Cam and compression ratio compatibility
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| − | 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. | + | |
| + | |||
| + | ==Matching cam to compression ratio== | ||
| + | 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. The camshaft is not a stand-alone piece. It must be coordinated with other motor and drivetrain parts in order to arrive at a combination of parts that will all work together toward a common goal. | ||
| + | |||
| + | 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. | 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. | ||
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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. | 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 | + | 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 less compression, down to an 8.50:1 cam. |
| + | |||
| + | [http://www.cranecams.com/faqview.php?s_id=9 Crane] says: | ||
| + | <blockquote> | ||
| + | '''Why is it necessary to know the Compression Ratio of an engine in order to choose the correct cam?'''</blockquote> | ||
| − | + | <blockquote>The compression ratio of the engine is one of three key factors in determining the engine's cylinder pressure. The other two are the duration of the camshaft (at .050" lifter rise) and the position of the cam in the engine (advanced or retarded). The result of how these three factors interact with one another is the amount of cylinder pressure the engine will generate. (This is usually expressed as the "cranking pressure" that can be measured with a gauge installed in the spark plug hole.)</blockquote> | |
| − | + | ||
| − | + | <blockquote>It is important to be sure that the engine's compression ratio matches the recommended ratio for the cam you are selecting. Too little compression ratio (or too much duration) will cause the cylinder pressure to drop. This will lower the power output of the engine.</blockquote> | |
| − | {| | + | <blockquote>With too much compression ratio (or too little duration) the cylinder pressure will be too high, causing pre-ignition and detonation. This condition could severely damage engine components.</blockquote> |
| − | !Static Compression Ratio (SCR) !!Intake Valve Duration (degrees @ .050" lift) !!Power Range (RPM) | + | |
| + | {| border="1" cellpadding="2" cellspacing="0" | ||
| + | |-bgcolor="#DDDFFF" | ||
| + | !Static Compression Ratio<br> (SCR) !!Intake Valve Duration<br>(degrees @ .050" lift) !!Power Range<br> (RPM) | ||
|- | |- | ||
| − | | align=" | + | | align="center" |8.00:1 || align="center" |185º || align="center" |Idle-4,000 |
| + | |-bgcolor="#F1F1F1" | ||
| + | | align="center" |8.25:1 || align="center" |189º || align="center" |Idle-4300 | ||
|- | |- | ||
| − | | align=" | + | | align="center" |8.50:1 || align="center" |194º || align="center" |800-4,500 |
| + | |-bgcolor="#F1F1F1" | ||
| + | | align="center" |8.75:1 || align="center" |200º || align="center" |900-4,600 | ||
|- | |- | ||
| − | | align=" | + | | align="center" |9.00:1 || align="center" |204º || align="center" |1,000-4,600 |
| + | |-bgcolor="#F1F1F1" | ||
| + | | align="center" |9.25:1 || align="center" |208º || align="center" |1,200-5,200 | ||
|- | |- | ||
| − | | align=" | + | | align="center" |9.50:1 || align="center" |212º || align="center" |1,600-5,400 |
| + | |-bgcolor="#F1F1F1" | ||
| + | | align="center" |9.75:1 || align="center" |216º || align="center" |1,800-5,600 | ||
|- | |- | ||
| − | | align=" | + | | align="center" |10.00:1 || align="center" |221º || align="center" |2,000-5,800 |
| + | |-bgcolor="#F1F1F1" | ||
| + | | align="center" |10.25:1 || align="center" |227º || align="center" |2,400-6,200 | ||
|- | |- | ||
| − | | align=" | + | | align="center" |10.50:1 || align="center" |233º || align="center" |2,800-6,400 |
