A full-speed thrust reduction or a difficult start of the engine are often the first alarm bells for the owner of water equipment. Many owners immediately begin to sin about the quality of the fuel or the condition of the candles, forgetting about the fundamental parameter - compression pressure in the cylinders. boat-engine compression It is a key indicator of the health of the piston group and the tightness of the combustion chamber.
Falling below the limits of the standard indicates wear on the rings, the appearance of bullies on the cylinder mirror or problems with valves in four-stroke models. Ignoring the symptoms can cause the engine to stop starting at the most inopportune moment, leaving you far from the shore without the possibility of self-repair.
In this article, we will discuss in detail what values are considered normal for different types of engines, how to take measurements yourself and what to do if the pressure gauge numbers disappoint you, and understanding these processes will allow you to diagnose the problem in time and avoid costly overhauls.
Effect of compression on engine operation
Compression pressure directly affects the combustion efficiency of the fuel-air mixture. If the cylinder does not have enough pressure, the mixture does not ignite with the desired force, which leads to loss of power and unstable idling. For two-stroke engines, low compression also means problems with the cylinder purging, since crank-pressure It is closely related to the degree of compression.
In four-stroke engines, the situation is aggravated by the presence of a valve mechanism. The loose fit of the valve plates or wear of the guide bushes lead to leaks of gases, which instantly affects the readings of the compressometer, in which case the engine can triple, and black smoke will come from the exhaust pipe due to unburned fuel.
⚠️ Warning: Operation of a critically low compression engine can cause a piston to burn or a rod to break due to detonation and overheating.
And the compression difference between cylinders is even more important than absolute numbers. If one cylinder has 10 atmospheres and the next cylinder has 7, the engine will be vibrating with a strong vibration, quickly wearing out the crankshaft bearings. crank-and-wheeler.
Compression standards for two-stroke and four-stroke engines
Owners often look for a single ideal number, but compression rates are highly dependent on the type of engine and its displacement. Two-stroke engines have higher requirements for tightness, since they do not have a separate valve lubrication system and rely on piston-ring It's a vacuum in the crankcase.
Four-stroke units, such as popular series Yamaha F or Honda BF, usually have slightly lower compression pressures due to the shape of the combustion chamber and the timing phases, but the range of values between cylinders should not exceed 10-15% of the maximum value.
Below is a summary table of indicative values for engines of different power, remember that accurate data for your model can always be found in the manufacturer's service manual.
| Motor type | Workload | Normal compression (kg/cm2) | Minimum permissible (kg/cm2) |
|---|---|---|---|
| two-stroke | 15 hp. | 9.0 - 11.0 | 7.5 |
| two-stroke | 40-60 hp. | 10.0 - 12.0 | 8.5 |
| four-stroke | 2.5 - 15 hp | 8.0 - 10.0 | 6.5 |
| four-stroke | 20 to 100 hp. | 9.0 - 11.5 | 7.5 |
It is important to consider that new engines can show values at the upper limit of the range, while engines with a lifetime of more than 500 hours will show stable, but slightly lower performance.
Measurement tools and preparation
For accurate diagnosis, you will need a specialized compressometer For two-stroke motors, it is critical to use a threaded tip device, because the pressure of scrolling with the starter can knock the usual clamping cone out of the candle hole.
The measurement procedure requires that the engine be warmed to operating temperature. A cold engine will show understated results due to thermal gaps between the piston and the cylinder. Before starting work, make sure that the battery is fully charged, since the starter must scroll the crankshaft at the same speed at each measurement.
You'll also need a candle key, pliers to remove high-voltage wires, and perhaps some engine oil for oil diagnostics (discussed below), and make sure you have a notebook to record the results for each cylinder.
☑️ Preparation for compression measurement
Step-by-step instructions for measuring compression
The first step is to turn all the spark plugs out, which will allow the crankshaft to spin freely and allow the starter to develop the maximum revolutions necessary to create pressure. If you can't unscrew all the candles (for example, because of the design of the casing), twist them in turn, but this is a less accurate method.
Put the compressometer tip in the candle hole of the first cylinder. Make sure the connection is sealed. Now you need to open the throttle completely. To do this, ask the assistant to squeeze the handle of the gas to the end or lock the trigger of the gas in the open position.
Procedure:1. Unwrapped the candles.
2. Insert the compressometer.
3. Open the throttle (gas to the floor).
4. Starter spin 5-8 seconds.
Turn on the starter and scroll the engine for 5-8 seconds or until the pressure gauge needle stops growing. Record the maximum reading. Repeat the procedure for all other cylinders, comparing the results. The reading difference should not exceed 1-1.5 kg / cm2.
What do I do if the arrow jumps?
If the pressure gauge arrow twitches unevenly, this may indicate valve jamming (in four-stroke motors) or problems with the ignition system, which is trying to work in a compression stroke.
Results analysis and oil diagnostics
If the measurements are low, you need to identify the source of the leak: the ring or the valve, and there is a proven method of "oil compression" to do this, pour 5-10 ml of engine oil into the low-pressure cylinder through the candle hole and repeat the measurement.
If after adding oil compression has increased significantly (by 2 or more atmospheres), then piston-ring If the readings are not changed, the problem is leaky valves or the cylinder head is laid.
For two-stroke engines, the increase in readings after adding oil can also indicate wear on the piston skirt. However, if the compression does not rise even with oil, this often indicates a breakdown of the gasket between the cylinder and the crankcase or damage to the crankshaft glands.
⚠️ Note: When conducting an oil diagnosis, do not exceed the dosage of the oil, otherwise a hydraulic shock is possible when trying to start the engine.
Use a syringe to enter oil into the cylinder - this will allow you to measure the exact amount (5-10 cubes) and not to stain the candle well.
Causes of pressure drop and methods of elimination
One of the most common causes of compression loss in two-stroke engines is overheating. Prolonged operation at full gas or improper adjustment of the carburetor (poor mixture) leads to thermal expansion of the piston and the appearance of a piston. bully on the cylinder mirror.
Four-stroke engines often have valve burns, which can be due to low-octane fuel or heat gaps, and it is also worth checking the condition of the HBC gasket, which often breaks down with antifreeze entering the oil or cylinders.
To correct the defects, it may be necessary:
- 🛠️ Replacement of the piston group (rings, piston, cylinder) in the presence of bullies.
- 🔧 Laundry or replacement of valves and valve seats in four-stroke motors.
- 🔩 Replacement of the cylinder head gasket and checking the adjoining plane.
- 💧 Replacement of the crankshaft glands with their wear (relevant for 2-strokes).
In some cases, especially for two-stroke engines with small bullies, the procedure of decoking or the use of additives helps, but this is a temporary measure. Full recovery of compression is possible only mechanically by replacing worn parts.
Regular oil change in four-stroke motors and the use of a high-quality mixture in two-stroke – the best prevention of compression loss.
Frequently Asked Questions (FAQ)
Can I drive a low compression engine?
The engine will run unstable, consume more fuel and warm up, and long-term operation will lead to the destruction of the piston group and expensive repairs.
Does the octane number of gasoline affect compression?
Directly, no, the compression depends on the mechanical state of the motor, but using low-octane gasoline causes detonation, which can physically damage the piston and valve, which eventually leads to a drop in pressure.
How often should I check the compression?
Preventive measurements are recommended every 50-100 hours or before navigation, and if you notice a drop in traction or difficulty starting, check immediately.
Why is the compression above normal?
Excessive compression is usually caused by the combustion chamber’s soot, which reduces its volume, or improper grinding of the cylinder head, which increases the risk of detonation.