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Old 02-25-2006, 06:16 AM
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How does the CC rating work on headers?

I am new to the engine scene and i have been trying to find out how the cc rating works with headers. I do understand that pistons help determine the cc rating of an engine but my question is:

What produces more horse power and more torque? A 65cc header or a 75cc header. Does the higher the number give you more horse and torque or does the lower the numbner? The only reason why i ask this question is because i am getting two different answers from people i have talked to at mechanic shops. :confused:

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Old 02-25-2006, 07:55 AM
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Sorry, i always do this i didnt mean headers, i meant the heads. anyway i got my answer to the question by plugging numbers into the mathimatical equation 8.5:1(cr) = 5738 (sv) / (CV) + 1. i just tried a few examples to get the answer i was looking for. the smaller the space in the combustion chamber the more power you get.
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Old 02-25-2006, 02:01 PM
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Here's the whole scoop. Compression ratio is simply the total volume inside the cylinder when the piston is at the bottom divided by the total volume when its at the top. If its 1000cc total at the bottom and 100cc total at the top, its a 10:1 engine. Reducing the combustion chamber size by 10cc, would make it 990cc at the bottom and 90 cc at the top which is 11:1.

The compression stroke of the engine does several things like putting the piston back to the top so it has somewhere to go, concentrating the mixture around the spark plug, and adding energy to the reaction. But its more important feature is adding energy to the mixture. If you're not familiar with the idea of adiabatic heating, its when you increase the temperature of something without adding any heat. When you compress the gasses in the cylinder, they get hotter. Its the reverse of why aerosol cans feel cold when you spray on your hand. High pressure to low "spreads the heat out" making the temperature decrease. Same heat, larger volume. I'm bad at describing it, but hopefully it makes sense.

There are several kinda complicated things happening, but the higher the compression ratio, the more energy you put into the reaction. The more energy in the reaction, the more you'll get out. Bigger boom. In turn more force from the combustion can put more pressure on the piston which makes more power. It has to do with BSFC or brake specific fuel consumption which is defined as the amount of stored energy (locked up in the gasoline's hydrocarbon bonds) is released as power in the engine. Higher CRs (which can be achieved with smaller head chambers or piston crown differences) are able to release more BTUs from the gasoline as power than lower CRs.

Energy in = Energy out. In the case of compression ratio, it takes less energy to more tightly compress the gasses than you get back as extra BTUs you get released from the fuel. That equals more cylinder pressure, which equals more power.

Before somebody says it, it DOES NOT necessarily mean more MPG. But that's a soapbox for another day.
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