| + | |-bgcolor="#F1F1F1" | ||
| + | | align="center" |10.75:1 || align="center" |236º || align="center" |3,000-6,800 | ||
|- | |- | ||
| − | | align=" | + | | align="center" |11.00:1 || align="center" |240º || align="center" |3,200-7,000 |
| + | |-bgcolor="#F1F1F1" | ||
| + | | align="center" |11.50:1 || align="center" |244º || align="center" |3,400-7,200 | ||
|- | |- | ||
| − | | align=" | + | | align="center" |12.00:1 || align="center" |248º || align="center" |3,600-7,400 |
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==Additional reading/engine theory== | ==Additional reading/engine theory== | ||
{{Note1}} | {{Note1}} | ||
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{{Note1}} | {{Note1}} | ||
*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. | *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. | ||
| − | *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). | + | *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). The relationship between SCR and DCR when different IVC points are used can be seen [http://www.crankshaftcoalition.com/wiki/Dynamic_compression_ratio '''here''']. |
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. | 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 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. | 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. | ||
| + | |||
| + | ====Estimating intake closing point==== | ||
| + | If the intake closing (IC) point isn't known, it can be calculated: | ||
| + | # Divide the intake duration by 2 | ||
| + | # Add the results to the lobe separation angle (LSA) | ||
| + | # Subtract any ground-in advance | ||
| + | # Subtract 180 | ||
| + | |||
| + | ===Dish volume=== | ||
| + | Most dished pistons have published volumes. Those that don't or with pistons that are unknown as to brand can be estimated by using the following formula: | ||
| + | |||
| + | {{Note1}}All measurements in inches. | ||
| + | *3.14 x [radius of the dish]² x depth of the dish x 16.4 (converts cubic inches to cc) = Volume in cc | ||
| + | **Add 2 or 3cc for the valve reliefs if they extend beyond and/or below the dish. | ||
===Compression calculators=== | ===Compression calculators=== | ||
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====Dynamic compression ratio calculators==== | ====Dynamic compression ratio calculators==== | ||
| − | *[http://www.empirenet.com/pkelley2/DynamicCR.html | + | *[http://www.empirenet.com/pkelley2/DynamicCR.html Dynamic compression calculator] by Kelly |
| − | *[http://www. | + | *[http://www.uempistons.com/calc.php?action=comp2 Dynamic compression calculator] by KB |
| + | *[http://www.wallaceracing.com/dynamic-cr.php Wallace Racing DCR calculator] | ||
| + | {{Note1}} Some dynamic compression ratio calculators (like KBs) ask for an additional 15 degrees of duration be added to the IVC @ 0.050" lift point nnbfigure. This works OK on older, slower ramped cam lobes, but the faster lobe profiles may need to have 25 degrees or more added to be accurate. | ||
===Cam phasing=== | ===Cam phasing=== | ||
| − | 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. | + | 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. Retarding the cam will cause the intake valve to close later, this will build less pressure in the cylinder and shifts the power band higher. |
| + | |||
| + | Modern engines with '''variable valve timing''' are able to phase the cam while the engine is running. This represents the best of both worlds because the same cam is able to optimize valve events based on engine RPM. This means the VVT engine is capable of: | ||
| + | *advancing the cam (early IVC) at lower RPM to produce more torque, less intake reversion, and less lope at idle | ||
| + | *retarding the cam (late IVC) at higher RPM to produce more horsepower | ||
