium plugs due to having the smallest tip size will promote a better ignition spark and be less likely to foul. By going for a slightly colder plug you will also have none of the heat issues mentioned in that article.

[jonowong]

what gap should a turbo fto have!

[MarkFTO]

You shouldn't need to GAP Iridium plus as they should be set from the factory, i think 1mm off the top of my head.

If you do gap them dont use a normal plug tool as its easy to damage the electrode just gently press and check.

[khunjeng]

Copper plugs are fine just chnage more often. Gap to 0.8/06 and get a cold heat rating of 7.

this is probably the last thing you should be worring about.