Math to figure out if your car will run on pump gas.
09-22-2003, 07:23 AM
#1
6600 rpm clutch dump of death Administrator
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Math to figure out if your car will run on pump gas. http://cochise.uia.net/pkelley2/DynamicCR.html
Dynamic Compression Ratio
(Will my engine run on pump gas?)
Dynamic Compression Ratio (DCR) is an important concept in high performance engines. Determining what the compression ratio is after the intake valve closes provides valuable information about how the engine will perform with a particular cam and octane.
Definition: The Compression Ratio (CR) of an engine is the ratio of the cylinder volume compared to the combustion chamber volume. A cylinder with 10 units of volume (called the sweep volume) and a chamber with a volume of 1 has a 10:1 compression ratio. Static Compression Ratio (SCR) is the ratio most commonly referred to. It is derived from the sweep volume of the cylinder using the full crank stroke (BDC to TDC). Dynamic Compression Ratio, on the other hand, uses the position of the piston at intake valve closing rather than BDC of the crank stroke to determine the sweep volume of the cylinder.
The difference between the two can be substantial. For example, with a cam that closes the intake valve at 70º ABDC, the piston has risen 0.9053" from BDC in a stock rod 350 at the intake closing point. This decreases the sweep volume of the cylinder considerably, reducing the stroke length by almost an inch. Thereby reducing the compression ratio. This is the only difference between calculating the SCR and the DCR. All other values used in calculating the CR are the same. Note that the DCR is always lower than the SCR.
Dynamic compression ratio should not to be confused with cylinder pressure. Cylinder pressures change almost continuously due to many factors including RPM, intake manifold design, head port volume and efficiency, overlap, exhaust design, valve timing, throttle position, and a number of other factors. DCR is derived from measured or calculated values that are the actual dimensions of the engine. Therefore, unless variable cam timing is used, just like the static compression ratio, the Dynamic Compression Ratio, is fixed when the engine is built and never changes during the operation of the engine.
Two important points to remember:
The DCR is always lower than the SCR
The DCR does not change at any time during the operation of the engine
Determining seat timing: Since the early days of the internal combustion gasoline engine, engineers have known that the Otto four stroke engine is compression limited and that the quality of the fuel used determines the CR at which the engine could operate. However, it is not the Static CR but the actual running CR of the engine that is important. Compression of the air/fuel mixture cannot start while the intake valve is open. It may start slightly before the intake valve is fully seated. However, there is no easy way to determine this point so using the advertised duration number provided by the cam manufacture is the next best thing. Most cam grinders use .006" of tappet lift (hydraulic cam), although some use other values, with .004" being a common one. This duration is often referred to as the "seat timing". We will used advertised duration for calculating the DCR.
The special case of solid lifter cams. Solid cams are usually speced at an abitrary lift value (often .015" or .020") determined by the designer to be a good approximation of the cam's profile. This lift spec is not always correct for a particular cam. The correct lift point to determine the seat to seat timing of the cam is: Lash / rocker ratio + .004". This accounts for the lash. A cam with a .026" lash (given 1.5 rockers) should be measured at .02133" (.026/1.5+.004= .02133>"). This cam lash, with seat timing speced at .020", is actually a bit smaller than advertised since the valve has yet to actually lift off the seat. How much is the question (.024" lash is the only lash that is correct at .020" with 1.5 rockers). Without knowing the ramp rate, and doing some calculations, or measuring with a degree wheel, it is impossible to know. Again, we have to use the mfg's numbers. Here is some Chevy factory cam help.
