Moparts Tech Archive

Motor General

Calculating Compression By MOPAULY

This procedure was done and the pictures were taken by Member Mopauly. I want to Thank him for taking the time to put this article together.I copied and pasted this info from his page at;
http://mywebpages.comcast.net/mopauly/compression.htm with his permission.

Calculating Static Compression - using the "1/2 inch" Downfill
Volume method

Preparation for measuring the block starts with the photo above. With the engine securly mounted to a stand and the heads removed, rotate the block to level out the cylinder to be measured.

In my particular case I used a floor jack with a block of wood under the front of the engine due to the engine stand sagging forward. It will help to leave the engine slightly leaning, as trapped air will be easier to expel while filling. Once the engine is in place, bring the cylinder you are measuring up to TDC (#1 in this case). Using a piston stop here can be beneficial in finding true TDC (if not already marked on your dampner, using a degree wheel to find true TDC.).

With the piston at TDC I took a few minutes and cleaned off the face, using a rag and some WD-40. This step insures the highest accuracy in measurement, by removing any carbon that had built up on the piston.

You will notice that this is a flat top style piston, however it does sit about .020 above the deck. The slight positive dish of the piston is somewhat offset by the depth of the valve reliefs.

With the piston still at TDC, affix a dial indicator to the block, and zero the dial on the piston. Ensure that you preload the indicator enough to allow the plunger to drop the distance that you will be measuring, without the indicator bottoming out. In this case, we will be measuring 5" down...thus I left close to .75" of travel on the indicator.

Begin rotating the crankshaft to bring the piston .5" down into the cylinder. In this case it is 5 full rotations of the indicator needle, or .500 thousandths.

This photo shows the piston at .5" down into the cylinder. It helps to be gentle when approaching 0 on the indicator for the 5th rotation. If you overshoot 0, reverse rotation at least .050, then come back up to the 0 mark.

When the piston is at .5" down, remove the dial indicator or move it out of the way. If at any time you bump the indicator while performing the measurement, bring the piston back to TDC, reset the indicator, and start over; even the slightest tap can throw the
indicator way off.

The next step is to seal the piston against the cylinder wall with a thin layer of sealant. In this case I used grease, however other products, such as petroleum jelly, can be used as well. This will ensure that no fluid leaks past the piston while filling the cylinder. I used a small paint brush, dabbing in grease and working around the entire piston.

In addition, the piston can be rotated past the downfill measurement, and grease applied around the rings; then bring the piston back up to the initial downfill mark. Doing this will assist in pushing the grease between the piston and cylinder wall, resulting in a better seal.

Once you've applied the grease, use a rag and wipe up any excess. You only want enough to seal the piston; clumps left laying on the piston will give a false volume reading later on.

This is just a close up photo of the grease seal between the piston and cylinder wall. Again, try to be as 'clean' as possible here in applying the grease seal.

With the piston sealed, put a small ring of grease around the outside top of the cylinder. Place the plexi-glass on top of the cylinder, pressing down on the outside to create a seal.

Take note that you only need enough grease to seal the plexiglass; over zealous use of grease will force it into the cylinder, occupying air space, which will result in an inaccurate volume measurement.

The next step is to setup the buret. This photo shows the buret, ready and filled with liquid, positioned on the cylinder block. Syringes and graduated cylinders can also be used for filling the cylinder.

I had been looking for a syringe, but happened across this setup on Ebay. The self supporting base and clamp make it easy to use; it's 100cc capacity also allows more use between re-filling. It has a precision nylon petcock that allows control of fluid flow from a full stream to single droplets.

In this photo you can see the tip of the buret as I began filling the cylinder.
Not shown is additional holes I had drilled in the plex-glass, to aid in removing trapped air while filling.
Once the chamber is full, minus any air pockets, record the total number of cc's used to fill the chamber.

Now that we have measured the cylinder volume, we can move onto measuring the heads. The photo on the right shows the head, with the valves and spark plug sealed with grease; this is a repeat process of what was done with the piston and cylinder.

This photo below shows the head with the plexi-glass in place, and the chamber filled with fluid. Note the additional holes in the plexi-glass to allow trapped air to escape. It helps to slightly tilt the cylinder head, opposite the placement of the holes. This insures that any trapped air makes it's way up and out of the 'vents'

As with the cylinder, a ring of grease is used around the chamber to seal the plexi-glass

Here is the measurement on the buret. It shows about 67.5 cc's; since I lost a few drops, and had a very small air bubble, I corrected it to an even 67 cc.

Now that we know the 1/2" down volume, and the cylinder head volume, we can begin to calculate our actual static compression. This particular engine is a 340, with a .030 overbore. We know our bore is 4.070, and the stroke is 3.31". This engine was also using .040" (crushed) head gaskets. So, we know that
The perfect 1/2" downfill volume for this bore = 106.54 cc's The 'perfect' downfill volume is calculated as such: V = Pi x radius2 x height; so, volume equals pi (3.14) times the radius squared, times the height (radius is equal to one half of the diameter).Thus, Volume = 3.14 x (2.035 x 2.035) x .5, which = 6.50 cubic inches. Take 6.50 x 13.387 to convert to cc's, and you get 106.54. I ended up with 113 cc during the cylinder measurement. This number includes the slight positive of the piston dish, and the negative of the valve reliefs. Now we subtract 106.54 from 113, and get 6.5cc. Due to the depth of the valve reliefs, we actually end up with 6.5 additional cc's over a 'perfect' 1/2" downfill for this cylinderSo, now we have 113 cc's for the cylinder, and 67 cc's for the head. We also need to know the gasket volume, which in this case would be 8 cc's for a 340 .040" gasket. Now we can calculate the combustion chamber volume (VTDC). So, we take the head volume (67cc) + piston dish (6.5 cc), + gasket volume (8.0 cc) which equals 81.5 cc. Our next step is to calculate the piston displacement.
V = 12.87 x bore2 x stroke, where V is piston displacement in cc's
So, V = 12.87 x (4.070 x 4.070) x 3.31, which equals 705.65 cc. Now we add the combustion chamber volume (81.5 cc's) to the piston displacement, which give us 787.15 cc (VBDC
Now we have the numbers to solve for compression
VTDC = 81.5 cc

Since the Compression Ratio equals VBDC divided by VTDC, we end up with a compression ratio of 9.6:1; give or take a little for a margin of error
Now that you know the compression ratio, you can closely calculate what it would be if you changed gasket thickness or shave a few thousandths off your heads. In my particular case, I needed to establish a baseline of where the engine's CR really was

Once you've done the cc'ing you will have an idea as to how little a cc is, and how quick they add up into a major volume.

To add some more accuracy to your measurments you need to figure the amount of cc's it would take to fill up the area between the piston and cyl wall above the top piston ring. (The area where the grease was to seal the piston to cly wall.)

Measure from the top of the top piston ring to the top of the piston and figure the volume of a cyl that tall, then figure the volume of the piston top above the top ring.

Then subtract the piston volume from the cyl volume and add that amount of cc's to your compression equation.

Using the formula above; V = Pi x radius2 x height. To calulate both volumes of the cly volume between the piston and cyl wall and the volume of the piston top from above the top ring. Then subtract one from the other.

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