Dynamic, Static Comp Ratio & Cranking Pressure Calculator

[Indycars]

This Excel file will calculate Dynamic and Static Compression Ratios for 5 different engines and show you the
results side-by-side. If you don't have the "Intake Valve Closing Angle", there is also a calculator for that.

I prefer this to online calculators. I can save my results and review them later, can't do that with the online
versions. You can only look at one calculation at a time when online, not so with this Excel spreadsheet...you
can see all 5 versions at the same time !!!

You will need Microsoft Excel 2007 or you can download "Open Office" for FREE here http://www.openoffice.org/.
Open Office can open and work with Excel spreadsheets. If you have an earlier version of Excel, then you can install the
Microsoft Office Compatibility Pack, it's also FREE from Microsoft.

Use this link for information on how to install Office Compatibility Pack:
http://support.microsoft.com/kb/923505

02/09/2011 (Version 02) - Edited Excel file to correct some text. Nothing changed that would effect a calculated value.
Previous Downloads = 5

01/17/2014 (Version 3) - Edited the Excel file to include the "Cranking Pressure", you will have to add the atmospheric pressure. This goes hand-in-hand with the DCR, since the lower the atmospheric pressure the lower the cranking pressure. Something to consider if you live at higher altitudes. With less atmospheric pressure you will be able to use a higher DCR. For the cranking pressure calculation, I used an Adiabatic Constant of 1.2, this seemed to be the best approximation for the real world. That number is based on the link below .....

http://victorylibrary.com/mopar/cam-tech-c.htm

Nothing changed that will effect the previous calculated values.

Also added some text and changed some colors. The different colors will help make the text clearer with the better contrast when posted in a topic. I only updated the first graphic below.

Previous total downloads = 154


Download the Excel file below:

DynamicCompressionRatioCalculatorWithCrankingPressure03.xlsx

DCRCalculator-1.jpg

IVCAngleCalc-2.jpg

IntermediateCalc-3.jpg

FullEngineDescription-4.jpg

[Indycars]

If you have downloaded this file and have version "DynamicCompressionRatioCalculator01", it's NOT important to download again. The only changes were minor, such as text. Nothing was changed that would effect a calculated value.

If you have version 1 or 2, then you will want to download the later version 3. This will give you "Cranking Pressure" in the cylinder.

[grumpyvette]

thanks for posting this,your a true valued asset on the site!
BTW when calculating dynamic compression you can make a ROUGH guess at the TRUE valve closing point by adding 15 degrees to the .050 numbers but remember that cam lobe ramps vary and while thats frequently close it won,t be exact
EXAMPLE
heres the same cam listed opening and close figures listed at .004 and at .050 lift notice adding 15 degrees doesn,t give you valid info but its better than the .050 figures

you might want to read thru this link carefully
http://victorylibrary.com/mopar/cam-tech-c.htm




use this calculator
http://www.kb-silvolite.com/calc.php?action=comp

[Indycars]

thanks for posting this,your a true valued asset on the site!
BTW when calculating dynamic compression you can make a ROUGH guess at the TRUE valve closing point by adding 15 degrees to the .050 numbers but remember that cam lobe ramps vary and while thats frequently close it won,t be exact

Thanks Grumpy ! I like to help where I can.

[Indycars]

Since the idea of Cranking Pressure has come up in discussion this week, the
spreadsheet for calculating compression ratios was a great fit for adding this
parameter to. The DCR and SCR will not change with a change in altitude, but
the cylinder pressure will most certainly change. So if you live at higher altitude,
then you might want to consider comparing your DCR/SCR to others at sea level.
When you don't see it mentioned in discussion you would have to assume they
are talking at sea level.

Let's look at the example below. With a 2000 foot change to a higher altitude,
you have to adjust the atmospheric pressure by 1 psi. (See the table below from
the Engineering Toolbox website.) That means you could decrease the combustion
chamber volume by 4.7 CCs to return to the same cranking pressure.

So at 2000 feet, a DCR/SCR of 10.65/8.34 is equivalent to 10.07/7.91 at sea level.

CrankingPressureExample01.JPG

http://www.engineeringtoolbox.com/air-a ... d_462.html

AtmosphericPressureVsAltitude.JPG

On the other side, if you are building a blown or turbo charged motor, the effect
would be just the opposite.

I cannot confirm just how well cranking pressures work in the real world and
adjusting the SCR/DCR for changes in altitude. If you have some real world
experience with the above please post your experiences ...... do they relate
well to the theory above ?????


To download the latest Excel file, go to the very first post,
it's the only download available.