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A Quick Compression Variation Test

You might not be aware of this, but you actually check your compression every time you start your car. You just need to know what to listen for.

I use this technique every time, before I would attempt to tune an engine. There is no point in performing a full blown tune-up on an engine that is down on compression in one or more cylinders. This quick test is used to confirm that compression variation is reasonable.

This is easy to do. You do not want the engine to start during this test so you must disable either the ignition system or the fuel system. If you have a carburetted vehicle, then disabling the ignition system is the easiest way. If you have a fuel injected model disabling either system will achieve the same result. Please note if you have a catalytic converter fitted on an EFI engine, then you must disable the fuel system to avoid damage to the converter. On a carburettored engine holding the throttle open during cranking will minimise fuel entry. I cannot cover the whole spectrum of ignition & fuel systems and how to disable them, but the end result is that you do not want the engine to start. Be aware however that dangerous voltages may be discharged from the ignition system depending on the method you use to disable it.  

Once you believe you have disabled the engine starting, jump in the drivers seat and press the accelerator pedal to the floor. This will allow free volume of air to be drawn into the cylinders to be compressed. Now crank the engine over. The engine may just quickly show a sign of life to start, this is just residual fuel in the inlet manifold or combustion chamber and once it is pushed out on the exhaust stroke no further pre-ignition will occur, this is only on carburetted models. While the engine is cranking you are listening to the compression beat of the starter motor. In a four cylinder the beat is easy to hear. The more cylinders there are, the closer the compression beats are together. Crank the engine for about five seconds or so. If the beats are similar in sound then the chances are you have reasonably even compressions. If you have a cylinder down on compression you will hear a distinct change in the starter motor noise or speed. This is because the resistance of the compression in the bad cylinder is less, so the starter motor speeds up until it hits the next good compression and then returns to normal speed until it comes back to the bad compression and so the cycle repeats. In TT3 I will cover how to diagnose the problem further if you believe you have variation in compressions.