[Guest]

hey all,

my question is what is the relationship between octane number and turning up the boost safely? i understand that a higher octane number will create a more violent combustion, resulting in greater energy release. but how does that relate to boost pressure? does the expanding ehaust gas result in less work necessary from the turbine wheel? any takers?

thanks

 

[Guest]

Actually, the higher the octane, the harder it is to ignite. You would use a higher octane because it doesnt ignite as easily as a lower number would.

This is how it ties in to boost: Detonation.

Detonation is when the mixture in the combustion chamber ignites prematurely, or before the piston completley finishes the stroke. When you turn up the boost, you increase the risk of detonation w/out enough fuel. So if you increase the octane, along with the boost, you are less likley to get knock. You really need some type of datalogger to know exactly what's going on.

That's about as simple as it gets.

-Steve

 

[Guest]

thanks for the reply, but still have questions. i know it is harder to detonate a higher octane, but it turn it releases more energy upon combustion. but detonation is a possibility in most all cars. how does the compressor wheel and turbine wheel effect the temperature inside the combustion chamber. as i understand it the compressor wheel "compresses" the air entering the the intake manifold to increase the volume of dense air in the chamber. combustion. the exhaust is purged and does work on the turbine wheel, turning the compressor wheel. so am i missing something? is some excess fuel purged with the possibility of detonation when in the turbine housing?

thanks again for the info!

 

[ATLSUPDAWG#2]

Detonation can be caused by exceeding the compressors efficiency range. If the compressor is rated at 76% efficient at a certain pressure ratio 24% of the compressor's work is in producing heat. When you crank the boost thus raising the pressure ratio the compressor can move out of that "efficient island" and start producing higher temps ie. 55% efficiency, which means 45% of the compressors work is in producing those high temps. This is what can lead to detonation. Also, the further you move out of the compressors prime efficiency range the density of the charge decreases. The high temps cause the compressed air molecules to expand apart hence the need for intercoolers, water/alcohol injection and/or NOS. The ideal situation would be to have the density ratio the same as the pressure ratio, but since physics exist in our world it doesn't quite happen that way and since luck doesn't seem to exist in my world I'll never win this damn lottery
Hope this kinda helps ya out.