Tips for Measuring Compression to Understand Engine Fundamentals

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The condition of the engine can be known by measuring the engine compression pressure just the same with the blood pressure measurement that gives a clue to know the physical condition. There's a problem somewhere in the engine if the compression is too low or too high just like human blood pressure.

The engine compression can be easily measured with a special gauge. So, if you have any concerns about your motorcycle, it will be easier to schedule the next maintenance or overhaul by having it measured regularly even if there is no problem at present.

What is the difference between compression pressure and compression ratio as an indicator of condition?

We humans can somehow feel sickness due to blood pressure, such as having a hard time waking up in the morning or having heart palpitations after a short run. However, the owner must somehow figure out the cause of the performance degradation such as the recent poor acceleration, increased white smoke, or a lack of punch since the engine of a motorcycle cannot report the defect on its own. The compression gauge is a reliable measurement device for this purpose. A compression gauge that connects an adapter to the plug hole and directly measures the pressure generated in the combustion chamber as the piston moves back and forth is like measuring blood pressure in human terms.

Incidentally, the "compression ratio" in the engine's main specs and the "compression pressure" that can be measured with a compression gauge are 2 different things. The compression ratio is the volume ratio of "cylinder volume + combustion chamber volume" and "combustion chamber volume", which is a static value calculated and has no unit. Compression pressure, on the other hand, is a dynamic measurement of the pressure when the piston actually moves to compress the mixture, and is expressed in kPa (kilopascals) (before the shift to SI unit system, it was expressed in kgf/cm2).

  1. Understand that compression ratio and compression pressure are two different numbers
  2. A compression gauge is a device that measures the compression pressure

The compression gauge consists of spark plug adapters, a joint tube, and a gauge. A joint tube can be made of a flexible rubber hose or metal pipe with low-pressure loss.

If the joint tube is a metal pipe, measurements may be taken while pressing the rubber adapter at the end of the pipe against the spark plug hole. This product has an adapter that is screwed into the cylinder according to the plug thread size, and features little compression leakage during measurement.

Turn the cell motor at full throttle when measuring.

In a 4-stroke engine, the pressure in the combustion chamber is highest when the intake and exhaust valves are closed and the pistons are in the upper dead center. However, since the pressure is measured while moving the piston, the pressure obtained will vary depending on the difference in piston speed between turning the crankshaft by hand with a socket wrench and when the engine is actually operating.

Therefore, in the case of a cell motor-starting engine, the cell motor is rotated for a few seconds with a compression gauge set so that the measuring conditions can be evaluated objectively with a constant value. In the case of a kick-starting engine, the kick pedal is stepped down a few times quickly.

In either case, the compression pressure indicated when the piston first passes through the upper dead center is small, and the indicated value gradually rises over repeated strokes, and does not rise any further up to a certain extent. The important thing to remember when measuring compression is to always keep the throttle fully open and the intake air maxed out while the cell motor is running (while stepping on the kick pedal). When the throttle is closed, the amount of air entering the combustion chamber is squeezed, so the displayed compression pressure is reduced.

Since the compression pressure shown in the service manual and other maintenance documents is the value at full throttle opening, it is not possible to make a correct decision if the measurement conditions are different. Also, all spark plugs are removed for engines with 2 or more cylinders, and measurements are taken. If any plugs are left in the cylinders other than the measuring part, the compression pressure will become resistance and the rotation of the crankshaft will not rise sufficiently, so the compression pressure will be displayed as low.

  1. Disconnect all spark plugs for engines with 2 or more cylinders
  2. Throttle fully open when measuring compression pressure

To measure the compression, press the cell button for a few seconds while the throttle is fully open. After a few seconds, it will show the highest value and not go any higher, so you don't have to hold down for 20 or 30 seconds. Keep the battery fully charged to ensure the crankshaft rotational speed.

Compression pressure STD values vary for each model

The STD value for compression pressure varies for each engine. For instance, the KAWASAKI Z400FX is 997 to 1490 kPa, while the ZEPHYR X is 775 to 1180 kPa.

When looking at the numeric value, the FX is more complex, but this is based on the engine specifications, so it's not because either one side is winning, but you have to make a decision based on the data of your beloved motorcycle.

Consider the cause and countermeasure if it doesn't fit in the STD range. If the compression pressure exceeds the upper limit, it is likely that the combustion chamber volume is smaller than in the STD condition, and more specifically, the carbon buildup at the top of the piston is often the cause. In this case, a cleaner chemical to clean the inside of the combustion chamber may be helpful if the accumulated carbon is soft. If it is solidified, you may need to remove the cylinder head and remove the carbon from the top of the piston and combustion chamber.

When the compression pressure drops below the lower limit, more specific symptoms, such as a lack of power in the engine or white smoke, are more likely to be felt. If the compression doesn't go up, it means that the mixture is escaping from somewhere, even if the piston goes up. Specific causes include piston ring wear, carbon entrapment in the intake and exhaust valve faces, and excessive clogging of valve clearances.

These are symptoms that show up as deterioration over time for motorcycles with long mileage, but on custom motorcycles that have the air cleaner removed and are running on funnel specs, the compression pressure may drop due to physical damage from inhaled foreign objects. Neither the upper nor the lower limit of the STD value is a good condition for the engine, but the important role of the compression gauge is to be able to visualize it.

  1. The compression pressure STD values vary for each model
  2. If it deviates from STD values, check for engine problems.

It is important to know the variation for each cylinder

As mentioned earlier, compression pressure is an important clue to the condition of an engine, but for engines with 2 or more cylinders, it also helps to compare each cylinder relative to the other.

If only 1 cylinder of a 4-cylinder engine breaks the piston ring, the engine oil does not suddenly get out of the way, but comes up into the combustion chamber from the crankcase and blow off the oil along with the exhaust gas from the exhaust system. You can determine the oil rise from a dirty spark plug, but if the compression pressure is low on a particular cylinder by measuring the compression, you can predict that there is a problem there.

In the case of the KAWASAKI Z400FX, the limit of variation of each cylinder is 100 kPa according to the service manual, and even if the compression pressure is within the STD value, if there is a difference of more than that, it is necessary to investigate the cause in detail. As you can see, cylinder compression pressure is an important clue to understand the engine condition in absolute and relative values, and the compression gauge, which visualizes the pressure, is one of the important measuring instruments for engine maintenance.

  1. For multi-cylinders, compressive pressures are compared relative to each other
  2. A compression gauge helps to understand the engine condition

When only certain plugs in the 4 cylinders are smoldering, if it's gasoline derived carbon, you may suspect a misaligned carburetor setting. If there is a large difference in compression pressure between the other cylinders as measured with a compression gauge, you need to suspect a cylinder or cylinder head failure.

The compression gauge shows the condition of the engine. I would also like to use it to know when to do maintenance.

See KAWASAKI Moto Index Page
See Accessories for KAWASAKI Z400FX
See Accessories for KAWASAKI ZEPHYR400X

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