How to Check Compression

  1. krazykuda
    Hello folks! The subject has come up on how to check compression. This is a thread to show you how I check MY engines for compression when I build them. I find it easy and simple to do this way, so I thought that I would share it with all. :D

    I do ask, since I am writing this on Labor Day 5/26/14 and it will take more than one post to complete, that you not post any questions or comments in this thread until I finish posting and say "That's all Folks". This is to keep the thread continuous and not get side tracked with questions. Your question may be answered by the time that this is done. I can answer them later if you have any. Please bear with me....

    I will now start with a new post.... Here we go.... Go get yourself a drink and a snack...

    The first thing that you will need to do is to make a cover plate. This will cover the cylinder and allow you to pour water in it to check the volume. This plate will be useful in measuring both the volume of the head and cylinder. I suggest you make one and keep it in a special place in case you need it in the future.

    I will be using a small block for the example in this thread, as that's what I have available and most of you probably are working on one also. You can do the same thing with a big block or any other engine, but the head bolt pattern may need to be "adjusted" for them.

    This plate is to cover the whole cylinder and be able to be secured with the head bolts. I suggest using a 1/4" thick piece of plexi-glass available at any hardware store. Go to the hardware store and ask for a 5 1/2" x 5 1/2" piece of 1/4" thick plexi-glass (maybe a couple of extras in case you make a mistake and have to try over - they're not that expensive). 5" seems too small, and 6" seems to big. Let them cut it for you.

    Ok, now measure the head bolt pattern and draw a diagram to show the dimensions like this. This is my drawing that I used for mine, or you can just use mine for a cheat sheet.

    Compression 2-1.jpg

    Now lay out/mark the bolt pattern onto the plexi-glass plate. I made mine so the bottom of the plate is about even where the bottom of the head surface on the block is and let the top go where it will. I used a fine point sharpee marker as the thinner your lines are the less room for error. Then test it on your engine block to make sure that they are close to being on center with the head bolt holes. Then center punch the holes once they line up with the block hole centers.

    Compression 2-2.jpg

    Clamp the plexi-glass down so you can drill it easier and not hold it in your hand (it may catch on the drill and injure you if you try to hold it. Safety, safety...) I also recommend using a piece of scrap wood below it to keep the hole from chipping when the drill gets to the bottom. The wood backing helps support the plexi-glass and you get better holes with it. I also used a scrap piece of wood on the top, so the clamp would not mark or break the plexi-glass. Like this:

    Compression 2-3.jpg

    Start out drilling the holes with a small drill and then gradually step up. Get a drill bit set that has many steps as the plexi-glass tends to chip sometimes if you try to take off too much at one time, so try to go in gradual steps to reduce this.

    Compression 2-4.jpg

    Continue using larger and larger drills until you get to 1/2" holes. As you get to the larger drill bits, they have more tendency to chip or become off center so be careful.

    Compression 2-5.jpg

    Then test fit the plate on the engine to make sure that the bolt holes are good. I went to the hardware store to get 1/2" coarse thread bolts by 1/2" - 1" length. You don't need grade 5 or 8 as you just need to snug the bolts when you use the cover plate the cheap grade 3 will work fine and save you money. If you tighten them too much, you will break your plate - it's only plastic.

    If the bolt holes do not allow the bolts to go through (slightly off center) after drilling the 1/2" holes, find out and mark which direction you need to grind off the plate to get it to fit and use a wood burr as pictured in the picture below this one (a carbide burr will do if you already have one, but is a bit overkill). Keep taking a little off at a time until you get the plate to fit with all 4 bolts able to be started cleanly. Try not to let the plate get too sloppy and wiggle when the bolts are in it - the tighter the better.

    After the bolt pattern is set, then place the plate on the engine block and put a mark for the fill hole at the top of the cylinder. Keep in mind to adjust for drilling the hole. I used a 3/8" hole for filling the water, so take half of that diameter to mark the center of the hole (3/16") so it is just at the top edge of the cylinder when drilled as shown in the next picture here. Too small of a hole won't let the water in fast enough, and it will spill out, making your results inaccurate and no good.

