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Engine Knock

"Knock Knock,.... Whose there?"

So what exactly is engine knock and how dangerous is it? What causes engine knock and how can it be avoided?

You may notice a knocking or rattling noise from your engine or a sudden loss of power under full throttle.

Immediately back off the throttle and only use low rpm engine speeds until you can get the engine investigated.

The chances are that you are suffering from a condition known as "engine knock".

Engine knock is a condition where the fuel in the engine ignites prematurely. It can cause serious damage to an engine. How does this damage occur?

A knock at the door means the arrival of a friend - a knock in your engine indicates the arrival of an enemy!

Firstly if the piston has not achieved its top dead centre position you risk forcing the crank to run backwards. This puts a lot of stress on the crank and other pistons as everything effectively seizes up.

Secondly if the intake valve has not closed the flame front will move through the intake of your engine causing damage.

Thankfully engines are fitted with a knock sensor that detects this as it happens and backs off the fueling causing the engine to run lean and avoid knock. The knock sensor will not save an engine from the effects of a serious knock and should not be viewed as a safety valve. It is an emergency only option and will only work effectively in mild situations.

The causes of engine knock are many and varied. Generally it is caused by high pressure inside the engine. As fuel is compressed it gets hot and this can cause it to ignite prematurely. Occasionally it may be caused by a misfire of the spark or even a hotspot inside the cylinder.

Over tuning an engine to a very high compression ratio will often cause this problem. Running  a high pressure forced induction setting will also increase the risk of compression based ignition.

Other causes of premature ignition include hot spots inside the engine. If the carbon build up is allowed to continue you will potentially have deposits of carbon glowing red hot igniting the fuel as it enters the engine.

The octane rating of fuel describes the fuels resistance to engine knock. In a lot of cases simply switching to a higher octane fuel will solve your engine knock problem. A lot of Japanese cars run high compression ratios and the manufacturers demand high octane fuel be used.

You may have filled up with a bad batch of fuel. In some countries the octanes of standard fuels can be lower than you expect so it is worth checking the octane before filling up. If you have no choice but to buy a poor quality or low octane fuel back off the throttle and run with low rpm engine speeds.

Avoiding engine knock. All engines are fitted with sensors which detect knock and retard the fueling to prevent engine damage.

Lowering the compression ratio of the engine. You can rebore the engine, fit low compression pistons or rebuilt the engine with a suitable stroker kit to alter the compression ration. Thicker gaskets make very little difference to the compression ratio and are generally much less reliable than standard gasket widths.

In turbo application where you need to wring out the maximum power you will find the solution in water injection. We have a detailed article on water injection if you need more information. Basically water is injected into the engine via the air/fuel intake. The water suppresses the ignition point of the engine allowing a more complete burn. Water has the benefit of vaporising on combustion and produces Hydrogen and Oxygen which will further help the burn of the engine.

Turn down the boost. If you are running a turbo and experience fuel cut off or the engine runs lean at wide open throttle you should suspect that the knock sensor is kicking in. This indicates that you are running just beyond the maximum and the pressure in the cylinder is causing the fuel to pre-ignite. Do not ignore this situation otherwise you risk serious damage to your engine. 

You should turn down the boost via a programmable boost controller. You may be able to modify the wastegate control also to achieve the ideal boost parameters. 

(Restore the engine to a lower boost/compression ratio setting before doing this.) The engine may be suffering from a build up of carbon deposits and these cause hotspots in the engine. Clean out the engine  to remove this build up. Ideally the engine should be stripped down and rebuilt, but this is not a cheap answer.

There are a number of effective engine cleaning methods that do not require a rebuild. Firstly add some injector cleaner such as Redex or better still some BG44k to the fuel tank and take the car on a long run with the engine rpms at motorway speeds. A couple of hours will suffice. Include some high rev work at lower speeds and on hills and get the engine under as much load as possible.

The second way is to use the Power Boost spray from Ekotec or sea foam. This is sprayed into the intake as a foam and it thoroughly cleans and removes the carbon/varnish deposit inside the engine. Follow the instructions on the tin to obtain the best results.

Join us in our forum to discuss engine knock and other tuning issues with our performance tuning enthusiasts.

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