How to measure your Uncorrected Compression Ratio

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How to Measure
Your Uncorrected Compression Ratio...

       [SIZE=+1]There are basically two commonly   utilized methods of stating a given engine's compression ratio:[/SIZE]
   [SIZE=+1]The "Uncorrected   Method"[/SIZE][/B] (sometimes referred   to as the Geometric or European method) which compares the volume   above the piston at Bottom Dead Center (BDC) to the volume above   the piston at exact Top Dead Center (TDC).  This method   is often criticized because it does not reflect the dynamics   that occur during the engine's actual running conditions, but,   as with steady state flow techniques used on a flow bench (which   also do not duplicate actual running circumstances) it has a   very useful place in the planning of an engine's tuning and application.
   The "Corrected Method" (sometimes referred to as the Trapped or Japanese   method) which compares the volume above the piston at the point   on the upstroke that the exhaust port roof is fully closed (on   a two stroke, exhaust valve closed on a four stroke) to the volume   above the piston at exact Top Dead Center (TDC).  This at   first seems to be the most sensible way of looking at the situation   since how could we really begin compressing fuel/air mixture   before all "leaks" are shut off, right?  Well,   not really...
         At elevated   engine speeds (rpm), the piston is moving so quickly that it   will actually "outrun" the fuel/air mixture to the   "leak" and "trap" a much larger volume of   fuel/air in the upper cylinder than just the static volume above   the exhaust port.  This "trapping efficiency"   improves with more rpm's.  Our engine steadily improves   with regard to how much of the fuel/air mixture that has been   ingested into the motor actually remains in the upper cylinder   area after exhaust port closing and doesn't get lost out the   exhaust port beforehand.  Thus, as engine speed increases,   dynamic trapping efficiency improves.  So, under actual   running conditions, our true compression ratio dynamically increases   with rpm!  It is rare to approach 100% efficiency at any   rpm in a "stock" motor, but with port alterations and   a well designed exhaust system that creates a "suction"   (or scavenging) pulse to assist in thorough evacuation of exhaust   gases AND negative pressure to pull extra fuel/air mixture up   through the transfer ports........ then returns a "stuffing"   (or positive) pulse just before exhaust port closing to reduce   fuel/air losses, over a narrow range of rpm operation we can   actually EXCEED 100% trapping efficiency!  This means that   your 125 cc motor over a narrowly defined "powerband"   can actually trap more than 125 cc's of fuel/air in the upper   cylinder and then "squeeze" it into the much smaller   volume above the piston just prior to ignition.  The problem   here is that this requires intake and exhaust system pulse timing   that only works over a narrow range of engine speeds.  At   other engine speeds outside the "powerband", the pulses   in the intake and exhaust systems are out of phase and will actually   contribute to a loss of trapping efficiency.  In stock motors,   the intake and exhaust system pulses are broader and thus are   effective over a wider range of engine speeds making the motor   more flexible and user friendly...... the cost is less trapping   efficiency overall and less peak power output.
         Now, knowing   what the real happenings are when the engine is in its' "powerband",   maybe you can begin to see that what REALLY matters when considering   compression ratio is:
   

• How big is the engine (swept volume     by the piston in the cylinder from BDC to TDC)?
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• What is the remaining volume at     TDC above the piston that (whatever "trapped" percentage     of) the cylinder volume will be squeezed into?
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• What kind of dynamic trapping efficiency     is anticipated given the engine's state of tune?  (The range     here can run from as low as 75% or so to 110% or a bit higher     in a sharply tuned rig.)
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• How large is the bore?  Larger     bore sizes tend to be less efficient as far as filling/trapping     of fresh fuel/air mixture and scavenging of residual exhaust     gases from the last combustion event.  Due to these facts,     they also tend to have much greater difficulty controlling the     combustion process without detonation and/or pre-ignition problems.      Mainly for these reasons, compression ratios cannot be     typically pushed as high in larger bore motors without risking     problems or having to take extra measures to ensure acceptable     reliability.  (Have you noticed that truly high rpm racing     motors tend to spread their total engine displacement out across     several smaller cylinders with short crankshaft strokes?  The     small bores are easier to keep efficient as far as filling/scavenging     and detonation control and the short stroke permits very high     engine rpm with lesser piston speeds than a comparably sized     longer stroke motor operated at the same RPM.)
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• What is the octane level of the     fuel that the engine will be fed?  Higher octane fuels and     exotic fuels such as methanol have a much higher resistance to     "pressure induced spontaneous combustion" (detonation)     meaning that they can withstand higher compression ratios and     still wait for spark from the spark plug to ignite their fuel/air     mixture rather than "detonating" on their own.       If you're going to utilize a strict diet of high octane fuel,     you can plan for a suitably higher compression ratio.
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• Note that in a two stroke engine,     compression ratio selection will have a large impact on the generated     squish velocity and must also be suitably planned.
  

