What should be the compression in the engine? Instrument for measuring engine compression

By compression in automobile engines is meant the level of pressure in the cylinders in the final compression phase, at the very moment when the crankshaft rotates with the help of a starter.

Why measure it?

If more recently, the power unit worked properly and without complaints, but suddenly its power dropped sharply, fuel and lubricant consumption increased, and at idle there were vibrations - these are signs that compression in the cylinders has decreased.

To check whether this is so, it is possible only with the help of measurements. Let's try to find out what compression should be in the engine and how to calculate it.

Checking the level of compression allows you to learn enough about what there are problems with the motor. There is no need to remove the engine or disassemble it.

Among the reasons for the lowering of the pressure level in the piston group can be the banal wear of the pistons or cylinders, the malfunction of the gas distribution mechanism and many other reasons. And the car owner, most likely, shines repair of the engine (VAZ or other car).

Levels of compression on gasoline cars

The degree of compression is the volume of the cylinder at the moment when the piston is at the bottom dead center. This is the maximum available space divided by the same volume with the piston at the top dead center, that is, the minimum available space.

What do you need to understand if the technical specifications indicate the compression ratio and compression? Let the compression ratio be 10. This means that the maximum available volume of the combustion chamber of one cylinder is ten times larger than the minimum volume. So, the fuel mixture will shrink 10 times from the amount that enters the cylinder.

There is a relationship between compression and compression ratio.

This dependence is determined by a special coefficient. For gasoline aggregates it is equal to 1,2-1,3. So, if the compression ratio is known, and these figures can be found in the documents for the car, then it is not difficult to calculate what compression should be in the engine.

Naturally, these calculations can significantly differ from the passport data claimed by the manufacturer. Because this characteristic strongly depends on how much the piston group is worn out . The faster the parts wear out, the less will be the level of compression.

Low compression of the gasoline engine and its serviceability

So, what is the line, when the level of the given value is very low, and the unit needs to be repaired? It is believed that this is 10-12% of the degree of compression, what the manufacturers considered and presented. So, if the passport indicates that the level should be - 12 kg / cm ² , it is easy to calculate that the compression at 10.5 says about the faulty motor. He needs major repairs.

Measurements on popular car models

Let's see what should be the compression in the engine of popular cars in Russia. On models from AvtoVAZ, from 2106 to 2110, this figure is from 11 to 13 kg / cm ² . Kia Rio, according to manufacturers, depending on the year of release has a level of 12.75 to 13. "Nissan Kashkay" has a compression of 12.5 to 12, depending on the volume of the engine. Models from Volkswagen are mostly from 12 to 13. The figure also depends on the volume of the unit.

Compression and diesel engine

In diesel installations, this parameter is of much greater importance.

Yes, of course, it shows the state of the motor components, but still it is the only possibility of its normal start-up at low temperatures overboard. Also do not forget that the compression ratio and compression for the diesel are a bit different. The compression ratio is constant. Compression has the property of changing.

In diesel, unlike gasoline units, fuel flares up not from a spark, but thanks to compression of diesel fuel in a cylinder. When the pressure is high, the fuel is heated and self-igniting. The lower the compression, the lower the temperature. So, it will be quite difficult to start the engine. The compression ratio in a modern engine can be from 35 kg / cm ² to 40 kg / cm ² . Let's consider further how to measure this indicator.

How to measure engine compression

Many experienced car owners try to eliminate them on their own in case of any problems. Pistons and cylinders are no exception. And if suddenly the engine lost the compression, then you need to perform a diagnosis and perform the necessary measurements.

Measure the compression level with a special device. This is a compressometer. Such devices are well used in SRT, and are good for independent use. All devices can be divided into gasoline, diesel or universal models. According to the principles of operation, these measuring devices also differ significantly.

Types of compressometer

Clamping devices are used primarily in the case of rapid measurements for gasoline aggregates. They are distinguished by the presence of a special rubber tube. It is universal. Working with this device requires a second pair of hands.

Threaded instruments can be used independently, without anyone's help. The meter must be fixed in the candle hole or in the nozzle hole. The device is additionally equipped with a rubber bushing, which gives this compressor a clamping device function.

Special thread on a flexible compressometer allows you to mount the device in the candle holes.

Also, a very flexible design provides an excellent opportunity to carry out measurements without the need to dismantle engine assemblies.

And finally, the universal compressometer allows you to find out what compression is in the diesel engine. Measuring and diagnostic work is carried out without the need to dismantle any nodes.

Recommendations of professionals in choosing a measuring device

To get particularly accurate readings, professionals recommend using a threaded device. Here the valve that fixes the pressure must be installed not under the manometer, but in the head, which, in turn, is screwed into the holes for the spark plugs. If this valve is positioned correctly, the results will not be underestimated.

Knowing what the compression ratio in the engine should be, you can see how much gas is compressed in the combustion chambers under the action of the cylinder. During operation, it is possible to identify a faulty or faulty cylinder.

If the compression level in the combustion chambers decreases, this means that replacement of the piston rings is necessary , the candles or flaps of the steel are worn out.

Secrets of correct measurements

To get the most accurate result, it is necessary to warm up the unit to its operating temperatures. In the process, it is recommended to turn off the fuel supply to the engine and twist the spark plugs. Before you measure the compression of the engine, it is also recommended to fully charge the battery and check the starter for serviceability.

Closed or open flapper

During these diagnostic procedures, the throttle can be opened or closed. If the measurements are carried out in order to establish malfunctions, then the position of the flap depends on what you reveal.

If the shutter is closed, a small amount of air will enter the piston group.

So the compression will not be too big. So leakage is detected. After all, even a small decrease in density can lead to a decrease in the level of compression.

If the flap is opened, the air volume will be large enough. The level of compression will drop, however it is insignificant. The leakage may increase. It is possible to detect breakage of pistons, breakage or coking of piston rings, deformation or burnout of valves, violation of the geometry of the cylinders.

Measurement algorithm for measurements

First, remove the candle wires and unscrew the candles. Work as carefully as possible - do not put dirt in the candle holes. Clean out all the sand and dust that accumulated there. Then screw the compressor in one of the holes.

Let your partner completely squeeze the accelerator pedal and at the same time start the starter. These operations must be repeated until the meter needle stops rising. Accelerate the accelerator to open the throttle.

Record the readings for each cylinder. Then find the standard data for your car and mileage. Now, knowing what compression should be in the engine, as well as the mileage of the car, analyze and compare the data. The indications for different cylinders should not be very different.

The reasons for the lack of compression

It happens that the pressure may disappear altogether. Provoke it can be a lot of different problems.

If there is no compression in the engine, the reasons can be in low quality fuel and lubrication. This leads to coking of the piston rings or cuffing of other motor parts.

Also, among the reasons, small amounts of chips can be distinguished, which appears in the cavity of the cylinder while the motor is running. It's not the chips themselves that are to blame, but the scuffs on the surface of the cylinder. Also, if the gasket under the cylinder head has run out, this too can provoke depressurization.

How to solve a problem?

If there is no compression in the engine, the causes of this phenomenon can be completely different. Can you solve these problems? The method exists, but no one guarantees its effectiveness. Although many helped.

Prepare the valve cleaning liquid. Twist the spark plugs, then pour 50 g of this mixture into the problem cylinder. You can determine it using a compressometer. Leave the car for 12 hours in complete rest. After scrolling the engine, clean the spark plugs, adjust the gaps and set everything in its place.

After that, go out of town and at a high speed ride about 30 km. Then check again. If the readings of the device remain the same, then you need urgent repair of the engine, VAZ in particular, since this is a common problem on these cars.