    This is how your cover plate should look finished:

    Compression 2-6.jpg

    Here is a wood burr similar to what I used to open up my holes to fine tune the bolt pattern and account for any variations. This one has the pointed tip and the one I used had the flat bottom, I can't find the other one at this time. They are available at any hardware/home improvement store.

    Compression 2-7.jpg

    Congratulations! You have now completed your cover plate and are now close to begin measuring your volumes.

    Ok, now lets explain our strategy for measuring the volumes needed to calculate compression. I hope that you all were paying attention in your high school geometry class, as this is where you are going to use it.

    Compression is how much the piston squeezes or compresses the air/fuel mixture in the cylinder. It is simply the volume of the cylinder above the piston at bottom dead center (BDC) (larger) divided by the volume above the piston at top dead center (TDC) (smaller).

    Here is a drawing showing the piston at BDC & TDC.

    Compression 3-1.jpg

    This drawing has the volumes shaded. The green shaded volume is the volume above the piston at TDC, and the red shaded volume is the volume above the piston at TDC.

    Compression 3-2.jpg

    This drawing shows the volumes together. The green volume shows the swept volume (this the volume in the cylinder that the piston displaces while moving - also your engine size divided by the number of cylinders). It is accurately calculated by using the measurements for bore and stroke, which I will get into later. I am going to save the calculations until we need them towards the end of this thread. We're collecting the information that we need to use them now.

    So we have the green volume, which is the swept volume, and the red volume is what's called the clearance volume which is simply the volume above the piston at TDC (the part that's not affected by the piston travel). This is the part that we are going to get into detail now.

    Compression 3-3.jpg

    Here is a drawing showing our reference points for figuring out the clearance volume. To keep everything simple and understandable, I am going to use the case where the piston is BELOW the top deck of the block.

    So you have the top of the piston as the lowest mark (TDC), then the top of the block next (which is also the bottom of the head gasket), then the top of the head gasket (which is also the bottom of the cylinder head), and then the combustion chamber volume (which the head guy can tell you), or we will measure it ourselves since we can do it easily.

    Compression 3-4.jpg

    Now here we have the three different pieces that we are going to measure. We will then use this information at the end to calculate compression.

    The top part with the red shading is the cylinder head combustion chamber volume. The next volume below that with the blue shading is the head gasket volume. Then the green shaded part is the volume above the piston to the top face of the engine block. With our cover plate, we can measure the red and green volumes and calculate the head gasket volume at the end. Once we know these three volumes, we can calculate compression.

    Compression 3-5.jpg

    Ok, now we're almost ready to measure the volumes with our cover plates, but we need to make sure that you have a few items.

    The hardest thing to find is a graduated beaker that is marked in cc or mL. A mL (milli-Liter) is equal to a cc (cubic centimeter). Most head volumes are measured in cc. I got mine from a place that I worked at - they were going to throw it away and gave it to me. It is measured in two cc increments. I would try going to a school supply store with science supplies (high school science) or an industrial supply place like Grainger or Production Tool Supply. Mine can hold up to 250 cc, I would recommend getting one with at least 100 cc and the finer the increments the better (ie. 1 cc would be good). If someone knows where to buy these, maybe post after I'm done with the calculation portion of this thread after I post "That's all folks".

    Here is a picture of my graduated cylinder/beaker with water and blue food coloring to show the water level better. You can use food coloring if you wish, it makes it easier to see.

    Compression 4-1.jpg

    Next you will need a small funnel. I looked all over at hobby stores and hardware stores, and couldn't find any. I finally found some at Bed Bath & Beyond on the wall where they have all of the kitchen supplies for only $1.50. You want a small one with a narrow tip to fit in the fill hole of the cover plate that we made earlier.

    Here's a picture of the funnels.

    Compression 4-2.jpg

    Here's the cover plate that we made above.

    Compression 4-3.jpg

    You will also need a dial indicator set up to find exact TDC.

    Then you will need to get 4 short 1/2" coarse thread bolts approximately 3/4" length, and grade 3 is all you need as we don't want these too tight and they can get expensive, especially for the harder bolts. Also get 4 washers to fit the bolts. These will be used on the block to measure the volume above the piston at TDC.

