How to calculate Engine Compression

Engine Heads ready for volume measurement

Why do you need to Calculate Engine Compression?

Any time that you make a change to an engine by machining the crankcase or machining the cylinder heads you change the compression relationships. If you change the relationship, you need to calculate engine compression.

Today with the limited availability of original pistons and cylinders the use of aftermarket pistons is becoming more normal. If you are working on an older engine it will take more than a surface clean to get the components back to new. To clean up the wear you will need to machine the engine. So add machine work and use of aftermarket pistons and it cements the need for accurate compression measurements.

In the first lesson of two, we show you how and what components will need to be measured. Then in the second lesson, we will show you how to use the measurements you collected. We show you how to build your own engine calculator. You will be using either Microsoft Excel or Google Sheets, we use Excel.

The best place to build an engine is on “paper” first. Work through these lessons and it will allow you to make decisions about piston dome sizes and shim pack thickness before you begin the assembly process.

Calculating Piston Deck Height

Piston deck height, (the height of the piston above or below the head gasket surface at TDC), is a critical calculation. Changing the deck height of the piston will affect the compression ratio and chamber clearances.

There are two ways to measure piston deck heights. The first way is to perform a dummy assembly of the engine. You would then physically measure where the piston is in relationship to the sealing surface.

This method only works if you build 100% stock on the engine. Sometimes it can be difficult to measure in place. Certain types of pistons make it hard to measure the deck height when installed. This is because of the dome size and the fact that there can be no-where to get an accurate measurement. You may not even have your pistons if you are ordering a custom set. To order you will need to specify a pin height and deck height.

The second method is to calculate the deck height by measuring the individual components. This is the method we prefer. This is what the lesson is about. Once we have determined the actual sizes of the engine’s physical parts, we can apply a calculation to determine the expected deck height.

How to Measure the Components

To calculate deck height, we will need to measure the following items.

Measure the cranckcase before it is together but after the machine work.
Measuring the crankcase half

Crankcase half thickness.

You need to measure the crankcase while it is still apart. The case should be machined already. If you look closely, the machinist may stamp the amount removed on the case.

The stroke is used to calculate engine compression
Engine Stroke

Crankshaft Stroke

The crankshaft stroke can be taken from the Porsche specifications book. Or you can measure as required. Measure from top dead center, TDC, to bottom dead center, BDC.

Measure the connecting rod from plans or real life.
Measuring the connecting rod

Connecting Rod

You will be measuring the connecting rod length from center to center. Measure from the little end from the center of the rod end. measure from the center of the crankpin at the big end. Or you can also get the specifications from the Porsche tech book.

You need to measure the components to calculate piston deck height
Measuring Piston Pin Height

Piston Pin Height

Measure the piston pin height from the piston top down to the pin centerline. The best way to do this is to put in the connecting rod pin and add back half the diameter.

Barrel Height

The Barrel height from sealing surface to sealing surface. We use a flat piece of granite for a more accurate staring surface. Which ever way you measure make sure you are nice and paralle for a good mesurement.

Shim Pack

To start you will have to estimate your shim pack thickness. Shims only come in a few sizes unless you have them custom ground. Sometimes the machine shop will supply shims equal to the amount they took off the case half. It is ok to stack shims at the base of the cylinders against the case on a Porsche engine. Shims are a part you can easily change using the engine compression calculator.

Doing the calculations for Piston Deck Height

Once you have all of the measurements, you can plug the numbers into the formula. This is the formula using the numbers we measured from our engine in the video.

Piston Deck Height Formula

(Case thickness – (stroke/2) + Barrel height + Shim Thickness) – (Rod length + Piston height) = Deck Height

             (111.41 – (66/2) + 85.47 + 1.02) – (130 + 33.91) = 0.99 mm

More to measure

Now you have the Piston deck height you need to measure the piston dome volume and chamber volume. So work along with this part of the video lesson. You will need to open your own Excel page or Google Spreadsheet. This will be the beginning of your engine calculator, so do not miss this step. After calculating deck height we still need to measure the volumes of the piston dome and the chamber volume.

Using fluid is the best way to measure piston dome volume
Measuring the piston dome volume

Measuring Piston Dome Volume

The dome design on a piston is usually an irregular shape. That makes it hard to estimate its size. With cut-outs for valve pockets and different dome designs the only way to get an accurate volume is to measure it using fluid.