| + | Of course the SCR remains constant despite cam phasing. The DCR will vary based on RPM since this effects intake charge velocity. | ||
| + | |||
| + | ==Compression ratio calculators== | ||
| + | ===Dynamic compression ratio=== | ||
| + | *[http://www.wallaceracing.com/dynamic-cr.php Wallace Racing DCR calculator] | ||
| + | *[http://www.empirenet.com/pkelley2/DynamicCR.html Kelly DCR calculator] | ||
| + | *[http://www.uempistons.com/calc.php?action=comp2 KB/Silvolite DCR calculator] | ||
| + | *[http://www.rbracing-rsr.com/comprAdvHD.htm RSR DCR calculator] | ||
| + | {{Note1}} Some dynamic compression rtatio calculators (like KBs) ask for an additional 15 degrees of duration be added to the IVC @ 0.050" lift point figure. This works OK on older, slower ramped cam lobes, but the faster lobe profiles may need to have 25 degrees or more added to be accurate. | ||
| + | |||
| + | {{Note1}}If the intake valve closing (IVC) point isn't known, it can be calculated: | ||
| + | # Divide the intake duration by 2 | ||
| + | # Add the results to the lobe separation angle (LSA) | ||
| + | # Subtract any ground-in advance | ||
| + | # Subtract 180 | ||
| + | This result does not need to have any amount added to the IVC point, like the KB calculator calls for. | ||
| + | |||
| + | ==Compression ratio calculators== | ||
| + | ===Static CR=== | ||
| + | *[http://www.wheelspin.net/calc/calc2.html SCR] | ||
| + | |||
| + | ===Dynamic CR=== | ||
| + | *[http://www.empirenet.com/pkelley2/DynamicCR.html Dynamic compression calculator] by Kelly | ||
| + | *[http://www.uempistons.com/calc.php?action=comp2 Dynamic compression calculator] by KB | ||
| + | *[http://www.wallaceracing.com/dynamic-cr.php Wallace Racing DCR calculator] | ||
| + | *[http://www.rbracing-rsr.com/comprAdvHD.htm RSR DCR calculator] | ||
| + | {{Note1}} Some dynamic compression ratio calculators (like KBs) ask for an additional 15 degrees of duration be added to the IVC @ 0.050" lift point figure. This works OK on older, slower ramped cam lobes, but the faster lobe profiles may need to have 25 degrees or more added to be accurate.<br> | ||
| + | |||
| + | {{Note1}}If the intake valve closing (IVC) point isn't known, it can be calculated: | ||
| + | # Divide the intake duration by 2 | ||
| + | # Add the results to the lobe separation angle (LSA) | ||
| + | # Subtract any ground-in advance | ||
| + | # Subtract 180 | ||
| + | |||
| + | ===Static compression ratio=== | ||
| + | *[http://www.wheelspin.net/calc/calc2.html Static compression ratio calculator] | ||
==Resources== | ==Resources== | ||
| + | *[[Dynamic compression ratio]] | ||
*[[How to choose a camshaft]] | *[[How to choose a camshaft]] | ||
*[[Camshaft/Compression Ratio relationships]] | *[[Camshaft/Compression Ratio relationships]] | ||
| + | *[[Quench]] | ||
| + | *[http://www.compcams.com/camquest/default.asp Comp Cams CamQuest] dyno simulation program | ||
| + | |||
[[Category:Engine]] | [[Category:Engine]] | ||
[[Category:Camshaft]] | [[Category:Camshaft]] | ||
| + | |||
| + | <pre style="border:none;"> | ||
| + | General Application Chart * | ||
| + | Advertised | ||
| + | Camshaft Duration Rear Gear Ratio Engine RPM Range & Compression Ratio Converter RPM Stall Speeds ** | ||
| + | Stock to 260° Stock to 3.23 1000-4600 | ||
| + | 8 to 9.5 1500 to 1700 | ||
| + | Stock to 265° Stock to 3.23 1400-5000 | ||
| + | 8 to 9.5 1600 to 1800 | ||
| + | 265° to 280° 3.00 to 3.73 1600-6000 | ||
| + | 9.5 to 10.5 2200 to 2400 | ||
| + | 280° to 300° 3.55 to 4.56 2000-6500 | ||
| + | 9.5 to 11 3000 to 3400 | ||
| + | 280° to 310° 3.73 to 4.88 2800-7000 | ||
| + | 10 to 11.5 3500 to 3800 | ||
| + | 290°+ 3.73+ 3400-8000 | ||
| + | 10.5 to 12 3500+ | ||
| + | </pre> | ||
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