Dynamic, Static Comp Ratio & Cranking Pressure Calculator

[Indycars]

This Excel file will calculate Dynamic/Static Compression Ratios and Cranking Pressures for 5 different
engines and show you the results side-by-side. If you don't have the "Intake Valve Closing Angle" from the
cam card, 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

Since the Excel file includes the "Cranking Pressure", you will have to add the atmospheric pressure. 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

Download the Excel file below:

DynamicCompressionRatioCalculatorWithCrankingPressure04.xlsx

(22.58 KiB) Downloaded 27 times

DCRCalculator-1.jpg (64.01 KiB) Viewed 195 times

IVCAngleCalc-2.jpg (134.96 KiB) Viewed 195 times

IntermediateCalc-3.jpg (115.71 KiB) Viewed 195 times

FullEngineDescription-4.jpg (88.33 KiB) Viewed 195 times

[Indycars]

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 altitudes,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 (59.41 KiB) Viewed 171 times

AtmosphericPressureVsAltitude.JPG (52.96 KiB) Viewed 171 times

From my only personal experience the calculated came out lower than the measured
cranking pressure. I calculated 171 psi and measured 190 psi, so your mileage may vary.

Hope this info can help someone build the right engine !

More Related Info:

heatvscpr.jpg (64.65 KiB) Viewed 171 times