    To measure the head volume, you will need to get bolts, nuts, and washers to hold the plate to the head. Keep in mind that you will need 3 different lengths; two of the short ones for the bottom of the head, 2 longer ones for the head bolts that go under the valve cover, and one longer one for the one thicker/higher head bolt boss position. Place the head on a flat surface and use a stick/wire to measure the depth of the holes in the head for each particular hole, then add about 1' to accomodate the 1/4" thick plate, washers, and nuts.

    So you will need a minimum of 2 short bolts, 2 medium bolts, 1 long bolt, 8 washers, and 4 nuts to measure the head volume. (All bolts and nuts to have 1/2" coarse thread).

    Oh yeah, don't forget axle grease. I like to use the colored transparent see through stuff (personal preference). Make sure that it is a fresh tub with no dirt or contamination, as that may cause leaking.

    Go gather all of the supplies and then you will be ready to start measuring. This is what we've been working for. :D

    It's time to measure the combustion chamber volume. Don't forget to install a good clean spark plug for two reasons. One - you will need to account for the volume of the spark plug in the head. Two - All of the water will leak out if you don't (you'll only make that mistake once).

    Ok. make sure that all surfaces involved are good and clean for accurate results and to prevent leakage.

    Now take the axle grease and spread a thin layer around the outside of the cylinder to help seal the plate to the surface. Just a thin layer is all you need, too much will squeeze into the cylinder and affect your volume readings. It should look like this:

    Compression 5-1.jpg

    Then bolt the plate to the bottom of the head like this. You just have to snug the bolts, not torque or you will break your plate, just tight enough to seal:

    Compression 5-2.jpg

    Have a pitcher of water ready to fill the beaker. Fill the beaker to a convenient level that will make the subtraction easier.

    Compression 5-3.jpg

    Record the level of the water in the beaker when you start. I snipped the tip of the funnel to make it shorter and easier to hold, but make sure it still can fit in the fill hole. Then slowly and carefully fill the cylinder with water until it reaches the top of the hole. (the water may not fill quickly as the surface of the combustion chamber is so close to the plate so be careful).

    Compression 5-4.jpg

    It will help to tilt the head so that the fill hole is higher than the rest of the cylinder. Fill up the cylinder with water until it looks like this. Try to stop when the water is just at the bottom of hole and not fill the hole. If you do, take a guess and subtract a half or one cc to make up for the extra if it fills the hole all the way. Look at and record how much water is left in the beaker. Then subtract the two readings to get your head volume. In this case it was 69 cc.

    Compression 5-5.jpg

    Here's a closer look of the head after filling the combustion chamber with the water. Now you can repeat this for each cylinder and compare the results (they all should be very close).

    Compression 5-6.jpg

    Congratulations! You have cc'ed your first head!

    Now we are going to check the volume above the piston at TDC.

    As before, make sure that all surfaces involved have nice clean surfaces for accuracy in measuring and sealing.

    The first thing to do is to find top dead center (TDC). Set up a dial indicator, making sure that the axis for the indicator is as close to parallel with the direction that the piston travels in the cylinder. Turn the engine over until the reading on the dail peaks and then starts to reverse. Note where that reading was and go one more complete turn in the same direction, going slow when you are coming around to the next TDC and stop when it reads the peak reading on the dial indicator that you observed.

    Here's what the set-up should look like:

    Compression 6-1.jpg

    Here's a closer view:

    Compression 6-2.jpg

    Take your finger and fill the gap around the perimeter of the piston to seal between the piston and cylinder wall above the rings. Try to make the grease even with the top edge of the piston. This will keep the water from leaking out of the rings (or minimize it to a reasonable level) while you are filling the cylinder with water. Don't worry about the volume that the grease takes up, it is so small that it won't make much difference if any in your volume measurements and compression calculations.

