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Old 09-20-2005, 02:53 PM   #1
Kreeger
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Default Detonation and the RS-T

Detonation and the RS-T


I have read alot of discussion about the reasons why turbo-ing an N/A Subaru motor (or any N/A motor, for that matter) can potentially lead to a catastrophic engine failure. Although detonation has been mentioned as a cause, I feel the need to elaborate more on this matter simply due to the fact that I am the type of person who likes to fully understand why and how things happen.

Detonation = knock. Detonation, simply put, is an explosion of the fuel/air mixture within the cylinder before it is supposed to happen. Just imagine a piston on its upstroke, trying to compress the charge for a powerful and productive explosion... only to have the charge detonate before the piston reaches the top. The piston head, ringland and connecting rod take all of that force from the prematurely "detonated" charge. If these components are not strong and tempered (i.e. forged), they will fail, melt disintegrate, explode, etc.



Things that can cause detonation are:


- Low octane fuel.

Higher fuel octane is important for a turbocharged motor because it resists pre-ignition. A good example is: 89 octane fuel compressed within a cylinder will ignite at a lower compression rate than 93 octane fuel.


- Inadequately cooled charge.

Cooler air is denser (the oxygen molecules are closer together, so... more oxygen is better), a warmer air/fuel mixture tends to pre-ignite more easily... before the sparkplug sparks.

- Highly compressed charge (high compression rate and/or too much boost).

A naturally aspirated gas engine has a normal compression rate of between 9.5:1 up to 10.5:1 and even as high as 12:1 in high performance N/A motors. The more that air is compressed, the more it heats up; and when air heats up, it expands. Expanding air that is already heated can prematurely detonate. Furthermore, if the charge is under-cooled, or the cooling apparatus is a heat-soaked one (i.e. a TMIC), then you are more at risk for detonation.

Other things to ponder:

Spark Plugs

Spark plugs use a ceramic insert to isolate the high voltage at the electrode, ensuring that the spark happens at the tip of the electrode and not anywhere else on the plug; this insert does double-duty by helping to burn off deposits. Ceramic is a fairly poor heat conductor, so the material gets quite hot during operation. This heat helps to burn off deposits from the electrode, but can cause detonation in a turbocharged engine simply due to the fact the the electrode is hot enough to ignite the mixture at a low compression rate.

Standard plugs are considered 'hot' plugs. This type of plug is designed with a ceramic insert that has a smaller contact area with the metal part of the plug. This reduces the heat transfer from the ceramic, making it run hotter and thus burn away more deposits. 'Cold' plugs are designed with more contact area, so they run cooler. Typically, the electrode of a 'cold' plug is usually made of Iridium. This is because Iridium transfers heat better than most other metals and, thus, reduces the chance of detonation.

Water/Methanol injection.

This brings me to my question of " Why haven't the aftermarket turbo kit companies offered an accompanying W-M/I system with their kits?"

Water/Methanol 101 : The primary function of water/methanol injection is to provide “chemical intercooling”. In gasoline engines, as with any intercooler, this suppresses detonation so more power producing boost and timing can be utilized. Water, with its high latent heat of vaporization cools the intake charge and combustion. Methanol cools the charge and combustion but also acts like an extremely high octane fuel (some researchers claim as high as 120 octane) as well as adding more oxygen to combustion. Water injection combined with the typical air-air or air-water intercooler further cools the charge, which further inhibits the chance of pre-detonation.

There are many kits available and there are many people who are fabricating their own kits (which is very easy to do, BTW).

Here is a linky... http://www.google.com/search?hl=en&l...ater+injection


Please remember that a good EMS (engine management system) is ALWAYS a must with any turbocharged setup looking for max performance.

Have fun... and be safe.
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Last edited by Kreeger; 09-20-2005 at 04:26 PM. Reason: forgot to mention engine mgmnt.
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Old 09-20-2005, 03:14 PM   #2
Kawikid20o2
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link dont work
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Old 09-20-2005, 04:06 PM   #3
Kreeger
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Fixed it.
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