How to Calculate Uncorrected Compression Ratio

By High Output

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 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 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.....