HOW TO:Calculating static compression ratio

[bobsmyuncle]

CALCULATING STATIC COMPRESSION RATIO:

This is my ATTEMPT to explain how static compression ratio is figured. I see many many people asking about this and nobody gets a straight answer. This will yield you an exact ratio if done properly.

Most people view the calculations of static compression ratio as being very confusing. The truth is, its simple Static compression is a simple ratio. It is the ratio of the volume of a cylinder at the bottom of its stroke, compared to the volume of a cylinder at the top of its stroke. How do we find these volumes? Through a series of easy to use formulas and math. Which I can guarantee anyone can do! After trying endlessly to find a calculator online that’s accurate, I decided to put pen to paper and do it myself., And now I want to show you how to!!

Before we begin we will need to know the following:

combustion chamber cc - 59cc
head gasket bore - 3.910
head gasket compressed thickness - .040
cylinder bore - 3.905
cylinder stroke - 3.622
deck clearance - -.010(above deck)
ring height - .30
piston volume - -4cc(valve reliefs)


First thing we have to do is figure out how much our cylinder volume is when our piston is at bottom dead center, or like this:



We will start at the top and work our way down, and remember all volumes must be converted to the same units! and we will add all of the volumes in the final step. It can be done with whatever you want cubic centimeters, cubic feet, even cubic miles, I’m going to demonstrate with cubic inches.

STEP 1:

Find the volume of your combustion chamber. In the diagram, this volume is represented by the pink shaded area. this is one of the easiest steps. to convert your head cc volume to cubic inches, use this formula:

1cc=0.061023744095 cubic inch

so, for my calculations, I am using a 59cc head. To find the cc’s of your head you can use water(search online for a how-to) or if you have stock chambers you can look up the sizes. So using 59cc heads, my calculations would look like this:

59cc x .061023744095 = 3.600400901605 < this is the cubic inch volume of my combustion chamber. We need to arrange a list of numbers. So this should be your first(you will see in final step).

STEP 2:

Find the volume that your head gasket bore will have. Represented in the diagram by the red area. The formula we will be using is a simple volume formula for the volume of a cylinder. Which looks like this:

Pi x r^2 x H = gasket volume or, 3.142 x R^2 x H = gasket volume

So, 3.142 is pi. R is your radius squared. and H is your height.

So the dimensions of the head gaskets we will be calculating volume for are as follows: 3.910 bore and .040 thickness. Now my formula should look like this:

3.142 x 1.955^2 x .040 = 0.4804831428571425

Now you may be wondering where I got these numbers. 3.142 is Pi obviously, 1.955 is the radius of our gasket bore. Bore or diameter is the same, but as most of you know, we need the radius(R). Which is simply half of our bore. And the .040 is the thickness or height of our gasket. Now add this number to your list as well.

STEP 3:

Calculating deck clearance. Show in the diagram by the light blue area. Now as most of you know, anyone owning most lsx series engines are blessed with negative deck clearance. For those of you who do not know what that means, it simply means that when the piston is at TDC(top dead center, or very top of stroke)it is actually out of the motor or block. Typically somewhere between 7-10 thousandths(.007-.010) I will be using .010 for my calculations. Do not add this number to the list.
Now you may be wondering why we are putting this number into our BDC(bottom dead center)volume calculations. When it has to do with the piston being at TDC. I will try and put it as simple as possible. When we are calculating our next step or step 4(cylinder volume), We cant use the stock stroke number of 3.622. why you ask? because if we physically go in and measure the distance from deck to piston face, it will not be 3.622. it will be 3.612, because the top .010 of our stroke, will be above the deck. We will call this our corrected stroke, (3.612)

STEP 4:

Calculating cylinder volume. Represented by the orange area in the diagram. For this we will be using the same formula as we did to calculate our head gasket volume:

Pi x r^2 x H = cylinder volume or 3.142 x R^2 x H = cylinder volume

so 3.142 is pi. R is your radius or half bore squared. and H is your height or corrected stroke(3.622 - .010 = 3.612). in this case its 3.612 as stated before. I am using a 3.905 bore, so my calculations would look like this:

3.142 x 1.9525^2 x 3.612 = 43.27673294999995

again you might be wondering where i got these numbers. 3.142 is Pi, 1.9525 is radius or half bore, and 3.612 is our corrected stroke or distance from piston face to deck surface. And the result is our cylinder volume in cubic inches. You can add this number to our list.

