well sorry for the newbie question, but what extacly is engine compression? Compression ratio? Im new into cars (espcially hondas) and just been reading here on honda-tech to learn. But what do people mean when they talk about compression ratio? from the threads I searched and read I usually see numbers (EXAMPLE- 10.4 : 1 ) what exactly do those mean? From what I read also u can lower and bump up the compression..?? by adding diff. pistons (like ctr/ITR ones) port and polish head, bored out block, gaskets etc etc, correct? When one building an engine (all motor) do u wanna lower ur compression? bump it? Thanks for the help guys. My future goal is to build a semi daily driver/track car (all motor) maybe a LS/Vtec or B20vtec to be put into my 97 LS integra, but Ill just take things one at a time haha, I gotta learn about small things before I try to read up ahead

 

Compression ratio is the ratio of the volume of the cylinder & combustion chamber with the piston all the way down; to the volume with the piston all the way up.

So lets say your total displacement is 1800 cc (not exactly a B18, but the arithmetic gets easier). Each cylinder's displacement is 450 cc. Lets also say your combustion chamber volume is 50 cc after you account for the space added or taken away by the shape of the piston's top.

So with the piston at BDC the volume is 450 + 50 = 500 cc.
With the piston at TDC it's 50 cc.

500/50 = 10 so your compression ratio is 10.0 : 1.

Changing pistons so they have more material projecting upwards, that takes volume away from the combustion chamber; which increases CR. Using a thicker headgasket leaves more volume which lowers CR. Thin gasket => higher CR. Milling some material off the bottom of the head makes the chamber smaller. Porting & polishing is outside the combustion chamber so it doesn't change CR. Boring out the cylinders increases the displacement which raises the CR.

Get it?

Generally you want to use a lower compression ratio for a turbo, or a higher compression ratio for non-turbo. But that's a general rule which has to allow for timing, fuel octane, & other stuff.

 

Sorry I dont totally agree with the previous answer

Compression ratio is simply the ratio of swept volume over compressed volume. Where:

Swept volume = bore area x stroke
Compressed volume = combustion chamber size with piston at TDC
(this approximates to: combustion chamber volume - piston dome volume)

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by jmcclell &raquo;</TD></TR><TR><TD CLASS="quote">Compression ratio is simply the ratio of swept volume over compressed volume. </TD></TR></TABLE>

But that completely neglects the volume in the head when the piston is at BDC. You must add that volume to the overall volume, as it fills with air when the pistons moves down the bore as well.

http://www.hotrod.com/techarticles/54258/

"Compression ratio is the relationship of cylinder volume (or displacement) with the piston at BDC to cylinder volume with the piston at TDC."

The combustion chamber is part of that volume.

 

Both those above definitions are correct. One is applied in theory, one is applied in actual useage. For example, in most ICE texts, they only include a diagram of a single piston within a combustion chamber (inside a closed system, i.e.: no head). The top of the cylinder is the top of the chamber. There is no domed area within the head to account for extra volume.

To make things easier (especially in our terms, since we are applying it), we should just say, "Compression ratio is the ratio of the volume of the combustion chamber when the piston is at BDC versus the volume of the combustion chamber when the piston is at TIC." This way, using the term combustion chamber, you include all areas of combustion.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by shamoo &raquo;</TD></TR><TR><TD CLASS="quote">Both those above definitions are correct.</TD></TR></TABLE>I don't think they can BOTH be correct. They contradict each other.

My textbook (Obert) gives it as (C+D) / C
where:
D = displacement = pi/4 * bore^2 * stroke (this is swept volume)
C = clearance volume above piston at TDC

Thermodynamics texts more often talk about the volume ratio or expansion ratio, which is really the same thing. Maybe part of the confusion comes from the definition of 'displacement' which doesn't include the clearance volume.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">Thermodynamics texts more often talk about the volume ratio or expansion ratio</TD></TR></TABLE>

SSHHHHH! You'll give away all the secrets! ER is very powerful if you really understand it and can apply it to an engine design.