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
|-
|}


===Advanced Reading and Engine Theory===
Intake valves close after the piston travels '''after''' bottom dead center (ABDC). This sounds counter-intuitive, but this design aspect was a break through in modern engines and has allowed them to make more power as well as run at higher RPM's. The reason why is all about the Dynamic Compression Ratio (DCR). Air is a gas and as such is compressible (unlike liquids or solids). When air enters the engine is does so at a high velocity, and when the piston reaches bottom dead center, due to momentum, the air wants keeps moving in and starts to compress. In short, the longer the intake valve stays open, the more air can be packed into the cylinder.

The extent to which intake valves stay open is measured in degrees. IVC (Intake Valve Closing), is one of, if not the most important determining factor in how the cam shaft impacts engine performance. Manufacturers will give this value in degrees ABDC (after bottom dead center). As an example, the Comp Cams XE275H cam's intake valve closes 64º ABDC.

Factory cams are low duration cams, this means that the intake valve closes much sooner ABDC and thus less of the intake charge is pushed (reversed) out of the intake valve. Since less is reversed, then the majority of it stays in the cylinder, and as such, the compression stroke will yield statisfactory DCR compression with 8.5:1 SCR. This type of engine will have a low SCR and a high DCR. These engines produce power in the lower RPM ranges because they cannot utilize the effects of DCR. Since there is little reversion, these engines idle very smoothly (no lope). If you were to put a compression tester on these engines you would see something in the 150-190 PSI range.

Performance cams with longer durations, are exactly the opposite. At low RPM's since the intake valve stays open longer ABDC, more of the intake charge is reversed. Hence there is less pressure build up on the compression stroke. To compensate for this, a higher SCR is used. As the RPM's climb, so does the intake charge velocity, thus more air is able to cram itself into the cylinder. Even though the piston is moving up ABDC, the air is compressing upon itself, and able to move into the cylinder because the intake valve is still open. As you can see, at higher RPM's these cams are able to more completely fill the cylinder with air and are able to make more power. These engines have a high SCR and low DCR. They make power at higher RPM's. Since there is a lot of reversion, these engines idle very rough (have a lot of lope). If you were to put a compression tester on these engines you would see something in the 125-150 PSI range.

===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]]