STEP 5:

Calculating piston to bore clearance volume. as you may notice in the diagram there is "space" between the piston and cylinder wall above the top ring. That is so the piston doesn’t seize into the cylinder when the temperature fluxgates. For these calculations we will need to know piston to bore clearance, as well as top ring height. Ring height is the distance from the piston face, to the first compression ring. For my calculations i will be using a clearance of .0015 or one and a half thousand. and a ring height of .300 or three hundred thousandths.

The easiest way to figure how much volume is around the piston above the top ring, is to find the total volume of the cylinder above the top ring but below the piston face, represented by the orange area, but only the orange below the piston face. And subtract the volume that the piston takes up(dark grey area). So theoretically we are taking the volume of a very short cylinder(.300x3.905), and subtracting the volume of a slightly smaller cylinder(.300x3.902) and getting the volume of the "ring" that is left over. like this:



To do this we use the same formula as we used for the gasket, and cylinder. like this:

Pi x r^2 x H = cylinder volume or 3.142 x R^2 x H = cylinder volume

3.142 x 1.955^2 x .300 = 3.6036235714285683<This is our total volume below piston face and top ring. do not add this number to the list.

3.142 is Pi, 1.951 is the diameter of the piston above the top ring, and .300 is distance from top ring to piston face, or ring height.

Now we must figure out the diameter of the piston above the top ring, you can do this easily by measuring the piston, or taking our bore of the cylinder and subtracting the clearance twice(because it has .0015 on both sides of the cylinder). I will be using 3.902.

Now we must find the volume of the piston above the top ring and subtract that from this number. like this:

3.142 x 1.951 x .300 = 3.5888923714285688

3.142 is Pi, 1.951 is the diameter of the piston above the top ring, and .300 is distance from top ring to piston face, or ring height.

So the total volume is 3.6036235714285683
and the piston volume is 3.5888923714285688
these two numbers subtracted will give us our volume around the piston. Which looks like this:

3.6036235714285683 - 3.5888923714285688 = .0147311999999995 <This is the “ring” that is left over, or clearance volume as we will call it. You can add this to our list.

STEP 6

Calculating piston face volume. Calculating piston face volume, or piston volume is easy. Piston volume will be 0 for a flat top piston. Most manufacturers will tell you what piston volume they are or how many cc's. We will use the same method as we did for our chamber. Like so:

1cc=0.061023744095 cubic inch

My pistons have 4cc relief’s(shown by the purple area in the first diagram), so it would be calculated like this:

4cc x 0.061023744095 = 0.244095 cubic inches, add this number to our list. and that leads us to our last step for BDC calculating.

STEP 7:

The last step to finding total BDC cylinder volume.

Adding volumes. After we have calculated all of our volumes, we simply add them to get a total volume for BDC. Now some of you may be thinking, “why didn’t we just take the 346ci and use that? The reason is, that’s the displacement that the pistons will displace, not the total amount of space. Our number is 346ci plus the volume of gaskets heads ect. If you do not recall, these are the numbers that we calculated:

03.600400901605 < combustion chamber volume(pink area)
00.4804831428571425 < head gasket volume(red area)
43.27673294999995 < cylinder volume(orange area)
00.0147311999999995 < clearance "ring" around piston(orange also)
00.244095 < piston volume(purple area in first diagram)


We simply add up all of these numbers and it will give us the total amount of "space” inside each cylinder at BDC. And we get:

47.616443194462092 <This number is 1 of 2 key numbers. now we will get our second number in part 2.

