[andwrong]

okay.... i have heard/read the things that say "engine ping" or "high compression and av gas" or whatever.

i dont get it? can somebody/everybody please explain to me why certain compressions must run certain gasses to avoid pingign? what exactly is pinging?
thank you

jason
p.s. go as detailed as you want... my girlfriend is sick so i'm bored

 

[Mach1Morgan]

Basically, compression creates heat. A diesel engine will use a 20:1 compression ratio, which creates enough heat to ignite the diesel fuel mixture without any other ignition source. High octane fuel resists this spontanious combustion better than low octane fuels do. Running high compression ratios with low octane fuel can cause the air fuel mixture to ignite on it's own, before the ignition spark occurs. When it does this it basically explodes violently and creates a loud knocking and pinging noise that can be heard by the driver. This is call pre-ignition. When the air-fuel mixture is ignited by ignition spark, as it should be, it usually burns in a controlled manner from the spark plug out in a basically circular pattern. If the octane level of the fuel is too low though, the increasing pressure and heat in the combustion chamber can cause the still unburned fuel near the edge of the combustion chamber and cylinder bores to ignite before the flame gets there. Again this causes a pinging sound. This is reffered to as detonation and is usually what is responsible for knocking ring lands off of the edge of the pistons. Both of these conditions can be reduced or eliminated by reducing cylinder pressure, cooler engine running temps, cooler intake air and higher octane fuels. More compact combustion chambers, tighter piston deck to clearances and heads that position the spark plug near the center of the bore but slightly closer to the hottest part of the combustion chamber (the exhaust valve) so that the fuel mixture in this area is burned first, before the cylinder pressure gets to high, will also help. I hope that explains what you were wondering about.

 

[CougarKid]

Great post Morgan! You pointed out a couple things I didn't know. This is why I come here! There can be a post on anything from the basic to the bost advanced and I know I'm going to learn something new no matter what. Thanks Morgan and thanks andwrong! I love FM !!

 

[ausTOM]

Pinging is caused by spark igniting the fuel mixture too early, piston still on its way up the bore. If the piston is too far down the bore as ignition occurs, the expanding gases push down on the rising piston causing the noises associated with pinging, and damage. Kind of like two forces opposing each other. To fix the problem we retard the timing, (later ignition). There are a lot of variables that cause pinging such as bad fuel , poor combustion chamber design etc. Hot spots caused by a piece of metal stuck to the com chamber can cause early combustion as well as run on.

 

[Brian M]

I'd just like to add that there is a difference between 'pinging' and 'detonation' Usually detonation is louder, becuase it is the mixture exploding before a spark, or shortly after the spark as mentioned before. 'Pinging' is basically when you are running too far advanced for your combo, heads and all. if your have 2 fuel that burn at the same rate, with your ignition too far advanced, changine fuels wont help at all, the flame is burning and creating the most pressure before the pistons has gone over tdc, slamming it back down onto the rod bearing, and piston pin. Very bad thing, it ruins bearings and other things. One example, look at an engine with a high advance rate, its hard to start, because the cylinders are trying to make power before tdc, and it is trying to reverse the motor. As the rpms speed up, the piston is going up faster, and with mechanical advance, in a perfect world, the most power comes just as the piston is rounding tdc, getting the most power from the combustion. So basically if you are running high compression, with a small cam its all octane related, if you are running a fairly mild engine, basically 8.5:1 compression, and you have just done your timing, it is probably too much advance, and you should back it off a little

 

[speedar]

As a followup question...
If you drop your octane and can still avoid detonation should you? I have heard that lower octane has a slower more complete burn that makes more horsepower but that detonation is the enemy. Against this I have tried octane variations on the performance trends analyser and shows lower octane = lower HP & Tq which would tend to contradict what I have heard. Can the experts indicate where the truth lies?

 

[ausTOM]

I have heard that the higher the octane rating, the slower it burns. Therefor it can take more spark advance. Fits in what others have said on this topic.

 

[tonyk]

Octane is the fuels resistance to detonation (uncontrolled ignition)
The higher comp you run the more the compression process itself wants to initiate combustion. The higher the octane rating of the fuel the more "stable" it remains under pressure. The most power per ounce of fuel is made when the "critical compression ratio" of the motor matches the fuel it uses. ie a 13.5:1 motor on 95 fuel will make a lot less hp than a 9:1 motor on the same fuel.

The engines power comes from the fuels energy rating (BTU), but again this is dependent on the motors specs being able to extract the power correctly, ie not over or under compressed.

There are a number of urls around which explain it better.
http://ubermensch.org/Cars/Technical/Octane
for example and the "How Stuff Works" website too

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<font size=-1>[ This Message was edited by: tony.k on 12/31/01 8:34am ]</font>

 

[speedar]

Thanks Tony, this board never lets me down:roll:

 

[lordviper13]

I would like to point something out that goes back to the issue of airport gas, a high octane is not always better say for instance a 9:1 stock motor. With that much octance you will have trouble getting something like 115/145LL av gas to burn unless you have the compression for it
say like a huge blower