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...&TID=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,

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) 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://www.empirenet.com/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.

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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.

 

I fully understand the difference and how DCR and SCR are reached, now. Thanks, JROD!

 

so what does it mean if u run a higher DCR ? ie the cam I just made up in my grill has a DCR of 10.8 ... and a SCR of 13 with my semi-right motor specs... motor isnt done yet

with cooling and IAT all being equal... would a 8 DCR be good for say 93 octane...

see where Im going...

all things being equal at what DCR do u need what octane?

 

I know someone looking into DCR relationships vs cu .inches. I need to dig a bit more to give a complete answer. Since I had a few minutes I added a DCR calculator to the VE spreadsheet in Excel. From that, you put in your cam specs, and basic engine specs and it will give you your valve events, and the SCR and DCR based on that cam. I'll post it up as soon as I have time.

 

damn..... that is awesome... thanx j-rod... NOTHING like that would ever been seen on ls1//2.com

mike

 

I think if J-Rod fills his head with any more of this technical information his head is going to explode and there will be a mess of cams, valves and rockers everywhere.

 

Here is the spreadsheet now it has a DCR tab, and it alos has a sheet with two separate DCR columns so you can see what your DCR is with your current cam and see how it changes if you change the cam specs or anything else.

Quick and Dirty VE calculator Spreadsheet