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Old 11-28-2011, 11:48 AM
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Static compression ratio is found by measuring the volumes of all components involved, converting some of them to cc's and doing the math. You will see 2 different constants throughout my math, (.7854) 25% of pi and (16.387) number of cc's in a cubic inch.

You will need 5 values for the math:

1. Volume of one cylinder in cc's. Measure the bore and the stroke of one cylinder. Let's say it is a 350 Chevy block that you have bored +0.030" (cylinder bore measures 4.030" diameter with your dial caliper) and the stroke is stock at 3.480". Begin by multiplying .7854 times 4.03 times 4.03 times 3.48 times 16.387 and find 727.41 cc's in the cylinder.

2. Chamber volume. You have a set of early production heads, #882. You set up your burette and pour them, finding an average of 78 cc's in the combustion chambers. You are so excited about using the burette that you also cc the intake and exhaust ports.

3. Piston Deck Height Volume: This can be found by first measuring the block deck height (the distance from the centerline of the main bearing bore to the flat surface of the block deck where the heads bolt on). You can do this yourself with a 12" dial caliper or you can have the machine shop that will do your machining to do it for you. The nominal block deck height of a 350 Chevy block is ~9.025". Let's say that this block measures 9.015", indicating that it has probably been surfaced by someone else at sometime in its past. Now, we will add up the stack of our reciprocating parts. Half the stroke will be 1.740", found by dividing the stroke by 2 (3.48 / 2 =1.740). A stock 350 will have a rod length of ~5.700" and the piston we will use will have a compression height (distance from the centerline of the wrist pin to the crown of the piston) of 1.560". Now, if we add the 3 values together (1.740, 5.700 and 1.560), we find that our reciprocating (stack) of parts measures to 9.000". Remembering that the block deck height was 9.015", we can subtract the stack from the block deck height (9.015 minus 9.000) and find that the piston will be down in the bore 0.015" from the deck of the block with the piston at top dead center. Now, we can calculate the volume by figuring it the same way we did for finding the displacement of one cylinder. (.7854 times 4.03 times 4.03 times .015 times 16.387) and find 3.13 cc's.

4. Piston crown volume. 99% of the time, we will be dealing with flat top or dished pistons on this forum, so I will use a flat top for this exercise. The manufacturer will post the volume of valve eyebrows and/or dishes, so we just have to look at the literature to find it. We'll say that the piston is a flattop that has 7 cc's of eyebrows, so there's our piston value, 7 cc's.

5. Compressed head gasket volume. We measure this the same way as we did the cylinder volume and piston deck height volume, using the gasket bore and thickness. Let's say we're going to use a 10105117 GM gasket that measures 4.000" bore and 0.028" compressed thickness
(.7854 times 4.00 times 4.00 times .028 times 16.387 = 5.76 cc's)
The reason for choosing this thickness of gasket is to set the squish properly (0.035" to 0.045") without having to cut the block decks. Personally, I would want to cut the decks a little just to make sure that each corner of the block is the same distance from the centerline of the main bearing bore. If it took, for instance, 0.015" to get it square, then the block deck height would be 9.000" and the stack would be 9.000", so the piston crown would be dead even with the block deck with the piston at top dead center and you'd want to use a little thicker gasket to set the squish (0.041" gasket would work).

Now, we will add the 5 totals together....
727.41 + 78 + 3.13 + 7 + 5.76 = 821.3 cc's
Now, we will drop out the cylinder volume and add the other 4 values....
78 + 3.13 + 7 + 5.76 = 95.89 cc's
Now, we'll divide the larger value by the smaller value....
821.3 / 95.89
and find a static compression ratio of 8.56:1 with a 0.043" squish (to arrive at this squish, we add the piston deck height of 0.015" and the gasket thickness of 0.028").

Now that you know the SCR of the motor, you are armed with at least one of the determining factors for choosing a camshaft, the relationship of SCR to the closing point of the intake lobe on the cam.
If we look at this chart that I put together, we'll find that the motor (at 8.56) will like a camshaft with around 194 degrees @ 0.050" tappet lift. This is not the only parameter you will use. Others would be engine size, intended use, torque converter stall, transmission used, rear gear used, tire and wheel size, etc. Now, you can move around on that chart by 3/4 of a point either way to tailor the cam to the purpose of the motor. But at least it will give us a starting point to find the proper Dynamic Compression Ratio.

The DCR takes into account the SCR, stroke, rod length and intake closing point on the cam to give you a DCR (an indicator of the relative strength of the motor and it's tolerance for different grades of fuel).
If we use the KB calculator...
We'll plug in our values of 8.560, 3.480, 5.700 and 36.000 and find a dynamic compression ratio of 8.039:1. This would work fine on crummy pump gas.
Here's how I got to the 36.000 figure. I looked at Crane Cams site and chose a camshaft with an intake closing point of 21 degrees after bottom dead center @0.050" tappet lift. You'll see the figures at the bottom of the timing card....I like to use Crane because the intake closing point @0.050 is easy to find....
You'll notice the instructions of the KB calculator that tell you to add 15 degrees to the intake closing @ 0.050 figure, so if we add 21 and 15, we will enter 36.000 on the calculator.

Now, armed with this information, anyone should be able to find the SCR and DCR of any motor combination that uses flat top or dished pistons. To figure SCR with a domed piston, ask me how. I'll expand on this post a little and make a new wiki article from it.
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