Why it matters: A 355 engine with a 9:1 static CR using a 252 cam (110 LSA, 106 ICL) has an intake closing point of 52º ABDC and produces a running CR (DCR) of 7.93. The same 9:1 355 engine with a 292 cam (having an intake closing point of 72º ABDC) has a DCR of 6.87, over a full ratio lower. It appears that most gas engines make the best power with a DCR between 7.5 and 8.5 on 91 or better octane. The larger cam's DCR falls outside this range. It would have markedly less torque at lower RPM primarily due to low cylinder pressures, and a substantial amount of reversion back into the intake track. Higher RPM power would be down also since the engine would not be able to fully utilize the extra A/F mixture provided by the ramming effect of the late intake closing. To bring the 292 cam's DCR up to the 7.5 to 8.5:1 desirable for a street engine, the static CR needs to be raised to around 10:1 to 11.25:1. Race engines, using high octane race gas, can tolerate higher DCR's with 8.8:1 to 9:1 a good DCR to shoot for. The static CR needed to reach 9:1 DCR, for the 292 cam mentioned above, is around 12:1.
This lowering of the compression ratio, due to the late closing of the intake valve, is the primary reason cam manufactures specify a higher static compression ratio for their larger cams: to get the running or dynamic CR into the proper range.
Caveats: Running an engine at the upper limit of the DCR range requires that the engine be well built, with the correct quench distance, and kept cool (170º). Hot intake air and hot coolant are an inducement to detonation. If you anticipate hot conditions, pulling some timing out might be needed. A good cooling system is wise. Staying below 8.25 DCR is probably best for trouble free motoring.
>>Unless you have actually measured the engine (CCed the chambers and pistons in the bores), these calculations are estimations, at best. Treat them as such. The published volumes for heads and pistons can, and do, vary (crankshafts and rods, too). It is best to err on the low side. When contemplating an engine of around 8.4 DCR or higher, measurments are essential, or you could be building another motor.<<
Details: Long duration cams delay the closing of the intake valve and substantially reduce the running compression ratio of an engine compared to the SCR. The cam spec we are interested in to determine the DCR is the intake closing time (or angle) in degrees. This is determined by the duration of the intake lobe, and the installed Intake CenterLine (ICL) (and indirectly by the Lobe Separation Angle (LSA)). Of these, the builder has direct control of the ICL. The others are ground into the camshaft by the grinder (custom grinds are available so the builder could specify the duration and LSA). Changing the ICL changes the DCR. Retarding the cam delays intake closing and decreases the DCR. Advancing the cam causes the intake valve to close earlier (while the pistons is lower in the cylinder, increasing the sweep volume) which increases the DCR. This can be used to manipulate the DCR as well as moving the torque peak up or down the rpm range.
It is necessary to determine the position of the piston at intake valve closing to calculate the DCR. This can be calculated or measured (using a dial indicator and degree wheel). Since compression cannot start until the intake valve is closed, it is necessary to use seat times when calculating the DCR. Using .050" timing will give an incorrect answer since the cylinder is not sealed. At .050" tappet lift, using 1.5 rockers, the valve is still off the seat .075" and .085" with 1.7 rockers. While the flow is nearing zero at this point, compression cannot start until the cylinder is sealed.
Another factor that influences DCR is rod length. It's length determines the piston location at intake closing, different rod lengths change the DCR. Longer rods position the piston slightly higher in the cylinder at intake closing. This decreases the DCR, possibility necessitating a different cam profile than a shorter rod would require. However, the effect is slight and might only be a major factor if the rod is substantially different than stock. Still it needs to be taken into account when calculating the DCR.
Calculating DCR: Calculating the DCR requires some basic information and several calculations. First off, the remaining stroke after the intake closes must be determined. This takes three inputs: intake valve closing point, rod length, and the actual crank stroke, plus a little trig. Here are the formulas: (See the bottom of the page for a way around doing all this math.)
Variables used:
RD = Rod horizontal Displacement in inches
ICA = advertised Intake Closing timing (Angle) in degrees ABDC
RR = Rod Distance in inches below crank CL
RL = Rod Length
PR1 = Piston Rise from RR in inches on crank CL.
PR2 = Piston Rise from crank CL
ST = STroke
1/2ST = one half the STroke
DST = Dynamic STroke length to use for DCR calcs
What's going on: First we need to find some of the above variables. We need to calculate RD and RR. Then, using these number, we find PR1 and PR2. Finally, we plug these number into a formula to find the Dynamic Stroke (DST).
