There are basically two commonly utilized methods of stating a given engine's compression ratio:
The "Uncorrected Method" (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)?
- What is the remaining volume at TDC above the piston that (whatever "trapped" percentage of) the cylinder volume will be squeezed into?
- 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.)
- 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.)
- 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.
- 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 degrees 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 degrees 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).......
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.