    Also spread some grease around the cylinder on the block head face to seal the plate to the top of the block. It should look like this when you are done:

    Compression 6-3.jpg

    Here's a closer view:

    Compression 6-4.jpg

    Now bolt the cover plate to the block with the short bolts and washers like this. Then hold the funnel and carefully fill the cylinder with water until it reaches the hole, trying to stop when the water reaches the bottom of the hole and the cylinder is completely filled with water. Try not to spill any water as it will throw off the accuracy of the measurement.

    Compression 6-5.jpg

    Here's what the cylinder looks like full. Ignore the small bubble at the top as that is what leaked out while I was recording my readings and getting the camera ready for the picture. It was filled to the bottom of the hole. As with checking the heads, record the volume of water when you start filling (try to fill it to an easy number for subtracting). Then record the volume of water left in the beaker after the cylinder is full. Subtract the two readings to get your volume above the piston up to the top face of the block.

    Repeat for as many cylinders as you wish to measure. They should be close to the same for every cylinder. The more that you measure, the better indication you will have for what your volume is.

    Our measurements here turned out to be 34 cc.

    Compression 6-6.jpg

    Ok, now we are done with the measuring part that we need for the compression calculations. These have to be measured like this as it is too difficult/impossible to predict with a formula or calculation. The next steps will involve calculating and some math.

    Now we are at the calculation stage for calculating the compression. I will start with the head gasket first, and then do the block calculations.

    If you already have a used head gasket like the one you plan to use, then make sure that it's clean enough to measure with a set of calipers, or get a new one just like you plan to use, and put the head on and torque it to spec to compress the gasket to where it will be on a built engine. The point here being, get a compressed head gasket like you plan to use.

    Now measure the thickness of the head gasket. For this we measure the thickness of the fire ring (the one that seals the cylinder) with a caliper. I have a nice digital caliper, analog will also do.

    Here's how you measure the thickness of the compressed fire ring on the gasket.

    Compression 7-1.jpg

    Then measure the diameter of the fire ring like this.

    Compression 7-2.jpg

    Now you need to use your high school geometry here to calculate the area and volume. Both the head gasket and swept volume use this formula.

    The area of a circle is calculated by multiplying Pi times the radius squared as I have drawn in the picture below. Since it is easier to measure the diameter than the radius, we measure the diameter and divide it by two to get the radius. Or using algebra, you can convert the equation to use diameter. This will convert to Pi divided by 4 times the diameter squared.

    Compression 7-3.jpg


    Let's get our symbols defined for everyone so they don't get confused.

    For multiplication, we just use an x. To write length times width, we write it like this:

    L x W

    Now to write a variable that is to be squared (the same number times itself), we use two asterisks like this:

    Diameter squared is written like this:



    I hope that I haven't lost any of you yet. Now let's calculate the volume of the head gasket.

    The head gasket above measures 4.178" diameter and is .0555" thick as measured in the pictures above.

    d = 4.178 inches
    t = .0555 inches

    Now we plug these into the equation for volume. Volume is the area of a circle times the thickness or length of the cylinder. This equation applies to any cylinder including the head gasket volume and swept volume of the piston.

    So the head gasket volume equation is this:

    V = Pi/4 x (d)**2 x t

    Now we substitute our measurements into the equation:

    V = (3.1416/4) x (4.178**2) x .0555= .7854 x 17.46 x .0555 = 0.761 Cubic INCHES

    Now we have to convert cubic inches to cubic centimeters. To do this just multiply cubic inches by 16.39

    0.761 x 16.39 = 12.5 Cubic centimeters or 12.5 cc

    So our head gasket is 12.5 cc

    Now let's calculate the volume of the cylinder. If you noticed in the pictures above my engine is .030" over bored. It is a 360 with stock stroke. So this gives us:

    Bore = 4.030"
    Stroke = 3.58 "

    Now plug into the formula:

    V = (Pi/4) x (B)**2 x S = (3.1416/4) x (4.030**2) x 3.58 = .7854 x 16.24 x 3.58 = 45.66 Cubic INCHES

    Now let's check our calculations by multiplying the volume of the cylinder by the number of cylinders in the engine.

    45.66 x 8 = 365.3 Cubic INCHES

    This seems right for a .030" over 360. So we're on the right track.

    Now we need to convert the volume of one cylinder from cubic inches to cubic centimeters.

