| Re: Cold compression test? |
Subject: Re: Cold compression test? by CentralCoaster on 2011/1/31 7:28:20 Also, heat of compression will be quickly absorbed by the cool heads, reducing pressure. The compression is not adiabatic, the work done adds heat, which expands the air and increases pressure. A car with 9:1 compression with 90% volumetric efficiency during cranking would only give you about 118 psi adiabatically, the rest is from heat. 14.7psi * 9 = 132 psia = 118 psig If you do a compression check on a warm engine, you have to repeat it in the opposite order. This is because the numbers will noticeably drop as the engine cools. Otherwise you'd be misled to believe the last cylinders are weaker. The last one I did, the compression dropped by almost 20 psi before I got to the last cylinder. Also as the battery gets weaker, the engine will turn over more slowly, generating less heat of compression. |
