A Simplified Way to Calculate Compression Ratio

 By Jim Williams
Heart O'Dixie Region


With the large number of components for Porsche 911 engines we have available, choosing the right ones for a performance engine is not always an easy task. Many of the components (pistons/cylinders, cams, heads) will work well together to produce good results, and conversely, some combinations can be disappointing. It is best to fmd someone who has tried and documented a particular combination, but this may not always be possible. If you believe you have hit upon a workable combination, and want to verify the compression ratio before completing your engine, you will need to make the required measurements in the assembly process after the case has been assembled  with the crankshaft and rods, but before you permanently install the heads, pistons and cylinders.


For a brief review of the definition of compression  ratio (CR), refer to the drawing below. The compression ratio is defined as the ratio of the volume in the combustion chamber plus that in the cylinder with the piston at  bottom dead center (BDC), V1, to the volume in the  combustion chamber with the piston at top dead center (TDC), V2.


Compression Ratio = V1:V2


In order to calculate the compression ratio with a minimum of measurements, the piston and cylinder total volume, V1, is divided into the three volumes shown in Figure 1: V(head), V(cylinder), and V(apd).  Volume V2 is obtained by subtracting from V1 the volume swept by the piston as it moves the length of the stroke from BDC to TDC.


V2 = V1-V(swept)


The cylinder swept volume is simply calculated from the bore and stroke:


V(swept) = pi x (D/2)2 x (stroke length) 
where D = Internal cylinder diameter


To obtain the measurements, you will need access to a few items which include a burette with 100 cc capacity and some clear plexiglass covers for the cylinder head and cylinder. Also required is a depth micrometer or vernier caliper capable of measuring the depth of the piston dome below the top edge of the cylinder with the piston and cylinder mounted on the engine and the piston at BDC.  In making the measurements, the first step is to determine the volume of the cylinder head (with valves and spark plug installed), by using the burette to fill the inverted head. There should be a means of leveling  the head so that air bubbles can vent through the plexiglass cover placed over the combustion chamber. The plexiglass cover should mate with the cylinder head on  the surface where the top of the cylinder contacts the head. The plexiglass cover must fit snugly enough to the head so that the fluid used will remain contained in the combustion chamber The cover may have either a single hole which is large enough to let the air bubble escape, or two small holes, one to fill through and one to let the air bubble out. The fluid used can be most any low volatility solvent which can readily be seen in the burette and in the cylinder head through the plexiglass cover. Kerosene or mineral spirits colored with automatic transmission fluid will work. Use a sufficient layer of grease between the cover and the head to seal in the liquid, but not so much that the grease intrudes significantly into the volume being measured and displaces the measuring fluid. The volume of the fluid required to fi11 the cylinder head is the required V(head).


The second volume determined with the aid of the tt burette is the volume around the piston dome, V(apd). The setup for this measurement is to take one piston and cylinder set, and place it on the work bench under the burette with the piston inserted into the cylinder so that the top of the piston dome is exactly level with the top edge of the cylinder. If you have chosen the size of your plexiglass cover for the head carefully, you can use this same cover to cover the top of the piston/cylinder as well. The mating surface between the  cylinder wall and the top ring should be sealed with  grease so that the measuring fluid doesn't seep past it while you are adding fluid from the burette. The plexiglass cover used for the head measurement can be used to press down on the piston dome to get the dome level with the top of the cylinder, provided that it is of sufficient thickness so that it doesn't deflect enough to push the piston dome past the top of the cylinder. It is important to place both the fill hole and the air vent hole on the cover off center so that the piston dome doesn't cover either hole. Note the position of the contact point of the dome with the cover and avoid this area when drilling the hole(s) in the cover. When filling the piston/cylinder from the burette go slowly enough that you can observe that all the air leaves the area you are filling. Be sure to slow up enough at the end of the filling process so you don't over-fill. (This caution applies equally to filling the cylinder head.)


After the above volumes have been determined, there is only one additional measurement to be made. Install one of your piston/cylinder sets on the engine on one of the rods and secure the cylinder firmly to the case. You can omit the piston rings and the wrist pin clip for this measurement. Turn the crank so that this piston is at BDC. Carefully measure the top of the piston dome (the same point which contacted your plexiglass cover over the piston) below the level of the top edge of the cylinder using either a depth micrometer or a good vernier caliper. An accuracy of about 0.l mm or 0.005 in. will suffice for this measurement, so the vernier caliper will work here. Note from your previous measurement of the piston/cylinder volume where the piston dome contacted the plexiglass cover, and measure down to this point on the dome. This measurement is called H, and allows calculation of the cylinder volume, V(cylinder). The volume down to the top of the piston dome can be calculated from the formula:


V(cylinder) = pi x (D/2)2 x H (in cm)


Now you are ready to determine the compression ratio from the formula:

Compression Ratio = V1/[V1 - V(swept)]



V1 = V(head) + V(cylinder) + V(apd), and

V(swept) = pi x (D/2)2 x stroke of the crank in cm, as previously described. 


This method produces the same results as other methods requiring more extensive measurements and will provide the engine builder with confidence that the selected components meet his compression ratio requirements.