    45.66 x 16.39 = 748.5 cc

    Ok in summary, we have:

    Head Gasket volume of 12.5 cc
    Cylinder volume of 748.5 cc

    Ok. now we have all of our numbers measured and calculated. It's time to move on to do some compression calculations.

    Ok, I hope you are all still following me and nobody has gotten lost. We're almost there.

    Now we need to calculate our compression.

    Let's do a little review to try to get everybody caught back up.

    First, we have the drawing below that shows compression. The first drawing shows the two different cylinder volumes involved. BDC & TDC. Compression is the volume in the cylinder at bottom dead center divided by the volume of the cylinder at top dead center. Easy way to remember is BIGGER divided by SMALLER.

    The volume of the cylinder at BDC is shown with the green shading in the picture on the left, and the volume of the cylinder at TDC is shown to the right.

    Compression 8-1.jpg

    This drawing shows the volumes all in one picture. This shows the two different volumes needed for calculating compression. The first volume is called the swept volume and is shown in green (this is what changes with piston travel). The other volume that we will need is the clearance volume (volume of the cylinder at TDC) that is shown in red (this volume is not altered by piston travel).

    Compression 8-2.jpg

    Since it is difficult/impossible to measure or calculate the clearance volume in one step, we broke it down into three different volumes that we are capable of measuring. These are shown in the picture below. The three different volumes are; volume of the combustion chamber in the head, the volume of the head gasket, and volume in the block above the piston to the head face of the block.

    Compression 8-3.jpg

    Now we have to calculate the clearance volume (volume of the cylinder at TDC). We have measured the volume above the piston in the block, and the combustion chamber in the head, then calculated the volumes of the head gasket. Now all we have to do is add these three volumes together to find our clearance volume. This is the volume of the cylinder that is not affected by the piston movement.

    So let's bring back the volumes that we have from the previous posts:

    Volume of the head combustion chamber = Vcc = 69 cc
    Volume of the head gasket = Vg = 12.5 cc
    Volume of the piston = Vp = 34 cc

    Compression 8-4.jpg

    To calculate the clearance volume, we just add the three volumes together:

    Clearance Volume = cv = 69 cc + 12.5 cc + 34 cc = 115.5 cc

    We calculated the swept volume in the last post:

    Swept Volume = sv = 748.5 cc

    Compression 8-5.jpg

    Now from the above drawing we see that the volume of the cylinder at BDC is the Swept Volume + Clearance Volume.

    So now we make the equation for compression. Swept volume + clearance volume divided by clearance volume:

    Comp = (cv + sv)/cv

    Or we can rewrite this as:

    Comp = (cv/cv) + (sv/cv)

    This reduces to:

    Comp = 1 + (sv/cv)

    So in our case we have

    sv = 748.5 cc
    cv = 115.5 cc

    Now we substitute these into the equation:

    Comp = 1 + (sv/cv) = 1 + (748.5/115.5) = 1 + 6.48 = 7.48

    So, as you can see, the compression in this engine is pretty low at 7.48 or we'll call it 7.5

    I would like to find out what I need to get to 9.5 compression. To do this, we rearrange the equation to solve for clearance volume as once we have decided on bore & stroke, we can't change that to tune in our compression. (this is also true with your head combustion chamber volume once the heads are done - and you can only mill the heads so much).

    Let's rearrange the formula with algebra to solve for cv:

    Comp = 1 + (sv/cv)

    Now bring the "1" over to the other side:

    Comp -1 = (sv/cv)

    Bring the clearance volume over to the other side:

    cv x (comp - 1) = sv

    Now solve for cv by dividing by (comp - 1):

    cv = sv / (comp -1)

    Now substitute in our numbers.

    Comp = 9.5
    sv = 748.5

    Now substitute them into the equation:

    cv = sv / (comp -1) = 748.5 / (9.5 - 1) = 748.5 / (8.5) = 88.1 cc

    So we will need a clearance volume of 88.1 cc to get to 9.5 compression.

    Once the bore and stroke has been determined, you cannot change that, so we will treat the swept volume as fixed.

