# noob question here.... What does 9:5:1 mean?



## 1.8TRabbit (Nov 15, 2004)

Ok, I am trying to figure out what the numbers indicate? Is it for example 9:5:1 Does the 9 represent the oxygen? And 5 represent gas? And the 1 represents combustion? Or what?


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## '89gli (Sep 2, 1999)

*Re: noob question here.... What does 9:5:1 mean? (1.8TRabbit)*

Compression ratio


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## gopher11 (Nov 24, 2004)

*Re: noob question here.... What does 9:5:1 mean? (1.8TRabbit)*

http://en.wikipedia.org/wiki/Compression_ratio


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## Agtronic (May 14, 2001)

*Re: noob question here.... What does 9:5:1 mean? (gopher11)*

The correct format for that number is actually 9.5:1 ... 9.5 to 1 in spoken english. 
It is the ratio between the cylinder's volume when the piston is at bottom dead center (BTC) and when it is at top dead center (TDC).


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## 1.8TRabbit (Nov 15, 2004)

*Re: noob question here.... What does 9:5:1 mean? (gopher11)*

That helps. But when its talking about volume the 9 is referred to as what? mm of air mass at the maximum compression?? 
And what does the 5 dictate? 
I know it means volume blah blah blah. But what do the numbers represent? BESIDES COMPRESSION!








Does anyone know what im trying to ask here?


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## GTijoejoe (Oct 6, 2001)

*Re: noob question here.... What does 9:5:1 mean? (1.8TRabbit)*

well, you almost answered your own question I would think
9.5:1 
meaning 9 as a whole number, .5 as a half of a whole put them together you got 9.5.
This CR means that you are compressing the air (generic term) 9.5 times from its orginial value.
So..... if you are allowing 1 atmosphere of pressure in a cylinder, after compression takes place your pressure would be....
pressure = 1 atm * (9.5/1) 
Dictating that the "compression RATIO" being (9.5/1).
I hope that helps or makes sense cause i'm pretty tired


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## 801pete (Apr 20, 2006)

It means if you have 9.5 CC's with the piston at the bottom, that gets squished down to 1 CC when the piston is at the top.


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## 1.8TRabbit (Nov 15, 2004)

*Re: (801pete)*


_Quote, originally posted by *801pete* »_It means if you have 9.5 CC's with the piston at the bottom, that gets squished down to 1 CC when the piston is at the top. 
Thanx Pete! So aiding more compression will give more power but the down fall you are more likely to detonate due to the amount of heat?


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## 801pete (Apr 20, 2006)

Higher compression results in much higher cylinder pressure when the fuel / air charge is lit off. You get the same amount of energy released but it is in a much smaller space, so the pressure becomes much higher. 
The pressure and tempurature due to compression also are much higher before the actual ignition takes place, so yes- the downside is the engine becomes more prone to pre-igition. 
Adding boost pressure to an engine has exactly the same effect. You need to balance desired boost pressure, compression ratio, and the fuel you intend to run the engine on to come up with a cylinder pressure which agrees with the fuel being used. 
The motor I'm building right now is a full 10:1 compression but will only see quite low boost on 91 octane.


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## 1.8TRabbit (Nov 15, 2004)

*Re: (801pete)*


_Quote, originally posted by *801pete* »_Higher compression results in much higher cylinder pressure when the fuel / air charge is lit off. You get the same amount of energy released but it is in a much smaller space, so the pressure becomes much higher. 
The pressure and tempurature due to compression also are much higher before the actual ignition takes place, so yes- the downside is the engine becomes more prone to pre-igition. 
Adding boost pressure to an engine has exactly the same effect. You need to balance desired boost pressure, compression ratio, and the fuel you intend to run the engine on to come up with a cylinder pressure which agrees with the fuel being used. 
The motor I'm building right now is a full 10:1 compression but will only see quite low boost on 91 octane. 
What are you building now Pete?








Or is this the MK2 1.8t motor?


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## GoKart_16v (Dec 17, 2004)

*Re: (1.8TRabbit)*

trying make sense of it...prolly like...
isn't its 5 times the atmospheric pressure (1) or 5 times something else
p.s. guy above answered it...


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## Agtronic (May 14, 2001)

*Re: (GoKart_16v)*

There are no units. It is a ratio. You can use any unit you'd like, the ratio would remain 9.5 to 1.


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