    So, by taking   each of these items into consideration, limitations should start   to become obvious when using the "corrected" method   of compression ratio calculation.......
        For example,   you can raise the exhaust port roof in a two stroke cylinder   and find that without touching anything else, if you use the   corrected method of compression ratio calculation your ratio   has dropped (due to less cylinder volume above the now higher   exhaust port roof). But has your engine really gotten smaller?   Of course not! And at some engine speed, trapping efficiency   will again rise. And if the exhaust port raising was a good idea   and proves to work, at a higher engine speed than before your   trapping efficiency might even be better such that dynamically   your engine traps MORE fuel/air mixture! In other words, your   "state of tune" port alteration has RAISED your dynamic   compression ratio at some now "higher than before"   engine speed.
        Tuners who   raise their "corrected" compression ratio every time   they raise their exhaust port can run into uncontrollable detonation   at some point in their endeavor! A 9.5:1 "corrected"   compression ratio may work just fine for a motor with an exhaust   port closing at 90° BTDC and running at say 85% trapping   efficiency at 9000 rpm, but could mean big trouble if it is duplicated   with an exhaust port closing at only 75° BTDC and 115% trapping   efficiency at 11,500 rpm. When the engine comes into its' "powerband"   and starts trapping fuel/air dynamically efficiently the 9.5:1   ratio may be way too high due to MORE than 100% trapping.
        Are you still   there? OK, so what the hell do we do? We look at the total cylinder   displacement (volume above the piston at BDC) and compare it   with the volume above the piston at TDC. Then we have a fairly   consistent "baseline" to compare engines of similar   size and state of tune......... apples to apples. We still have   to consider trapping/scavenging efficiency, bore size, rpm and   fuel octane to be used, but it gives us a much more consistent   reference value that proves to be more real world enlightening.   As a footnote, "corrected" compression ratio calculation   has its' usefulness, too... in planning squish velocity, for   example.
         Mild (stock)   motors tend to operate quite happily at moderate rpm's on pump   gas with "uncorrected" compression ratios typically   in the 10:1 to 11.5:1 range or even a bit higher in some cases.   Medium hot rods using 100 octane or so and bore sizes that are   sub-70 mm can frequently tolerate as high as 13.5:1 "uncorrected".   Dragsters used for short bursts on 110 octane or better with   well designed combustion chambers to discourage detonation can   tolerate 15.5 or 16:1 and sometimes higher. Methanol motors and   those using a blend of methanol and nitromethane can tolerate   16:1 and up (especially in smaller bore sizes).......
       

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How to calculate?   Quite simply, it is (volume of cylinder at BDC + volume of combustion   chamber at TDC) divided by (volume of combustion chamber at TDC).       The volume   of the cylinder is easy....... (radius of bore in millimeters)   X (radius of bore in millimeters) X (3.14159) X (stroke in millimeters).   Then divide your answer by 1000 to get the cylinder volume in   cc's.
        The volume   of the combustion chamber at TDC is not a simple cylindrical   shape so its' calculation is not so direct. One way is to remove   the head and coat the upper cylinder area with a thin layer of   high quality (CLEAN) grease. Then rotate the motor by hand to   EXACT top dead center (use a solidly mounted dial indicator)   and wipe off ALL the excess grease. This will leave a thin coating   between the cylinder wall and the uppermost ring creating a leakproof   seal. Reinstall the head and level the engine referencing the   spark plug gasket surface...... DO NOT rotate the motor from   exact TDC!! Fill the combustion chamber with Marvel Mystery oil   (CLEAN!) from a graduated burette (available through medical   supply stores or notably from an outfit such as Powerhouse Performance   Products in Memphis) just up to the bottom most thread of the   spark plug hole. Note how much fluid was drained from the burette   from its' original "before filling the combustion chamber"   volume. Now use this volume in the formula described above. Presto!
        You say your   engine is disassembled and you don't want to fully assemble to   do this? Or maybe leveling the engine while referencing the angled   spark plug gasket surface is a big pain in the butt with the   engine still in the frame? You can evaluate the components of   the chamber volume at TDC individually, but it involves a bit   more work...
        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 round   and big enough to totally cover up the combustion chamber, 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   (preferably the compressed thickness from a used head 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. VERY IMPORTANT! 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. If it is a positive number, your piston "protrudes"   upward while your piston is "recessed" at places in   the crown if the number resulting is negative. 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
        I hope that   this will be of some help to those who take the time to read   it and follow it carefully. This is substantially written around   a two stroke engine, but the reader should be able to apply all   of this to a four stroke engine as well (even the port timing   can be compared to valve timing in a four stroke with respect   to efficiency and improved fuel/air mixture "trapping"   at higher RPM's). It is a very standard procedure embarked upon   with each professional engine modification plan.
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Full credit for this article goes to RICHARDS' ENGINE DEVELOPMENT
you can see the original post here ---> http://www.sacoriver.net/~red/uccr.html