Calcs:
RD = 1/2ST * (sine ICA)
RR = 1/2ST * (cosine ICA)
PR1 = sq root of ((RL*RL) - (RD*RD))
PR2 = PR1 - RR
DST = ST - ((PR2 + 1/2ST) - RL)
This result is what I call the Dynamic Stroke (DST), the distance remaining to TDC after the intake valve closes. This is the critical dimension needed to determine the Dynamic Compression Ratio. After calculating the DST, this dimension is used in place of the crankshaft stroke length for calculating the DCR. Most any CR calculator will work. Just enter the DST as the stroke and the result is the Dynamic CR. Of course, the more accurate the entries are the more accurate the results will be.
Using this information: DCR is only a tool, among others, that a builder has available. It is not the "end all" in cam or CR selection. However, the information provided is very useful for helping to match a cam to an engine or an engine to a cam. It is still necessary to match all the components in an engine and chassis for the best performance possible. Pairing a 305º cam with milled 882 heads just won't cut it even if the DCR is correct. The heads will never support the RPM capabilities of the cam.
A good approach when building an engine is to determine the duration and LSA needed for the desired RPM range. Once this is know, manipulate the chamber size and piston valve reliefs (and sometimes the cam advance) to provide a DCR around 8.2:1. Now that the correct piston volume and chamber size is know, enter the actual crankshaft stroke in your CR calculator to see what static CR to build to. Often the needed SCR is higher that you would expect. Note: The quench distance (piston/head clearance) should always be set between .035" and .045" with the lower limit giving the best performance and detonation resistance.
Alternatively, with the SCR known, manipulate the cam specs until a desirable DCR is found. When the best intake closing time is derived, look for a cam with that intake closing timing, that provides the other attributes desired (LSA and duration). Often times the best cam is smaller than one might expect. Sometimes a CR change is needed to run a cam with the desired attributes.
The information given here should be used as a guideline only. There are no hard and fast rules. It is up to you, the engine builder, to determine the correct build of your engine. And remember, unless accurate measurements are taken, these calculations are approximations.
Here is a link to a discussion in which Jim McFarland discusses some issues regarding compression ratios and combustion problems.
http://www.n2performance.com/cgi-bin...=54&SID=96
Here is an article on High Compression by David Vizard
http://www.motortecmag.com/archives/...JUN010101.html
I hope you find this information helpful and useful,
Pat Kelley
--------------------------------------------------------------------------------
Automation, ain't it great: I have written a Visual Basic program to automate the calculations. It includes the Dynamic Stroke Length Calculator, plus a Valve Timing Calculator (to determine the intake closing point from the advertised duration), and a Compression Ratio Calculator.
There are two version. The larger file contains the required Visual Basic 6 runtime files. If you don't have these files on your system, this is the one to download. It will install these files for you. These runtime files do not come with any version of Windows and can be downloaded from Microsoft's site, if you prefer. If you have the VB6 runtimes, download the smaller file. It does not have the runtimes. If you have successfully run VB6 programs before, you have these files. If you have never ran a VB6 program before, you need the larger version. Un-Zip with your favorite archive program and run "setup.exe". This will install the program and register it with Windows. These files were compressed using WinZip 7.0. You can download a free demo copy of WinZip at www.winzip.com. If you have any problems, email me (my address is on the Home page http://cochise.uia.net/pkelley2/index.html ) and I will try to help. You can take a look the the
DCR FAQ's, the answer to your question could be here. http://cochise.uia.net/pkelley2/DCR_FAQ.html
DCR Calculator with VB6 Runtime files 1.55 MB
http://cochise.uia.net/pkelley2/dcrvb6.zip
DCR Calculator without VB6 Runtime files 423 KB
http://cochise.uia.net/pkelley2/dcr.zip
*A note to users outside the United States.* The DCR Calculator was written with the Regional Setting of Windows set to the "English (United States)" setting. To run properly, you may need to change the Regional Setting of your Windows operating system to "English (United States)". This is due to the way various regions use the "," and "." place and decimal separators (there may be other factors I'm not aware of, also). After running the DCR Calculator, you should return the setting to your original region settings to insure the proper operation of your system. The Regional Setting applet should be located in Control Panel.