[bobsmyuncle]

PART 2: Finding TDC cylinder volume

TDC volume is how much "space" is remaining when your piston is at the top of its stroke or TDC. We will be using a lot from what we learned in part 1. Part 2 we will need to know 4 things, or 4 steps. Most of which has already been done in part 1. Here is a diagram to show you what goes on when a piston is at TDC. So here we go:




Pink - combustion chamber
Red - head gasket bore volume
Yellow - the area that the piston protrudes into the head gasket volume
Blue - crankcase

STEP 1:

We need to find the volume of our combustion chamber in cubic inches(pink area shown). If you don’t recall how, refer to step 1 of part 1. Our volume from part 1 was 59cc or 03.600400901605 cubic inches. Put this number in a second list. This will be your first number of the second list.

STEP 2:

We need to find how much volume is above the face of the piston(red area above face, not beside or around) Now if you remember our gasket thickness is .040 or 40 thou. and our deck clearance is .010 or 10 thou. This means that at TDC the piston is protruding into the volume of the head gasket(yellow area). The easiest way to do this is to take the head gasket thickness and subtract the deck clearance. Like this:

.040 - .010 = .030

That means for this calculation, we will be treating the head gasket as if it were .030 or 30 thou thick. And you calculation is the same as prior. A cylinder volume formula that should look like this:

3.142 x R^2 x H

Pi, Radius squared and height

so my calculations would look like this:

3.142 x 1.955^2 x .030 = 0.36036235714285686

Pi, radius of gasket bore squared, height or thickness, Add this number to the second list.

STEP 3:

Calculating clearance volume around the piston. As in part 1, we need to know the volume around the outside of the piston(red area beside or around piston), below the face, and above the top ring, if you don’t recall how, refer to part 1. The clearance volume “ring” is:

.0147311999999995 Add this to our second list

STEP 4:

Calculating piston face volume, or piston volume. As stated in part 1 most manufactures’ will tell you this number but we need to convert it from cc's to cubic inches, like so:

1cc=0.061023744095 cubic inch

my pistons have 4cc reliefs(valve reliefs not shown), so it would be calculated like this:

4cc x 0.061023744095 = 0.244095 cubic inches, Add this number to our second list as well.

STEP 5:

Our last step to part 2. Adding volumes. There are 4 different volumes we need to add together to get our total cylinder volume at TDC. and they are as follows:

3.600400901605 < combustion chamber volume
0.36036235714285686 < theoretical head gasket volume
0.0147311999999995 < piston clearance "ring" volume
0.244095 < piston face volume

Add these up and you should get this:

4.21958945874785636 <this is our total TDC volume, now for our last and final step.

[bobsmyuncle]

PART 3:

Part 3 is simple but yet it is the step that yields the most results, our final static compression ratio. Before I show how to get the final result id like to explain how it works. Just for explanation purposes, say we have a cylinder that is 9 cubic inches, and a chamber volume of 1 cubic inch, and 0 deck clearance(piston is flat with deck at TDC). at BDC the cylinder will have 10 cubic inches of total volume(9 in the cylinder and 1 in the chamber), and at TDC it will have 1 cubic inch(just chamber). There-for it will have 10:1 compression. Because it takes the 10 cubic inches and "smashes" it if you will, into the 1 cubic inch. the easiest way to figure this final number is to simply divide the BDC total volume by the TDC volume. As follows:

If you remember correctly, our BDC volume is the first number, and our TDC is our second.

47.616443194462092 divided by 4.21958945874785636 and you should get this:

11.284615164573866176111056984825

Now we can round this to an easier 11.29 or 11.30, This is our static compression ratio.

11.30:1

So in the entire 1st 2nd and 3rd parts, we only used 2 formulas, one converting cc’s to cubic inches, and one to find volume of a cylinder. And you can find both of these online. They have calculators you can type in the dimensions and they calculate volume and convert units automatically!!!

Hope this helps a lot of you, it really is easy once you do it once or twice. And although they have calculators online, it is hard to find one that is accurate. But this is a sure-fire way to do it the correct way. Thanks for reading.

[mark21742]

good write up! also if anyone wants just about any resetup calculations, http://www.wallaceracing.com/Calculators.htm has LOTS of info and plug in your number figures