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Here is another write up




       How to Calculate Uncorrected Compression       Ratio

       By High Output       [/HR]There are basically two       commonly utilized methods of stating a given engine's compression       ratio: "Corrected Method" (sometimes referred to as       the Japanese method) which compares the volume above the piston       at the point on the upstroke that the exhaust port roof is fully       closed (on a two stroke, exhaust valve closed on a four stroke)       to the volume above the piston at exact Top Dead Center (TDC).       This at first seems to be the most sensible way of looking at       the situation since how could we really begin compressing fuel/air       mixture before all "leaks" are shut off, right? Well,       not really. At elevated engine speeds (rpm), the piston is moving       so quickly that it will actually "outrun" the fuel/air       mixture to the "leak" and "trap" a much larger       volume of fuel/air in the upper cylinder than just the static       volume above the exhaust port. This "trapping efficiency"       improves with more rpm's. Our engine steadily improves with regard       to how much of the fuel/air mixture that has been ingested into       the motor actually remains in the upper cylinder area after exhaust       port closing and doesn't get lost out the exhaust port beforehand.       Thus, as engine speed increases, dynamic trapping efficiency       improves. So, under actual running conditions, our true compression       ratio dynamically increases with rpm! It is rare to approach       100% efficiency at any rpm in a "stock" motor, but       with port alterations and a well designed exhaust system that       creates a "suction" (or scavenging) pulse to assist       in thorough evacuation of exhaust gases AND negative pressure       to pull extra fuel/air mixture up through the transfer ports........       then returns a "stuffing" (or pressure) pulse just       before exhaust port closing to reduce fuel/air losses, over a       narrow range of rpm operation we can actually EXCEED 100% trapping       efficiency! This means that your 125 cc motor over a narrowly       defined "powerband" can actually trap more than 125       cc's of fuel/air in the upper cylinder and then "squeeze"       it into the much smaller volume above the piston just prior to       setting the whole mess on fire. The problem here is that this       requires intake and exhaust system pulse timing that only works       over a narrow range of engine speeds. At other engine speeds       outside the "powerband", the pulses in the intake and       exhaust systems are out of phase and will actually contribute       to a loss of trapping efficiency. In stock motors, the intake       and exhaust system pulses are broader and thus are effective       over a wider range of engine speeds making the motor more flexible       and user friendly...... the cost is less trapping efficiency       overall and less peak power output. Now, knowing what the real       happenings are when the engine is in its' "powerband",       maybe you can begin to see that what REALLY matters when considering       compression ratio is:       A) How big is the engine (volume in the       cylinder with the piston at "Bottom Dead Center" (BDC)?
       
      B) What is the remaining volume at TDC above the piston that       whatever percentage of the cylinder volume that gets "trapped"       will be squeezed into?
       
      C) What kind of dynamic trapping efficiency is anticipated given       the engine's state of tune. The range here can run from as low       as 75% or so to 110% or a bit higher in a sharply tuned rig.
       
      D) How large is the bore? Larger bore sizes tend to be less efficient       as far as filling, trapping and scavenging of residual exhaust       gases from the last combustion event. Due to these facts, they       also tend to have much greater difficulty controlling the combustion       process without detonation and/or pre-ignition problems.
      Mainly for these reasons, compression ratios cannot be typically       pushed as high in larger bore motors without risking problems.       (Have you noticed that truly high rpm racing motors tend to spread       their total engine displacement out across several smaller cylinders       with short crankshaft strokes? The small bores are easier to       keep efficient as far as scavenging and detonation control and       the short strokes permit very high engine rpm with lesser piston       speeds than a comparably sized longer stroke motor.)
       