Home
http://cochise.uia.net/pkelley2/index.html
I have received several request regarding what is the best DCR for lower octane fuels. At this time, I don't know. If you are running 87 or 89 octane successfully and know what your DCR is, I'd be interested in hearing from you (email address is on my home page). This would help those that want the best performance on lower octanes for drivers and tow vehicles.
Dynamic Compression Ratio
(Will my engine run on pump gas?)
Dynamic Compression Ratio (DCR) is an important concept in high performance engines. Determining what the compression ratio is after the intake valve closes provides valuable information about how the engine will perform with a particular cam and octane.
Definition: The Compression Ratio (CR) of an engine is the ratio of the cylinder volume compared to the combustion chamber volume. A cylinder with 10 units of volume (called the sweep volume) and a chamber with a volume of 1 has a 10:1 compression ratio. Static Compression Ratio (SCR) is the ratio most commonly referred to. It is derived from the sweep volume of the cylinder using the full crank stroke (BDC to TDC). Dynamic Compression Ratio, on the other hand, uses the position of the piston at intake valve closing rather than BDC of the crank stroke to determine the sweep volume of the cylinder.
The difference between the two can be substantial. For example, with a cam that closes the intake valve at 70º ABDC, the piston has risen 0.9053" from BDC in a stock rod 350 at the intake closing point. This decreases the sweep volume of the cylinder considerably, reducing the stroke length by almost an inch. Thereby reducing the compression ratio. This is the only difference between calculating the SCR and the DCR. All other values used in calculating the CR are the same. Note that the DCR is always lower than the SCR.
Dynamic compression ratio should not to be confused with cylinder pressure. Cylinder pressures change almost continuously due to many factors including RPM, intake manifold design, head port volume and efficiency, overlap, exhaust design, valve timing, throttle position, and a number of other factors. DCR is derived from measured or calculated values that are the actual dimensions of the engine. Therefore, unless variable cam timing is used, just like the static compression ratio, the Dynamic Compression Ratio, is fixed when the engine is built and never changes during the operation of the engine.
Two important points to remember:
The DCR is always lower than the SCR
The DCR does not change at any time during the operation of the engine
Determining seat timing: Since the early days of the internal combustion gasoline engine, engineers have known that the Otto four stroke engine is compression limited and that the quality of the fuel used determines the CR at which the engine could operate. However, it is not the Static CR but the actual running CR of the engine that is important. Compression of the air/fuel mixture cannot start while the intake valve is open. It may start slightly before the intake valve is fully seated. However, there is no easy way to determine this point so using the advertised duration number provided by the cam manufacture is the next best thing. Most cam grinders use .006" of tappet lift (hydraulic cam), although some use other values, with .004" being a common one. This duration is often referred to as the "seat timing". We will used advertised duration for calculating the DCR.
The special case of solid lifter cams. Solid cams are usually speced at an abitrary lift value (often .015" or .020") determined by the designer to be a good approximation of the cam's profile. This lift spec is not always correct for a particular cam. The correct lift point to determine the seat to seat timing of the cam is: Lash / rocker ratio + .004". This accounts for the lash. A cam with a .026" lash (given 1.5 rockers) should be measured at .02133" (.026/1.5+.004= .02133>"). This cam lash, with seat timing speced at .020", is actually a bit smaller than advertised since the valve has yet to actually lift off the seat. How much is the question (.024" lash is the only lash that is correct at .020" with 1.5 rockers). Without knowing the ramp rate, and doing some calculations, or measuring with a degree wheel, it is impossible to know. Again, we have to use the mfg's numbers. Here is some Chevy factory cam help.