    Since the heads are already done, and I also prefer not to keep milling my heads which may then lead to having to mill the intake to match, I like to keep my parts interchangeable with other engines/parts, so we will treat the head combustion chamber as "fixed" at 69 cc. And you can only mill them so much anyway.

    Now there are a few different head gaskets available. You can get all of your options and then calculate the head gasket volumes of them, and choose which one that you like (some gasket manufacturers may also list or be able to tell you the volume of the different gaskets). Regardless, pick a gasket. For this example, I will keep the gasket that we have been using at 12.5cc.

    Now the only variable that is left that we can tune is the compression height of the piston. We need to calculate our target volume for the particular compression that we need, and then back that into the piston compression height.

    head gasket = Vhg = 12.5 cc
    Combustion chamber = Vcc = 69 cc
    clearance volume needed for 9.5 compression = 88.1 cc

    So now lets get a formula for the volume above the piston to get to 9.5 cc:

    cv = Vhg + Vcc + Vp

    Now rearrange to find Vp:

    Vp = cv - (Vhg + Vcc) = 88.1 - (12.5 + 69) = 6.6 cc

    So to get to 9.5 compression, I would need 6.6 cc above the piston in the block.

    If I were to have custom pistons made, I would tell the manufacturer that I want 6.6 cc above the piston. Or to figure out how far down in the cylinder the piston should be (we will assume a flat top piston to make the calculations easier), we take the volume and convert it back to cubic inches and then divide by the area of the bore (that we have in inches).

    Here's how to do that:

    Since we multiply cubic inches by 16.39 to convert to cubic centimeters, we would then do the opposite and divide the cubic centimeters by 16.39 to get to cubic inches:

    6.6 cc / 16.39 = 0.3998 cubic INCHES

    Now we need to find how far down a flat top piston will need to be, we just divide by the area of the cylinder. Area of the cylinder is Pi/4 times diameter squared:

    A = (Pi/4) x (d)**2

    Our bore is 4.030":

    A = (3.1416/4) x (4.03**2) = (.7854) x 16.24 = 12.76 Inches squared

    Now divide the volume needed by the area:

    .3998 / 12.76 = 0.031"

    So for a flat top piston to achieve 9.5 compression in this engine, it will need to be .031" down in the bore at TDC.

    Or for custom pistons, you tell the manufacturer that you need to have 6.6 cc above the piston in the block including valve reliefs.

    All of the information and pictures in this thread have been written by me for the reference of members for information only and not to be published in any way shape of form, except for (FABO). Any publication for profit is prohibited and will be pursued legally.

    It is to be used for reference by FABO members to help them build better engines. FABO members have permission to print it for personal reference only to help with their engine builds.

    I have just spent almost 10 hours writing it up, not to mention the time to take all of the pictures and convert them for posting on FABO only. I do not wish for my information and hard work to be stolen.

    Use this information at your own risk. I will not be held responsible for any mishaps from misapplication of these concepts or misunderstandings.

    Thank you for your cooperation.

    Happy motoring.... :D

    Ok, I would like to thank all of you who looked at this thread while I was working on it, for not posting while I was working and interrupting the flow (185 views).

    I have checked each post twice for errors and have tried to fix them as best I can. Feel free to PM me if you see any mistakes or corrections that I need to fix.

    This is as simple as I have been able to make checking compression at home in your garages by breaking it down into easy realistic steps that can be measured by the average guy/gal and is applicable in real life (you are able to use in a practical manner), using readily available materials that you should be able to buy and use at home.

    I have spent lots of time and effort for your benefit. Please reward me by clicking on the thanks button if this has helped you, so I can see if it has helped anyone and has been worthwhile.

    Please post any information that you would like to add, especially if you know where to buy a graduated beaker/cylinder.

    Using this information will allow you to accurately measure and calculate your true static compression on your own engines without having to rely on any other information which may not be as accurate for your specific application. Don't rely on print, measure it yourself and know what you have for sure.

    I may add more information at a later date. (If I can figure out a simple way to apply this to a piston that travels above the head deck for higher compression pistons).

    Happy motoring....

    "That's All Folks..." (for now)

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