I got a knock

[Tony91Sport]

I have a knock in my engine at full open throttle after 3000+ rpms.. If i put better fuel in, then it doesent knock as bad. Any suggestions?

 

[97V8]

Your detonating.

 

[Tony91Sport]

What do you mean i am "detonating"?

 

[Tony91Sport]

P.S....I also just changed the O2 sensor (to a bosch) and the fuel filter. I recently (like a week ago) custom installed a conical K&N air filter from a 97 explorer. So what can i do now?

 

[97V8]

Detonating is pre ignition. Many things can cause this. like cheap fuel, to much timing, there are more cause but I hope you get the idea. Did all this happen before you put the K&n on your truck? I would bet that it is the cause, b/c its getting all the hot air from under your hood and feeding it to your motor. Hope this helps.

 

[97V8]

I didn't mean pre ignition as you explained it. I ment the ignition is lighted before it is supossed to. The trem Pre ignition never came to mind. Sorry for the screw up, guess I need to watch my wording more carefully.
Im well awary of what detonation is, I have to be vary careful of it in my supercharged mustang. I do think you explained the two very well thou.

 

[Ira]

97V8,
You are wrong. Pre-ignition and detonation are not the same thing. Pre-ignition takes place when an object in the cylinder is heated to the extent that it glows cherry red and this is used as the source of ignition for the cylinder instead of the timed spark. This could be from a carbon build-up on a head or an electrode from a spark plug which is of too hot of a heat range. The spark plug is actually too far down in the cylinder and when the combustion occurs in the cylinder, the spark plug cannot dissipate the heat and cool enough before the next compression stroke and the air fuel mixture ignites from the cherry glow.

Detonation is actually more violent than pre-ignition. When an engine is running, the ignition timing is advanced more as the RPM and acceleration of the engine increases. This is done to compensate for the time that it takes for the air fuel mixture to burn. When an engine is at idle, the time between sparks is less than when running at high speeds so the fuel has more time to burn completely. When at higher RPM's, the timing actually is advanced so that the spark is given well before the piston reaches top dead center (TDC). If the timing is not advanced, the combustion would occur after the piston has reached TDC and has started back down which would result in less power. If the timing is advanced too much, the combustion would occur prematurely and cause the piston to be pushed back down in the cylinder before it reaches the top and causing the piston to actually try to stop and reverse before it is ready to. Thus resulting in detonation. This is what is commonly known as knock. Things that can cause it are, improper timing, low fuel quality, etc. I have actually seen pistons self destruct because of detonation.

Pertaining to the differences in fuel octane, regular unleaded actually burns at a faster rate than premium. This is one reason for knock because the flame front from the combustion is more violent and done sooner and can be excessive when the piston has reached TDC. The higher octane fuel helps reduce the knock because it burns at a slower and more controlled speed.

I hope maybe this has helped to explain the difference. It is not easy to put into words, but I did my best.

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Ira

91 XLT 4x4

 

[Tony91Sport]

You guys have explained this very well for me...but still....what should i do to correct this? I have heard numerous people tell me i cannot change the timing...I need some repair info...

 

[Tony91Sport]

P.S. I heard the knock before i put in my air filter.

 

[Ira]

First thing I would try is to put a fuel system cleaner in that will help remove carbon deposits in the engine. Anyone at a parts house should be able to reccommend a good one for you. Try to get one that will work in only one tank of fuel.

If that does not work, I would check the spark plugs for a couple of things. First to be sure they are of the correct heat range. People do make mistakes and you could have the wrong ones in there. I would also check the condition of the electrode on the spark plug. If they are severely worn, it could be thin enough that it could be glowing and causing pre-ignition. It is a long shot, but it is possible.

Hope this helps you.

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Ira

91 XLT 4x4