Why it matters: A 355 engine with a 9:1 static CR using a 252 cam (110 LSA, 106 ICL) has an intake closing point of 52º ABDC and produces a running CR (DCR) of 7.93. The same 9:1 355 engine with a 292 cam (having an intake closing point of 72º ABDC) has a DCR of 6.87, over a full ratio lower. It appears that most gas engines make the best power with a DCR between 7.5 and 8.5 on 91 or better octane. The larger cam's DCR falls outside this range. It would have markedly less torque at lower RPM primarily due to low cylinder pressures, and a substantial amount of reversion back into the intake track. Higher RPM power would be down also since the engine would not be able to fully utilize the extra A/F mixture provided by the ramming effect of the late intake closing. To bring the 292 cam's DCR up to the 7.5 to 8.5:1 desirable for a street engine, the static CR needs to be raised to around 10:1 to 11.25:1. Race engines, using high octane race gas, can tolerate higher DCR's with 8.8:1 to 9:1 a good DCR to shoot for. The static CR needed to reach 9:1 DCR, for the 292 cam mentioned above, is around 12:1.
This lowering of the compression ratio, due to the late closing of the intake valve, is the primary reason cam manufactures specify a higher static compression ratio for their larger cams: to get the running or dynamic CR into the proper range.
Caveats: Running an engine at the upper limit of the DCR range requires that the engine be well built, with the correct quench distance, and kept cool (170º). Hot intake air and hot coolant are an inducement to detonation. If you anticipate hot conditions, pulling some timing out might be needed. A good cooling system is wise. Staying below 8.25 DCR is probably best for trouble free motoring.
>>Unless you have actually measured the engine (CCed the chambers and pistons in the bores), these calculations are estimations, at best. Treat them as such. The published volumes for heads and pistons can, and do, vary (crankshafts and rods, too). It is best to err on the low side. When contemplating an engine of around 8.4 DCR or higher, measurments are essential, or you could be building another motor.<<
Details: Long duration cams delay the closing of the intake valve and substantially reduce the running compression ratio of an engine compared to the SCR. The cam spec we are interested in to determine the DCR is the intake closing time (or angle) in degrees. This is determined by the duration of the intake lobe, and the installed Intake CenterLine (ICL) (and indirectly by the Lobe Separation Angle (LSA)). Of these, the builder has direct control of the ICL. The others are ground into the camshaft by the grinder (custom grinds are available so the builder could specify the duration and LSA). Changing the ICL changes the DCR. Retarding the cam delays intake closing and decreases the DCR. Advancing the cam causes the intake valve to close earlier (while the pistons is lower in the cylinder, increasing the sweep volume) which increases the DCR. This can be used to manipulate the DCR as well as moving the torque peak up or down the rpm range.
It is necessary to determine the position of the piston at intake valve closing to calculate the DCR. This can be calculated or measured (using a dial indicator and degree wheel). Since compression cannot start until the intake valve is closed, it is necessary to use seat times when calculating the DCR. Using .050" timing will give an incorrect answer since the cylinder is not sealed. At .050" tappet lift, using 1.5 rockers, the valve is still off the seat .075" and .085" with 1.7 rockers. While the flow is nearing zero at this point, compression cannot start until the cylinder is sealed.
Another factor that influences DCR is rod length. It's length determines the piston location at intake closing, different rod lengths change the DCR. Longer rods position the piston slightly higher in the cylinder at intake closing. This decreases the DCR, possibility necessitating a different cam profile than a shorter rod would require. However, the effect is slight and might only be a major factor if the rod is substantially different than stock. Still it needs to be taken into account when calculating the DCR.
Calculating DCR: Calculating the DCR requires some basic information and several calculations. First off, the remaining stroke after the intake closes must be determined. This takes three inputs: intake valve closing point, rod length, and the actual crank stroke, plus a little trig. Here are the formulas: (See the bottom of the page for a way around doing all this math.)
Variables used:
RD = Rod horizontal Displacement in inches
ICA = advertised Intake Closing timing (Angle) in degrees ABDC
RR = Rod Distance in inches below crank CL
RL = Rod Length
PR1 = Piston Rise from RR in inches on crank CL.
