Originally Posted by The Clevite Kid

Let's see what I can do. Refer to the attached thumbnail (click on it to open in a new tab): the red circle and line at the bottom represent the crankpin, Theta is the crank angle at which the intake valve closes (0.006" off its seat), the blue line is the connecting rod, and the green shape is the piston in a red cylinder.

If the connecting rod did NOT lean over during the stroke, (see dotted blue line to the right of the cylinder) the piston would actually be somewhat higher in the cylinder (dotted green line) by a value I call omega. The formula in the spreadsheet in my OP takes this into account and computes omega as a function of theta, and of the ratio of the rod length to the stroke of the crankpin.

Note that if we call the angle between the actual connecting rod and the straight line between the main bearing and the wrist pin beta, then

L*sin(beta) = (stroke/2)*sin(theta)

and you can use Pythagoras' Theorem to find omega as a function of theta.

Since omega always lowers the piston compared to the straight-rod scenario, the DCR is increased due to this affect. Ignoring it would underestimate the TRUE DCR and could lead to detonation and all its nasty consequences.

There will be a quiz on Monday .

Originally Posted by MrDrezzUp

The consensus (from SpinMonster, of course!) is that with a tight quench (0.030" or so) the heads for the LS2 (and LS3) can run up to 8.7 to 1 DCR on pump (93 octane) gasoline.

Any thoughts on what the DCR should be for 91 octane at sea level?

Originally Posted by The Clevite Kid

Thanx for repeating your experience here, Guy. I had seen a similar post of yours before.

According to the data Joe_G sent me, and assuming the ramps on his LG cam are similar to COMP XE-R ramps, his theta is about 75 degrees, and his DCR is about 8.2 to 1.

If he can avoid having the valves get friendly with the pistons in his new heads, he can bump up his static compression ratio from around 11.3 to 12 to 1 and still have a DCR of 8.7. Interesting, no?

Originally Posted by J-Rod

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&amp;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



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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.

Originally Posted by Justinjor

This is a pretty amazing thread but wow, a lot of it is over my head.

Taking what I've learned from reading, I'm trying to figure out my current DCR and what is needed to get to the optimal 8.7 DCR as recommended.

Looking at the spreadsheet, do I need to change the stroke or rod length? My car is an LS3 and not an LS2.

Also, looking at my cam doctor report for the cam in my car, I see 46.82 as the ABDC measurement at .050 lift. Is that the correct number to use for Theta? If so, my DCR is not even on the chart How do I fix that? I was going to try to adjust the cells in the spreadsheet to get back to a 46 Theta but I'm not 100% sure that's correct. If it is, that puts my current SCR at something like 9.8:1, could that be correct? I know I would have lost compression due to the new overlap of my cam, but I didn't think I'd lose that much.

Any help is appreciated. Attached is my cam doctor sheet.

Originally Posted by kjcmusic17

Here I am to revive a 10year old thread 😁

I've been looking at this excel sheet trying to adapt for LS3. I input the stroke and rod length, and assume things like bore and combustion chamber are automatically accounted for in the static compression ratio. Summit's cam calculator with "stock LS3 cam" specs of 204/211 117, show a closure at 39° ABDC. This gives me a DCR of 9.9:1. Surely this isn't right. What am I missing?

Reason I'm interested is for calculating DCR with Spins cam. I keep getting crazy high DCR values. According to summit, spins spec should close 49° ABDC
Plz help 😁

Originally Posted by old motorhead

The calculator I use requires "advertised" duration (measured at .006 lift) to arrive at your engine's DCR. Spin's cam initially was ground by Comp on different lobe styles for the intake and exhaust. I know Tx Speed will grind it and probably Cam Motion too. If it's ground by Cam Motion, I can figure it for you as I know to add 54* to duration at .050 lift to get to duration at .006. That duration at .006 info is also available from Comp if you know the lobe styles and want to do some digging. I have no idea how to get from .050 duration to .006 duration using Tx Speed lobes.

I use this calculator Jeep Strokers - Dynamic\Static Compression Ratio Calculator. It's the first one I learned how to use....so I still use it. I've had Pat G spec several cams for me. He includes the DCR with his cam specs. His DCR numbers agree with the numbers I get using this calculator.

Originally Posted by kjcmusic17

Thanks! Cam motion supplied me with what the advertised duration would be. I'm almost assuredly gonna go through them. My experience with TSP so far has been "use our cam, Spinmonster doesn't work" instead of them actually answering the questions I ask. I'll check out that calculator.

Originally Posted by old motorhead

I'm running a couple of Cam Motion cams now. Lots of miles and smiles with no valve train troubles. My LS3's cam in my Vette has .620 lift on both sides. Stock rocker arms and BTR springs were changed at 30K+ miles. It all looked perfect. I'm a fan of Cam Motion's stuff.

So, is advertised duration 284 on the intake for Spin's cam built by Cam Motion?