      E) What is the octane level of the fuel that the engine will       be fed? Higher octane fuels and exotic fuels such as methanol       have a much higher resistance to "pressure induced spontaneous       combustion" meaning that they can withstand higher compression       ratios and still wait for spark from the spark plug to set them       afire rather than "detonating" on their own. If you're       going to utilize a strict diet of high octane fuel, you can plan       for a suitably higher compression ratio.
       
      So, by taking each of these items into consideration, limitations       should start to become obvious when using the "corrected"       method of compression ratio calculation....... For example, you       can raise the exhaust port roof in a two stroke cylinder and       find that without touching anything else, if you use the corrected       method of compression ratio calculation your ratio has dropped       (due to less cylinder volume above the now higher exhaust port       roof). But has your engine really gotten smaller? Of course not!       And at some engine speed, trapping efficiency will again rise.       And if the exhaust port raising was a good idea and proves to       work, at a higher engine speed than before your trapping efficiency       might even be better such that dynamically your engine traps       MORE fuel/air mixture! In other words, your "state of tune"       port alteration has RAISED your dynamic compression ratio at       some now higher than before engine speed. Tuners who raise their       "corrected" compression ratio every time they raise       their exhaust port can run into uncontrollable detonation at       some point in their endeavor! A 9.5:1 "corrected" compression       ratio may work just fine for a motor with an exhaust port closing       at 90ø BTDC and running at say 85% trapping efficiency       at 9000 rpm, but could mean big trouble if it is duplicated with       an exhaust port closing at only 75ø BTDC and 115% trapping       efficiency at 11,500 rpm. When the engine comes into its' "powerband"       and starts trapping fuel/air dynamically efficiently the 9.5:1       ratio may be way too high due to MORE than 100% trapping.

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Are you still there? OK, so what do we do? We look at the total       cylinder displacement (volume above the piston at BDC) and compare       it with the volume above the piston at TDC. Then we have a fairly       consistent "baseline" to compare engines of similar       size and state of tune......... apples to apples. We still have       to consider trapping/scavenging efficiency, bore size, rpm and       fuel octane to be used, but it gives us a much more consistent       reference value that proves to be more real world enlightening.       Mild (stock) motors tend to operate quite happily at moderate       rpm's on pump gas with "uncorrected" compression ratios       typically in the 10:1 to 11.5:1 range or even a bit higher in       some cases. Medium hot rods using 100 octane or so and bore sizes       that are sub-70 mm can frequently tolerate as high as 13.5:1       "uncorrected". Dragsters used for short bursts on 110       octane or better with well designed combustion chambers to discourage       detonation can tolerate 15.5 or 16:1 and sometimes higher. Methanol       motors and those using a blend of methanol and nitro methane       can tolerate 17:1 and up.......
     
      How to calculate? Quite simply, it is (volume of cylinder at       BDC + volume of combustion chamber at TDC) divided by (volume       of combustion chamber at TDC).
       
      The volume of the cylinder is easy....... (radius of bore in       millimeters) X (radius of bore in millimeters) X (3.14159) X       (stroke in millimeters). Then divide your answer by 1000 to get       the cylinder volume in cc's.
       
      The volume of the combustion chamber at TDC is not a simple cylindrical       shape so its' calculation is not so direct. One way is to remove       the head and coat the upper cylinder area with a thin layer of       high quality (CLEAN) grease. Then rotate the motor by hand to       EXACT top dead center and wipe off ALL the excess grease. This       will leave a thin coating between the cylinder wall and the uppermost       ring creating a leakproof seal. Reinstall the head and level       the engine referencing the spark plug gasket surface...... DO       NOT rotate the motor from exact TDC!! Fill the combustion chamber       with Marvel Mystery oil (CLEAN!) from a graduated burette (available       through medical supply stores or notably from an outfit such       as Powerhouse Products in Florida) just up to the bottom most       thread of the spark plug hole. Note how much fluid was drained       from the burette from its' original "before filling the       combustion chamber" volume. Now use this volume in the formula       described above. Presto!
       
      You say your engine is disassembled and you don't want to fully       assemble to do this? Or maybe leveling the engine while referencing       the angled spark plug gasket surface is a big pain in the butt       with the engine still in the frame? You can evaluate the components       of the chamber volume at TDC individually, but it involves a       bit more work........ and I'll need a bit more time and coffee       to tell you how! But I will, in another post, after I get Sunday's       domestic chores done to keep the little woman happy.....

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Measuring Disassembled Components       for UCCR Calculation

       By High Output       [/HR]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.