would someone like to explain compression ratio for me, because no matter where i search on this forum, all i find are questions about personal compression ratios and such.

i would like to know exactly what it is and how you measure it or where it comes from etc etc???

thankz

 

It is the ratio between the volume of the cylinder, when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. A high compression ratio allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. High ratios place increased oxygen and fuel molecules into a reduced space; thus, they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree. Higher compression ratios, however, also make engine knocking, also known as detonation or pre-ignition, more likely and this can reduce an engine's efficiency and even physically damage it.

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

In other words, the higher the compression the smaller the area for the explosion in a internal combustion engine to happen in. The smaller the area for this to happen in the more force it will produce. The more force in the cyl ultimately means more power but it comes at a cost. High compression engines need good clean, higher octane fuel to subside pre-detonation. Pre detonation is when the cyl becomes hot enough to ignite the air/fuel mixture prematurely(before the piston moves to top dead center) therefore you have a pston moving up through the cyl on a compression stroke but before it rounds the turn to head back down the air/fuel ignites so you have 2 forces meeting eachother and in all cases when 2 forces meet the weaker one will break, or give in. Being the piston is being pushed by a crank and a rod and that air pressure along with the nature of air in a compressed state needs a place to go and can not "break" the weakest link is first. In some engines this is the rod, the rod berring, some it is the piston, and some it is even the crank.

Low compression gets rid of alot of chances for pre-detonation but there is still chances for this to happen but it is alot less likely being the cyl pressure and temps tend to run lower the lower the compression ratio you run. But there is a down side also, the lower thecompression the lower the power which is understandable but what really suffers is low and mid range torque.

 

Awesome explaination! /\ Heres another good link to help you understand how compression is calculated. http://www.homemadeturbo.com/forum/i...?topic=55794.0

Btw, this REALLY should be made into a sticky! This is very good information that everyone should read and understand!

 

thankz guys, that was crystal clear and really easy to understand.