The method that we are going to use to measure the piston dome is called the “down fill method”.

Using the down fill method

The Down Fill Method is done by installing the piston into the cylinder. You then set the piston at a certain depth below the top of the cylinder. Because the cylinder is a known diameter, we can calculate what the volume should be based on the depth of the piston. You then fill the cylinder and subtract the actual measurement from what it should be. The difference between the two is the actual dome volume.

Setting up the Piston and Cylinder

First, install the piston with at least the top piston ring installed into the bottom of the cylinder. You need to leave the piston at the bottom of the bore at first so that you can spread a small amount of grease around the bore. The grease is going to seal the piston to the bore so that you do not lose fluid while filling the cylinder. After applying the grease push the piston up and set it at a height low enough so that the dome is covered but not so low that takes more fluid to fill than the burette tube can hold.

Calculating Dome Height

Make sure that the height you set the piston is measured to the piston compression height and not the dome height. To find the dome height we would take our piston compression height number that we found earlier when calculating piston deck height. That number is taken from the center of the wrist pin to the top of the piston below the dome. Then measure from the center of the wrist pin to the top of the piston dome. Subtract the compression height from the overall height and the difference will be the dome height.

Calculating dome hieght to calculate engine compression
Dome height

Dome height = Overall height – Compression height

Now that you have the dome height you can set the down fill height. We are using a down fill height of 20 mm, to find the installed height I would subtract my dome height from my installed height and the difference will be the height from the top of the barrel to the top of the piston dome.

Installed Measured Height = Down Fill Distance – Dome Height

Now push the piston up to the correct measured height. Wipe out any excess grease that may be above the compression height. Do not leave excess grease in the chamber it will change the dome value making an inaccurate reading. Install the CC plate onto the barrel top. Making sure to seal it to the barrel using a light coating of grease.

Filling the cylinder with fluid

Fill and zero the burette tube. Make sure to use a fluid similar to “Brake Clean” or “Rubbing Alcohol”, these types of fluids have a high evaporation rate and will not leave any residuals on the part being measured. Using a fluid similar to ATF oil is not advisable as it is too thick and can result in trapped air bubbles in the fluid. It is also much harder to clean up after measuring. Do not use water. Using water can result in corrosion to the cylinder bore and piston rings.

Place the cylinder on a slight angle and fill the cylinder to the top. As you are filling, make sure that all the air is removed from the chamber. Read off and record the amount of fluid required to fill the cylinder. We will use the actual measured number in our spreadsheet to calculate the piston dome volume.

Measuring Combustion Chamber Volume

Filling the combustion chamber to measure volume to calculate engine compression
Use a plate when filling the combustion chamber

To be able to calculate the engine compression ratio we need to measure the cylinder head chamber volume. Because the chamber is an irregular shape it would be extremely hard to calculate the actual volume mathematically. The only way to get an accurate measurement is to fill the chamber with fluid and measure the amount of fluid required to completely fill it.

Differences in Cylinder Heads

An early 2.0 Liter cylinder head on Porsche is different from a later 2.7 Liter cylinder head. When measuring the 1965-69 2.0L cylinder heads it is important that the sealing plate is set at the correct height. This is because on the 2.0L engines the barrel sits into the cylinder head. If you measure the combustion chamber from the sealing surface down you will get an incorrect calculation. You need to make a plate that sets into the combustion chamber.

Setting up the Combustion Chamber

Do not overfill the chamber. If you just filled the chamber with fluid and no plate, surface tension would make it hold more fluid than actual real volume. For this reason, you need to use a piece of plastic to seal off the chamber.

To measure the chamber volume, you must first install the spark plug. It is preferable to use the same spark plug that you plan on using in the engine. Then using a small amount of light grease seal the cover plate to the top of the cylinder head. Set the cylinder head on a slight angle on the measuring table. The slight angle will allow the air to come out of the fill hole.

Filling the Combustion Chamber

Just like the last time you filled the barrel, fill the burette tube then set it to zero. To do this over-fill the tube slightly. The level can be brought down to zero by draining some fluid out. Make sure to use a fluid similar to “Brake Clean” or “Rubbing Alcohol”, these types of fluids have a high evaporation rate and will not leave any residuals on the part being measured. Using a fluid similar to ATF oil is not advisable as it is too thick and can result in trapped air bubbles in the fluid. Do not use water. Using water can result in corrosion to the engine valves, seats, and chamber.

