Compression Ratio

If you don't know the compression ratio of an engine, how do you determine it? Assume there is no published info on the engine. TIA.

Put the timing mark on TDC for #1, then measure the amount of gas/oil/kerosene is takes to fill the cylinder. That will give you the compression volume, divide the displacement by the compression volume to obtain the compression ratio.

If it's a stock motor, the specs are probably listed on the web somewhere, so you could use Google and save a lot of mess.

Post edited by: steell, at: 2005/11/29 09:59

steell wrote:

divide the displacement by the compression volume to obtain the compression ratio.

Divide the displacement + the compression volume by the compession volume to obtain the compression ratio.

CR=(piston displacement+combustion chamber volume)/combustion chamber volume

Ron

One of the nice things about this place, if I screw up there is always someone that will correct my mistake

Thanks!

I was just about to correct that when I scrolled down and saw Rons post.

Jay

If stock you might find the info listed here www.bikez.com/brands/index.php

It will still only give an approximate as you also need to know the VE (volumetric efficiency) of the engine.

For example, a 600cc 4-stroke with a 14cc head area is going to have rough compression ratio of about 10.7:1 ratio. However, if the VE is only 90% then it means that the compression ratio will drop to about 9.7:1 or so.

It means less mix being sucked into engine, so less to be compressed.

It will give you an exact (static) compression ratio, not an approximate one. Calculating the Volumetric Efficientcy in order to determine the dynamic compression ratio is a whole nother ball game, and I doubt that was the question.

volumetricly speaking, the dynamics of the efficiency of the compression are not subjects of displacement?

Volumetric efficiency is stated as a percentage of the engines total airflow potential. (Total airflow potential, or pumping potential = displacement X rpm.) Volumetric efficiency is determind by measuring airflow into the engine while it is running.

For more info:

www.yawpower.com/martech.html