Recent update, added new equation at bottom of page sent to me by Mitch Stewart. Allows you to figure out CC's of the combustion chamber if you know Compression Ratio, (say it isn't published like many of the L6 cylinder heads). I also included some links to Compression Ratio java pages. They allow you to input your values and figure out your compression ratio. I included a unit conversion site because it only allows units of inches.
I have had a lot of people ask me how to compute compression ratio. I have personally heard lots of different ways, but many don't accurately predict the static compression,(the dynamic compression depends on the camshaft, engine configuration, etc... more than I can explain). Anyways, The following will yield an accurate compression ratio, and take into account all your variables.
I use the following equation to compute the compression ratio, or CR:
I then divide that number by the following:
Let's start with a stock 280ZX,(I am choosing the ZX because it came with flat top pistons, which will simplify the calculations, for the dish size varied in the run of the Z's). It has a piston diameter of 86 mm or 8.6 cm. The radius is 4.3 cm. The stroke of a L28 is 7.9 cm, which will yield a swept area of (Pi x 4.3^2) x 7.9 = 458.9 cc. Multiplying that by the number of cylinders would yield the total displacement of (458.9 x 6) = 2753 cc.
Moving on, the next step would be the gasket thickness volume. This is the one area many people fail to consider, and consequently, end up with an artificially high CR. You must know the compressed thickness of the head gasket, and the diameter of the gasket opening. For a stock L6 gasket, the compressed thickness is 1.2 mm. The opening, or bore, is 89 mm. For reference, the 1 mm and 2 mm HKS metal gaskets do not compress, so that is the thickness for computations, and they have a 91 mm bore, which will further change the calculation.
So back to the stock gasket. With a diameter of 89 mm, which is a radius of 4.45 cm, we compute the area, (Pi x 4.45^2) = 62.2 cc. That must now be multiplied by the thickness of the gasket, which is 1.2 mm or .12 cm. 62.2 x .12 = 7.47 cc.
And now for the last part, the chamber volume. To be 100% accurate, you should have your builder measure each chamber with a dropper, for there are manufacturing variances, which will change each head slightly. But for everything but the fanatic, the factory specs are more than adequate. The Texas Z Car club head page has the chamber volumes of each head listed. Jump to their page and to their About The Z-Car page and then the L28 Bore & Stroke Combinations to find the measurement of your head. For the stock 280ZX computation, I will use the P-79 head, which has a chamber volume of 53.6 cc,(incidentally, the P-90 and the P-90A have the same chamber in both design and size).
Now, adding all these together, will yield the total start volume:
458.9 cc + 7.47 cc + 53.6 cc = 519.97 cc
Now, we must compute the final or compressed volume, which, is, again:
We already know the gasket volume, which is 7.47 cc, and the head volume, 53.6 cc. Since I am using a later ZX motor, which has flat top pistons, there is no dish volume to take into account. It should also be noted that I am assuming zero deck height, or that the top of the piston is flush with the top of the block. If you deck,(plane, make flat, lap, machine, etc...), the block, you reduce the overall block height, and the piston may stick over the top of the block, which would reduce your gasket volume. Or, conversely, you could have a piston/rod combo where the top of the piston never reached the top of the block, which would add volume to your final volume. But as I said before, I am assuming zero deck height, to simplify the calculations.
Which brings us to the final volume, using the equation above:
With these two figures, we can now calculate the CR. We take the total volume, and divide it by the compressed volume, as follows:
So the compression ratio for a 280ZX non-turbo with the P-79 head is 8.5 to 1.
Now for some fun. Lets look at what changing the stock gasket for the HKS gaskets will do. Here are the gasket volumes for the HKS metal head gaskets:
So plugging those figures into the previous equations, yields the following:
And now dividing by each other:
And for the 2 mm HKS, it yields:
As you can see, the 2 mm significantly drops the CR, which is advantageous on Forced induction motors, or when using early heads with larger displacement motors.
Increasing either the bore or stroke will Increase your CR. You are increasing your start volume, but keeping the end volume the same,(we are assuming, for it will change with different stroke cranks, different length rods...), and are compressing more air.
Now, what if you have a motor which you cannot find the CC's of the combustion chamber? Sure, most of the Datsun L6 engines have them published on the web, but what if you are working on an L4, or some other model? Mitch Stewart sent me an equation to find out CC's if you know the compression ratio and Displacement. Sure, you could work through the equations above, but his equation makes it much eaiser.TPD = Total Piston Displacement (one piston - piston area x stroke) CR = Compression ratio cc's = Cubic centimeters of cylinder head PD = Piston dish (if necessary)
Formula : (TPD / CR-1) - gasket thickness -(PD) = cc's
I'll use solid numbers for piston displacement and compression ratio (from an 83 280zx) CR= sweep volume + gasket volume + head cc's/[gasket+head cc's (+piston dish)] since gasket+head cc's=gasket+head cc's we can call that "y".
So, y + sweep / y = compression ratio then we simplify for "y". multiplying both sides by y:
We get 8.5 x y = y + 458.9 cc's subtracting y from both sides we get (y x 8.5) - y = 458.9 cc's simplifing 8.5y - y = 458.9 cc's 7.5y = 458.9 cc's y = 61.186 cc's compensating for gasket thickness of 7.465 cc's y = 61.186cc's - 7.465cc's:
y = 53.72 cc's which agrees with the P-79 and P-90 combustion chamber published figures.
This equation would be more useful for an engine which you know the factory published compression ratio, but can't find the published CC of the combustion chamber. Or, it will allow you to change the CC's of your chamber to see what effect it will have on CR.
Here is a Java page which you can calculate your compression ratio immediately. First you will need to convert our metric measurements to English. Online Conversion will allow you to immediately convert about anything.
And the Falcon 6 page has a good compression calculator. It also has a theoretical horsepower - remember that is for the Ford inline 6 motor from the 60's, not all 6 cylinders. Compression calculator
I hope this helps. If you see any gross, or even not so gross errors, please let me know.
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