Position the tube over the fill hole and begin filling the chamber. When filling the chamber make sure that no trapped air bubbles remain in the chamber. Make sure that the fluid level stops at the bottom of the fill hole and not the top. Record the amount of fluid used to fill the chamber.

Ready for Lesson Two

Now you have all of the measurements to move onto lesson two. This lesson is all on Excel. Work along with the video, pause where you need to. You should open the Deck Height Calculator from lesson one. This will be the base to start your complete engine calculator.

Calculating Engine Compression, The Math

Calculating Engine Compression Ratios

The best way to keep track of all the calculations required to get the compression ratio is by creating a sheet in Microsoft Excel or by using google sheets. By using either one of these programs you input all your measurements and keep track of your important engine data.

By using excel you can also save the sheet as a template. This allows you to keep track of multiple engines. By simply saving the sheet under a new name, that of your customer, or by the engine number, you will have a record of their build. Keep track of your engine compression builds whichever way is best for you.

The formulas you will need to know are

  • Deck Height = (Case thickness- (Stroke/2) + Barrel Height + Shim Thickness)-(Rod length + Piston Pin Height)
  • Dome Volume = (bore² x down fill distance x pi/4) – measured volume
  • Compression Ratio = (cylinder volume + deck height volume + chamber volume)/ (chamber volume + deck height volume -dome volume)
  • Swept Volume = (bore² x stroke x pi/4)
  • Deck Height Volume = (bore² x Deck Height x pi/4)
  • Crevice Volume = (bore diameter-piston diameter) x bore circumference x top ring land
  • Compression Ratio with crevice volume = (cylinder volume + deck height volume+ chamber volume) / (chamber volume + deck height volume +crevice volume – dome volume)
  • Displacement = (number of cylinders x swept Volume)
  • Millimeters to Centimeters = millimeters ÷ 10
  • Pi (π) = 3.1415

When calculating Cubic Centimeters, you need to convert from millimeters to centimeters. To convert mm to CC’s just divide by 10.

Conclusions to Calculating Engine Compression

The Air-Cooled Porsche engine is getting old. The last air-cooled in the 993 stopped production in the mid-’90s. That makes the young engines 30 years old or so and the early motors are 50 years old. Because of their age, not many of these engines are un-touched. And even if the engine has sat untouched in storage, it will also need a re-build. In conclusion, this means that many engines will be needing machine work. And non-stock parts are often all that is available for older motors. Two good reasons to calculate engine compression!

If you know your engine compression numbers you can make better decisions before you re-assemble your engine. Having the correct compression is essential to a quality build. Getting the engine compression you want is possible if you take the measurements and do the work. Remember the best way to learn a new skill after you have seen it explained, is to go out and do it.

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3 Responses

  1. This process seems like the most accurate way to calculate Comp. Ratio. In the four cam manual it shows the technician filling the combustion chamber (head bolted to cyl. with piston installed at tdc) for a cc measurement. (It says to minus 2cc for the missing plug) Would you consider this an accurate enough measurement for clearance volume?

    1. Daniel,
      yes, I’m familiar with the technique and the engines. The issue is when you try to CC an assembled engine, is getting the spark plug hole to be the highest point. Usually what will happen is you will end up with an air pocket in the chamber somewhere that will throw out your compression calculations. Another issue is where you stop the flow at the base of the spark plug hole or at some point up the spark plug threads. Changes in the stopping point will change the calculation number. It is also hard to judge a repeatable stopping point every time. Another point to keep in mind is the piston rings will allow seepage that will also create a miss measurement. As a general rule, I would never use this type of measurement for anything I would consider accurate or use it for a basis to make an engine calculation on.
      Now if you are using this to check piston to head clearance? then NO the only way to do that is by accurate individual component measurement or the rough and ready way using plasticine to measure chamber to piston clearance.
      Kurt

      1. Thanks a lot for the information Kurt! great stuff. Really appreciate your replies and all the time you have spent on this website. I have been using the method I described for calculating compression ratio. Always feared a possible unseen air bubble.. I’ll be using your process next time, the spreadsheet is just too cool.

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