Cam and compression ratio compatibility

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===Advanced Reading and Engine Theory===
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==Additional reading/engine theory==
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Intake valves close after the piston travels '''past''' 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 it 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.
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{{Note1}}
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*'''top dead center''' (TDC) is when the piston is at the very top of the stroke
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*'''bottom dead center''' (BDC) is when the piston is at the very bottom of the stroke
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Modern engines are designed so that the intake valve doesn't close until the piston travels '''after bottom dead center''' (ABDC). This sounds counter-intuitive, but this aspect of camshaft design was a breakthrough when introduced, allowing modern engines to make more power as well as run at higher RPM.  
  
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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 XE274H cam's intake valve closes 64º ABDC.
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The reason closing the intake valve ABDC works to make more power is because air is a gas, and as such is compressible (unlike liquids or solids). When air enters the engine it does so at a relatively high velocity. When the piston reaches bottom dead center, the air stream will still continue moving into the cylinder due to the momentum of the mass of the charge. Even though the piston has passed BDC and is now moving ABDC (back towards TDC, in other words), the air is compressing upon itself, and is able to move into the cylinder because the intake valve is still open. So up to a point, the longer the intake valve stays open, the more air can be packed into the cylinder.
  
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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.
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===Intake valve closing point===
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The length of time the valves stay open is measured in degrees. The '''intake valve closing''' point (IVC for short) is one of, if not ''the'' most important determining factors in how the camshaft impacts engine performance. Manufacturers give the IVC point in degrees '''after bottom dead center''' (ABDC). As an example, the Comp Cams XE274H cam's intake valve closes 64º ABDC. The other opening and closing points as well as lift, '''lobe separation angle''' (LSA), valve lash (where applicable), etc. are all called out on the cam card (see example below) that's supplied with the camshaft or in the manufacturers information.
  
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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.
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[[File:Cam card 3.jpg]]
  
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You can fine tune a particular cam by advancing or retarding it. This procedure does nothing to alter the lift, duration, lobe seperation, etc of the cam. All you do is change the phase of the cam shaft in relation to the crank. If you advance the cam, then as the name suggests you close the intake valve sooner (this will build more pressure in the cylinder and shift the power band lower). Advancing the cam will cause the intake valve to close later (this will build less pressure in the cylinder and shift the power band higher).
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===Static compression ratio vs. Dynamic compression ratio===
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{{Note1}}
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*The '''static compression ratio''' (SCR) is computed using the full swept volume of the cylinder from BDC to TDC compared to the volume of the combustion chamber, valve reliefs (or dome), and all the other volumes not occupied by the piston stroke.
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*The '''dynamic compression ratio''' (DCR) is computed from the position of the piston at the point of IVC to TDC compared to the volume of the combustion chamber, etc. (instead of using BDC to determine the swept volume of the cylinder). Some SCR vs. IVC points for the 350 SBC can be seen [http://www.crankshaftcoalition.com/wiki/Dynamic_compression_ratio '''here'''].
  
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===Dynamic compression ratio calculators===
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Most factory production cams are considered to be short duration cams. This means that the intake valve closes much sooner ABDC than a high performance cam might, and thus less of the intake charge is pushed (reversed, thus the term ''reversion'') past the intake valve back into the intake tract. Since less of the intake charge is lost to reversion, more of it stays in the cylinder, so the compression stroke can yield a satisfactory DCR with a relatively low SCR.
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An engine having a lower SCR and an early IVC figure will produce power in the lower RPM range. Since there is little reversion, these engines idle very smoothly (no lope) and will have high idle vacuum around 20 in/Hg. If you were to put a compression tester on these engines you would see something in the 150-190 PSI range.
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Performance cams with longer durations begin to exhibit the opposite traits of a short duration cam. At low RPM more of the intake charge is lost to reversion because the intake valve stays open longer ABDC. Hence there is less cylinder pressure built up on the compression stroke. To compensate for this, a higher SCR is used. As the RPM climbs, so does the intake charge velocity. Now the later closing intake valve will allow more air to cram into the cylinder. At higher RPM this type of cam is able to more completely fill the cylinder, so will make more power.
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An engine having a high performance cam (later IVC point) needs a higher SCR in order to keep the DCR within an optimum range. Because the late IVC point can cause reversion (along with an increase in overlap and/or a tighter LSA), this type of tune can cause the engine to idle rough (have a lot of lope). Idle vacuum will be lower and if you were to put a compression tester on these engines you would see something in the 125-150 PSI range.
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===Compression calculators===
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{{Note1}} Different calculators use different ways of expressing piston dome or dish volumes. Some calculators assign a negative number for a dish or dome, others will use a positive number. The same thing goes for advertised figures published by different manufacturers when they describe piston dish or dome volumes. So be sure of the sign (negative or positive) that is needed to get the correct results from the calculators.
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====Static compression ratio calculator====
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*[http://www.wheelspin.net/calc/calc2.html Wheelspin.net]
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====Dynamic compression ratio calculators====
 
*[http://www.empirenet.com/pkelley2/DynamicCR.html Kelly calculator/info]
 
*[http://www.empirenet.com/pkelley2/DynamicCR.html Kelly calculator/info]
 
*[http://www.kb-silvolite.com/calc.php?action=comp2 Keith Black calculator]
 
*[http://www.kb-silvolite.com/calc.php?action=comp2 Keith Black calculator]
  
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===Cam phasing===
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You can fine tune a particular cam by advancing or retarding it. This procedure does nothing to alter the lift, duration, lobe separation, etc. of the cam. What does happen is the phase of the camshaft is advanced or retarded in relation to the crankshaft position. Advancing the cam closes the intake valve sooner. This will build more pressure in the cylinder and shift the power band lower. Advancing the cam will cause the intake valve to close later, this will build less pressure in the cylinder and shifts the power band higher.
  
 
==Resources==
 
==Resources==

Revision as of 08:43, 27 September 2012

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