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Mach1 Driver

Static Compression Calculator

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Are you one of those people that has an inherent distrust of on-line calculators? I’m never quite sure if I have the correct information, in the way they want it, or what the calculator is doing, so I decided to make one myself. The formulas and bulk of definitions are from rbracing-rsr.com. Once you understand what is in the formulas, it’s easy to see that the results are accurate.

I started this because I wanted to know which pistons and heads to get for my 1969 351W. The calculations are simple math, although there is a fair amount of them, so like any good engineer I turned to Excel. I laid the formulas out so it is easy to see what is happening. Basically it calculates the volumes of several different “math cylinders” within an engine cylinder. You also need to know the cylinder head volume and piston head volume for any reliefs or what not. There are a total of 10 different things to enter. Piston and head manufacturers usually provide the answers on their websites, and if they don’t, you probably don’t want to use their stuff anyway. Ixnay on Ebay.

Initially I was confused about deck height because of what is shown in the formulas. Here I refer to the engine’s deck height as “deck height 1”, and “deck height 2” is from a calculation and provides the distance from the top (flat part) of the piston to the top of the deck. Also, until I saw the formulas I didn’t understand why an area cut out of the top of the piston should be expressed as a positive number, while a projection from the top of the piston should be a negative number. The way the formulas are written, you are adding or subtracting these volumes from the totals.

The spreadsheet has two sheets, the first of which is definitions. You need to be certain of these so you feed the correct numbers into the calculator on the second sheet. Fill in the green cells to get the answer in the blue cell. It allows you to pick and choose which heads, pistons, head gaskets, rod lengths, and stroke to use for a desired compression ratio.Compression formulas.xlsx

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16 hours ago, RPM said:

Dang Terry, you get bored? Thank God for engineers. 


You are welcome. Yeah that's literally what happened, so I decided to do something that I find interesting and that I will eventually need. Using this spreadsheet I have preliminarily selected AFR 185cc heads with 58cc combustion chambers, and Icon 1C9925-03 forged pistons that have +11cc top volume. Its a little higher compression than I was shooting for at 10.39:1, but so far its the only combination of parts that comes close- I was shooting for something a little lower around 10:1. 

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