Measuring Disassembled Components for UCCR Calculation

Measuring Disassembled Components for UCCR Calculation

By High Output

To figure out the "trapped volume" at TDC with the components disassembled you will have to determine each of the following:

1) The "Flat Plated Volume" of the combustion chamber.

2) The "Head Gasket Volume".

3) The "Deck Height Volume".

4) The "Piston Crown Displacement Volume".

To check the "Flat Plated Volume" (FPV) of the combustion chamber, start by scraping the head gasket surface clean of gasket material, cleaning the combustion chamber of excess carbon deposits and the like (gently with a wire brush) and installing the normally used spark plug. Position some wood or similar supports under the head so that it is combustion chamber up on a bench with a slight tilt in one direction referencing the gasket surface (not level). Apply a narrow border of grease about 3 mm's from the edge of the combustion chamber totally encircling it on the gasket surface. Using a piece of plexiglass (should be at least 1/4" thick or more) with a 3/8" "fill" hole in it at one edge, position the hole at the "high" side of the tilted combustion chamber and press it firmly against the gasket surface smashing the grease and creating a seal. Make sure to press it firmly so the grease does not become a spacer. Now carefully fill the chamber with Marvel Mystery oil from your burette again noting the beginning reading so you'll know how much has been used to fill the chamber after your done. Write your reading down. This is your chamber's FPV.

To figure out the "Head Gasket Volume" (HGV) simply use the same formula as you used up above to figure out the cylinder volume, just substitute the radius of the head gasket ID (usually BIGGER than the bore, so measure it!) and use the thickness of the gasket as a substitute in the above equation for "stroke". Divide your answer as before by 1000 and you'll have the HGV. Write this down also.

The "Deck Height Volume" (DHV) is again calculated using the same basic formula. But you must either note the Deck Height during disassembly or put the piston temporarily back on the rod, slide the cylinder down over the piston (onto a fresh base gasket but you don't need rings) and use a couple of head or base nuts to pull it down firmly against the cases. Bring the piston up to TDC and use the depth measuring probe from a vernier or dial type caliper to determine how much below or above the top of the cylinder the piston crown edge is. Check it inline with the wrist pin so the piston will not tilt on its' wrist pin axis during measurement. In the formula, use bore size again and substitute the deck height for the stroke. If the deck height was ABOVE the cylinder at TDC, put a negative sign (-) in front of your calculated answer. If the deck height was BELOW the top of the cylinder at TDC, leave the calculated answer as is (positive). Record this number as DHV.

To determine the "Piston Crown Displacement Volume", first put the top ring only back on the piston. Make sure you have cleaned all the excess deposits from the piston crown so you will get an accurate measurement of volume. Next coat the last inch at the inside top of the cylinder with a layer of grease about 1/16" thick all the way around. Carefully compress the ring and install the piston from the bottom of the cylinder. Push the piston up the cylinder to within approximately 1/2" from the top of the bore. Make sure you don't push it so far that the top of the crown protrudes above the top of the bore. Carefully hold the piston in place and wipe all the remaining grease from the top of the piston crown with clean rag(s). The ring tension and grease will normally maintain the piston's position in the bore after you have cleaned the crown. Now use the depth probe on your dial type caliper to measure from the bore top down to the piston crown edge. Do this in three or four places around the bore and "square" the piston in the bore as required to make the distance down the bore equal all the way around the cylinder. Record this distance down the bore to the piston's crown edge. Now put a little border of grease all the way around the top of the cylinder and tilt the head gasket surface again slightly (as you did before when cc'ing the head) on a bench using blocks of wood (or whatever) to support it. Press your plexi plate firmly into the grease to create a good seal and again position the "fill hole" at the high side of your tilt. Fill the upper cylinder area completely up to the fill hole with fluid from your graduated burette once again noting the "before filling" reading. Determine how much fluid you poured into the cylinder when done and record it. NOW, do a calculation using our formula from above again. In the formula, use your cylinder bore size and substitute the distance down the cylinder your piston was positioned for the stroke measurement. The answer is the volume in the upper cylinder above your piston if the piston had a FLAT TOP. Of course, it doesn't, which is why we just cc'd the thing! Subtract from this FLAT TOP calculation your actual cc measurement you just made on your piston. The difference is your actual Piston Crown Displacement Volume (PCDV) for your piston. Record this figure.

OK, now you're ready to figure out your actual combustion chamber's "trapped volume" at exact top dead center. Calculate as follows:

Trapped Vol. = (FPV of combustion chamber) + (HGV) + (DHV) - (PCDV)

Whew! Now go back and calculate your Uncorrected Compression Ratio.......

(Cyl. Vol. + Trapped Vol.) / (Trapped Vol.) = UCCR:1

Hope that made sense. When I have some more time, I will give you a quick easy way to measure "squish clearance" with the motor assembled or to calculate it with the motor disassembled.