Editing Cam and compression ratio compatibility

Sometimes it's difficult trying to explain to fellows who are new to hot rodding that you have to match the characteristics of the camshaft to the static compression ratio of the motor along with the operating range where the cam makes power. This chart is nowhere near scientific or definitive in its scope. Larger or smaller cubic inches, different lobe separation angles and other variables will alter these figures, but at least it's a starting point in the explanation of using a certain range of cam with a certain range of static compression ratio. The figures shown in degrees are camshaft degrees of duration measured at 0.050" tappet lift.

The chart will show why it is foolish to install a 230 degree cam in an 8.00:1 motor in an attempt to get a lope out of the motor. It might lope, but it will lack the power to pull the hat off your head. There seems to be an obsession with lope these days. Lope is simply the sound of the motor being inefficient at low rpm's because the ascending piston is pushing fuel/air mixture back up the intake tract through the still open intake valve and disrupting the metering abilities of the carburetor.

On the other hand, it's equally as foolish to install a short cam into a motor with a fairly high static compression ratio. You get into an area of such high cylinder pressure that no fuel short of alcohol or racing gasoline will prevent detonation.

You can move 1/2 point of SCR either way and be in the ballpark. In other words, if you have a 9.00:1 motor and you want a little more cam, you can move up to a cam that would be used in a 9.5:1 motor and be ok. On the other hand, if you wanted to go with a little less cam, you might use 1/2 point the other way, down to an 8.50:1 cam.

Here's what Crane has to say about it....
http://www.cranecams.com/faqview.php?s_id=9 

 

{| style="color:black; background-color:#ffffcc;" cellpadding="5" cellspacing="0" border="1"
!Static Compression Ratio (SCR) !!Intake Valve Duration (degrees @ .050" lift) !!Power Range (RPM)
|-
| align="right" |8.00:1 || align="right" |185º || align="right" |500-4,000
|-
| align="right" |8.25:1 || align="right" |189º || align="right" |650-4300
|-
| align="right" |8.50:1 || align="right" |194º || align="right" |800-4,500
|-
| align="right" |8.75:1 || align="right" |200º || align="right" |900-4,600
|-
| align="right" |9.00:1 || align="right" |204º || align="right" |1,000-4,600
|-
| align="right" |9.25:1 || align="right" |208º || align="right" |1,200-5,200
|-
| align="right" |9.50:1 || align="right" |212º || align="right" |1,600-5,400
|-
| align="right" |9.75:1 || align="right" |216º || align="right" |1,800-5,600
|-
| align="right" |10.00:1 || align="right" |221º || align="right" |2,000-5,800
|-
| align="right" |10.25:1 || align="right" |227º || align="right" |2,400-6,200
|-
| align="right" |10.50:1 || align="right" |233º || align="right" |2,800-6,400
|-
| align="right" |10.75:1 || align="right" |236º || align="right" |3,000-6,800
|-
| align="right" |11.00:1 || align="right" |240º || align="right" |3,200-7,000
|-
| align="right" |11.50:1 || align="right" |244º || align="right" |3,400-7,200
|-
| align="right" |12.00:1 || align="right" |248º || align="right" |3,600-7,400
|-
|}

===Dynamic compression ratio calculators===
*[http://www.empirenet.com/pkelley2/DynamicCR.html Kelly calculator/info]
*[http://www.kb-silvolite.com/calc.php?action=comp2 Keith Black calculator]

==Resources==
*[[How to choose a camshaft]]
*[[Camshaft/Compression Ratio relationships]]
[[Category:Engine]]
[[Category:Camshaft]]