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WOT-Tech pistons and the CR Calculator

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  • WOT-Tech pistons and the CR Calculator

    With all of the different top-end swaps and bottom-end builds out there, I was talking to Ben at WOT-Tech about the Diamond pistons and how different setups would affect the compression ratios (CR) of the pistons he was offering. I wanted to pass this piston information on to the users so that they could then determine what their final CR would be with any setup. Due to marginal sales, WOT-Tech has decided to no longer list or stock specific CR pistons anymore. What does this mean to everyone? It means you can tailor the pistons to suit your specific application, no matter what components you are using.


    So why make a site article about this instead of just posting in the forums? Well, I want to let people know how to use the site's CR Calculator to the best of its ability in order for them to be able to make decisions on components they can use. The information from the calculator can be used to order custom pistons from anyone that can make them for these engines.

    Ok, so for those of you that haven't messed with the calculator much, let's look at what the screen looks like when you first open it up.

    Click image for larger version

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    The first drop down is your engine type. This will give you a basic start with the various values required for the calculation. Next, select your overbore from the list. These are standard overbore sizes. If you have something different, you can always just change the bore as needed during the calculation. There is also notes section, but this isn't necessary for anything. If you are doing multiple calculations, you can enter a note to describe each different calculation. This will show up at the very bottom of the calculation screen. The final selection is just if you want to determine default CR (possibly with an overbore), or custom entered values, which is what we will mainly discuss here. Before we go on, though, I do want to mention that you can use the default selection to copy down values from other engines if you are working with components from various engines.

    In the image below, I selected a 3400 with a 0.020" overbore, and went with default values.

    Click image for larger version

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    Let's go through the various values to get a better understanding of what each part of the equation is.
    • Bore - This is the diameter of the cylinder. Pretty basic information. If you need to do a non-standard overbore, you would add to this number.
    • Stroke - This is the amount the piston moves up and down inside the cylinder. This is based off of the design of the crankshaft. For the most part, this is a number that is going to match the default, unless you do custom work to the crank, such as offset grinding.
    • Head Gasket Bore - This is the diameter of the head gasket around the cylinder. Again, this is fairly standard and will usually match the default, but when using gaskets, such as the MLS ones available from WOT-Tech, make sure you are using the values given.
    • Head Gasket Thickness - Same as above. One thing to remember, this is the COMPRESSED thickness of the head gasket.
    • Combustion Chamber Volume - This is the volume of the cylinder head combustion chamber. The values listed are values which we have determined to be accurate throughout the various years. This should be good for most calculations, but if you are looking to get closer to exact final numbers, you really need to measure your own combustion chambers. This is above and beyond the scope of this article, but I may look into adding an article about the procedure at some point.
    • Piston Dome Volume - This description may confuse some people, but it is the volume of the dish (positive number) or the dome (negative number) of the piston. For most of the 60V6 engines, pistons are either flat top, or dished, so the numbers will almost always be positive. But, there are some pistons which do have portions that stick up above the edge of the piston. Dished pistons can be measures the same way as the head's combustion chamber. Domed pistons require a bit of ingenuity to be measured, but it is still possible.
    • Piston Deck Clearance - This can be another confusing number. This is the height that the pistons sits in the cylinder (positive number) or protrudes the deck surface (negative number) when the piston is at Top Dead Center. As shown in the example above, the 3400 pistons actually come out of the block 0.020".

    The next few numbers are the results of the above values when used in the compression ratio equation.
    • Final Displacement - This is the final displacement (in cubic centimeters) of the engine you just entered values for. Since this calculator is specific for this site, this number always assumes a 6-cylinder engine.
    • Calculation Compression Ratio - This is the number we've really been looking for. There are really too many variables to take into account to decide what CR's are ideal, including if the engine has forced induction, tuning of the PCM required for optimum operation, etc.
    • Quench - Quench is the distance between the top of the piston and the bottom of the cylinder head. The smaller the quench, the more compression, and the more compacted the explosion of gases inside the engine. This results in higher output, as it'll cause the piston to be pushed down with more force. But there is a limit, thus the listed color band. Too small of a quench will result in potential contact of the piston with the valves, especially when the rods begin to stretch at higher RPMs. Keep out of the red, and all should be good.
    • User Notes - If you made any notes at the beginning, they will show up here. As stated above, if you do multiple calculations, and you want to keep track of them, enter a description, take some screen shots, and save them for later. Maybe someday I'll add an option to export the output as an image that you can save.

    Next, let's look at the other option, of using user selectable values.

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    This view is basically the same thing, but it lets you enter your own values for the various areas. It also shows you the default values for the engine you chose. This is where you can begin to experiment with mixing and matching components from different engines. If you want to use 3500 heads on a 3400, first calculate the default CR for a 3500, and write down the combustion chamber volume for the 3500 heads. Then, select the 3400, whatever overbore you plan on using, a note if you want, and then Use User Entered Values. Type in each corresponding value for all of the components, and when you get to the combustion chamber volume, enter the value for the 3500 heads. Finish it up, hit calculate, and see what you come up with.

    Now that I've explained all that, you are probably wondering what all of that has to do with the Diamond pistons from WOT-Tech. Well, I'll tell you. Say you want to have an engine with 10.5 compression, but you want to use parts from all different types of engines. Enter all of the known values for the various components, and just change the value for Piston Dome Volume until you get your desired CR of 10.5. Write this number down. When you go to order your pistons from WOT-Tech, you can provide this volume to Ben, and he will custom order pistons to match. As a note, when doing this, provide a list of all of the bottom-end components you plan to use for the build, and this will allow Ben to double check your calculations with his engine simulator, and make any tweaks necessary to the volume to get you exactly what you are looking for.

    While we are on the topic of the CR calculator, I also want to explain another equation that is related and may prove useful for some people. I have talked with several people who would like to try to use pistons from other engines in their 60V6. I, too, have looked into this in the past. But how do you know what the results will be? When looking at pistons online, such as from Summit or Jegs, you will usually see one of the specifications listed as compression height. Besides matching the bore, the compression height is needed to determine the CR. But how is this number used? Here is an equation:

    Piston Deck Clearance = Block Height - 1/2 * Stroke - Rod Length - Compression Height

    The values for block height and rod length can be found in the specifications for the various engines, in their respective sections. For a 3400, the block height is 8.818", and the rod length is 5.7". The stroke is listed in the CR Calculator, and the compression height you got from the piston listing. While I was chatting with someone a while ago, he found a piston with a compression height of 1.090", and was asking me if it would work or not. So let's run it through the equation and see!

    Piston Deck Clearance = 8.818" - (0.5)(3.31") - 5.7" - 1.090" = 0.373"

    Looking back at the definition of Piston Deck Clearance, that means the piston will sit 0.373" below the surface of the block. This will result in a CR of around 5.3, which is obviously less than ideal. This equation gives you the option to explore other options out there (albeit very limited options).
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