PR2 = Piston Rise from crank CL
ST = STroke
1/2ST = one half the STroke
DST = Dynamic STroke length to use for DCR calcs
What's going on: First we need to find some of the above variables. We need to calculate RD and RR. Then, using these number, we find PR1 and PR2. Finally, we plug these number into a formula to find the Dynamic Stroke (DST).
Calcs:
RD = 1/2ST * (sine ICA)
RR = 1/2ST * (cosine ICA)
PR1 = sq root of ((RL*RL) - (RD*RD))
PR2 = PR1 - RR
DST = ST - ((PR2 + 1/2ST) - RL)
This result is what I call the Dynamic Stroke (DST), the distance remaining to TDC after the intake valve closes. This is the critical dimension needed to determine the Dynamic Compression Ratio. After calculating the DST, this dimension is used in place of the crankshaft stroke length for calculating the DCR. Most any CR calculator will work. Just enter the DST as the stroke and the result is the Dynamic CR. Of course, the more accurate the entries are the more accurate the results will be.
Using this information: DCR is only a tool, among others, that a builder has available. It is not the "end all" in cam or CR selection. However, the information provided is very useful for helping to match a cam to an engine or an engine to a cam. It is still necessary to match all the components in an engine and chassis for the best performance possible. Pairing a 305º cam with milled 882 heads just won't cut it even if the DCR is correct. The heads will never support the RPM capabilities of the cam.
A good approach when building an engine is to determine the duration and LSA needed for the desired RPM range. Once this is know, manipulate the chamber size and piston valve reliefs (and sometimes the cam advance) to provide a DCR around 8.2:1. Now that the correct piston volume and chamber size is know, enter the actual crankshaft stroke in your CR calculator to see what static CR to build to. Often the needed SCR is higher that you would expect. Note: The quench distance (piston/head clearance) should always be set between .035" and .045" with the lower limit giving the best performance and detonation resistance.
Alternatively, with the SCR known, manipulate the cam specs until a desirable DCR is found. When the best intake closing time is derived, look for a cam with that intake closing timing, that provides the other attributes desired (LSA and duration). Often times the best cam is smaller than one might expect. Sometimes a CR change is needed to run a cam with the desired attributes.
The information given here should be used as a guideline only. There are no hard and fast rules. It is up to you, the engine builder, to determine the correct build of your engine. And remember, unless accurate measurements are taken, these calculations are approximations.
Here is a link to a discussion in which Jim McFarland discusses some issues regarding compression ratios and combustion problems.
http://www.n2performance.com/cgi-bin...=54&SID=96
Here is an article on High Compression by David Vizard
http://www.motortecmag.com/archives/...JUN010101.html
I hope you find this information helpful and useful,
Pat Kelley
--------------------------------------------------------------------------------
Automation, ain't it great: I have written a Visual Basic program to automate the calculations. It includes the Dynamic Stroke Length Calculator, plus a Valve Timing Calculator (to determine the intake closing point from the advertised duration), and a Compression Ratio Calculator.
There are two version. The larger file contains the required Visual Basic 6 runtime files. If you don't have these files on your system, this is the one to download. It will install these files for you. These runtime files do not come with any version of Windows and can be downloaded from Microsoft's site, if you prefer. If you have the VB6 runtimes, download the smaller file. It does not have the runtimes. If you have successfully run VB6 programs before, you have these files. If you have never ran a VB6 program before, you need the larger version. Un-Zip with your favorite archive program and run "setup.exe". This will install the program and register it with Windows. These files were compressed using WinZip 7.0. You can download a free demo copy of WinZip at www.winzip.com. If you have any problems, email me (my address is on the Home page http://cochise.uia.net/pkelley2/index.html ) and I will try to help. You can take a look the the
DCR FAQ's, the answer to your question could be here. http://cochise.uia.net/pkelley2/DCR_FAQ.html
DCR Calculator with VB6 Runtime files 1.55 MB
http://cochise.uia.net/pkelley2/dcrvb6.zip
DCR Calculator without VB6 Runtime files 423 KB
http://cochise.uia.net/pkelley2/dcr.zip
*A note to users outside the United States.* The DCR Calculator was written with the Regional Setting of Windows set to the "English (United States)" setting. To run properly, you may need to change the Regional Setting of your Windows operating system to "English (United States)". This is due to the way various regions use the "," and "." place and decimal separators (there may be other factors I'm not aware of, also). After running the DCR Calculator, you should return the setting to your original region settings to insure the proper operation of your system. The Regional Setting applet should be located in Control Panel.
