compression

[Don Dulmage <big-d@xxxxxxxxxxxx>]

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"I put in a 509 cam and now the compression is 130"
Give the man a cigar!
You are a very observant fellow. Why did that happen? Because the valves with the new cam are open longer . No compression takes place until the intake valve closes at the end of the induction stroke. Actually it is held open into the early part of the compression stroke as the air/ fuel mixture is flowing into the cylinder at a high rate and does't want to stop (Intertia) So we can hold the valve open there for a tiny bit as the flow coming in is strong enough that the slight upward movement of the piston doesn't upset the flow. Eventually it does and we close the valve just before that point . Unfortunately we lost a few pound per square inch of compression while we were waiting for the intake flow to slow up. 
What do we do now? Can we get that back? YES. That is why we raise the compression when we lengthen the camshaft intake duration. The two are inseperable and this is the key to making horsepower. We don't raise compression to get more compression (after we reach about 10 to 1) we raise it to get back what we lost from holding the intake valve open longer. 
Once you understand this the rest is a piece of cake. 
There is a book which is more of an engineering type book than an fun to read book called Design and Tuning of Competition Engines. It is by Philip H Smith and I believe it was written across the pond (aka England) It is a very very good book and if you can stand that type of intensive reading and study will help you understand many of the things that go on in the engine. It has few photos and those that are are engines we would have little interest in but it is good solid engine sceince unlike much of the Bullschlage that we are often subjected to from our magazine articles. 
Actualy compression can tell you a lot more than one would think. It could take about 4 or 5 pages to cover the subject but i am impressed with your direction of thought! NEVER stop being curious.
Don 


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