Home
http://cochise.uia.net/pkelley2/index.html
I have received several request regarding what is the best DCR for lower octane fuels. At this time, I don't know. If you are running 87 or 89 octane successfully and know what your DCR is, I'd be interested in hearing from you (email address is on my home page). This would help those that want the best performance on lower octanes for drivers and tow vehicles.
09-22-2003, 11:39 AM
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Re: Math to figure out if your car will run on pump gas.
Alternatively, with the SCR known, manipulate the cam specs until a desirable DCR is found. When the best intake closing time is derived, look for a cam with that intake closing timing, that provides the other attributes desired (LSA and duration). Often times the best cam is smaller than one might expect. Sometimes a CR change is needed to run a cam with the desired attributes.
08-03-2004, 11:55 AM
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Originally Posted by J-Rod
http://cochise.uia.net/pkelley2/DynamicCR.html
Definition: The Compression Ratio (CR) of an engine is the ratio of the cylinder volume compared to the combustion chamber volume. A cylinder with 10 units of volume (called the sweep volume) and a chamber with a volume of 1 has a 10:1 compression ratio.
Definition: The Compression Ratio (CR) of an engine is the ratio of the cylinder volume compared to the combustion chamber volume. A cylinder with 10 units of volume (called the sweep volume) and a chamber with a volume of 1 has a 10:1 compression ratio.
On the dynamic compression ratio, he (and others) speak of the intake valve closing time as the point at which compression really starts, but don't talk about how to get that from cam specs. Is that the 0.006 figure? It seems like very little air would escape at high RPM with the valve 0.010 off the seat. Or maybe even 0.020 off the seat. And how about inertial effects? How does that affect the dynamic compression ratio?
It is obvious that the theory is correct since it is a fact that you can run more static compression by moving the IVC event further out in time (say from 40* ABDC to 50* ABDC). But it seems to me that dynamic compression ratio is very hard to quantify. Does anyone have better information on this?
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08-03-2004, 04:47 PM
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Originally Posted by critter
It is obvious that the theory is correct since it is a fact that you can run more static compression by moving the IVC event further out in time (say from 40* ABDC to 50* ABDC). But it seems to me that dynamic compression ratio is very hard to quantify. Does anyone have better information on this?
Looky what I found: http://victorylibrary.com/mopar/cam-tech-c.htm
08-03-2004, 08:08 PM
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I don't know if everything in that can be right. My wife's old mustang had a mild 306, .030 over stock block with domed SRP's, 5.4" eagle rods, stage 3 trick flows, and a smallish 242/234 hydro roller. Static compression checked in right at 11:1, and it ran beautifully on 91 octane. This thing tells me the dynamic compression is 9.21:1, and he recommends that anything over 8.5:1 is going to require race fuel?
08-04-2004, 12:29 AM
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maybe the formula is not for fords.. 
j/j their are a lot of different things that can be factored in this ....hell i just learned when useing a micrometer you can get different readings with different weight oils on the metal ??i guess you can get as in depth as you want to with anything .....i think if it works use it....
j/j their are a lot of different things that can be factored in this ....hell i just learned when useing a micrometer you can get different readings with different weight oils on the metal ??i guess you can get as in depth as you want to with anything .....i think if it works use it.... 
08-05-2004, 12:31 AM
#13
Camaroz28.com has a ton of threads on this.....
Mr. Horsepower, Midgame, RKrause, Old SStroker, MaxRaceSoftware all have some good things to say on this.
http://web.camaross.com/forums/showt...pression+ratio
http://web.camaross.com/forums/showt...pression+ratio
http://web.camaross.com/forums/showt...pression+ratio
http://web.camaross.com/forums/showt...pression+ratio
http://web.camaross.com/forums/showt...pression+ratio
IMHO if you are doing it right you can run 9:1 DCR on a street motor with 91-93 octane, easy on a LS1 with cubes, not on a stock cube motor.
Bret
Mr. Horsepower, Midgame, RKrause, Old SStroker, MaxRaceSoftware all have some good things to say on this.
http://web.camaross.com/forums/showt...pression+ratio
http://web.camaross.com/forums/showt...pression+ratio
http://web.camaross.com/forums/showt...pression+ratio
http://web.camaross.com/forums/showt...pression+ratio
http://web.camaross.com/forums/showt...pression+ratio
IMHO if you are doing it right you can run 9:1 DCR on a street motor with 91-93 octane, easy on a LS1 with cubes, not on a stock cube motor.
Bret
12-08-2007, 06:15 PM
#17
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question for J-Rod on scr vs dcr J-Rod in one of your paragraphs you stated that unless variable valve timing is used that dcr does not change and is set like scr. I find that statement to be back-wards. Variable valve timing will bring dcr closer to scr by means of closing out negative effects. One would be minimizing the effects of reversion on valve to valve over lap and the second would be removing reversion on the begining of the compression stroke. if you did not not have variable valve timing dcr would be constantly changing depending on rpm. Low rpm will not provide a ram charging or stacking effect and would lower the dcr by pushing some of the fill right back out of the cylinder before the intake valve('s) has a chance to close. As well without valve timing the valve to valve over lap would allow a small period of time when exhaust gases rush up the intake tract and at some point they will have to stop and turn around when the intake charge is fianally pulled in by the piston. That leaves you with lost time to fill the cylinder. Add those two problems together and you are left with a lowe dcr.
12-18-2007, 11:31 AM
#18
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DCR is a mechanical measurement of effective compression ratio based on valve closure and not the volumetric efficiency of the engine, which as you say changes based on rpm, map pressure, intake temps etc, etc.
12-20-2010, 09:14 AM
#19
I have a question regarding maximum DCR number statements and recommendations. I know a few people who have been racing their stock-cube NA LS1 motors with a DCR of well over 9.5 for well over a year (milled/high-SCR heads). The spark timing has also been advanced over stock, and yet they run fine (and stronger than stock) on 91 octane pump gas. I understand that DCR is important, yet these motors with an abnormally high DCR seem to run fine, and lowering the DCR would not leave them with a performance advantage. They absolutely do run on pump gas. Efficiently? Reliably?? As far as I can tell...
So:
Is DCR for LS motors more important under forced induction applications than NA?
If these motors run just fine on pump gas with a 10.0:1 DCR with proper tuning (even with advanced spark timing over stock), are there new possible high DCR pump gas limits due to advances in engine technology since 1955 when the Gen 1 was introduced??
So:
Is DCR for LS motors more important under forced induction applications than NA?
If these motors run just fine on pump gas with a 10.0:1 DCR with proper tuning (even with advanced spark timing over stock), are there new possible high DCR pump gas limits due to advances in engine technology since 1955 when the Gen 1 was introduced??
Last edited by ZexGX; 12-20-2010 at 09:22 AM.
12-20-2010, 07:23 PM
#20
DCR is exactly that... dynamic. You'll see a change in effective compression ratio at different RPMs and loads. The detonation resistance of the motor has a lot to do with the chamber and piston design, bore, cylinder head temp, etc. So I wouldn't get too caught up in a specific DCR number, there are too many variables in cylinder heads and camshafts to simply figure on IVC. There's guys running >12.5:1 static on pump gas with 236/244 cams, however those are antiquated LT1s 