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Detonation...

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Old 05-10-2012 | 11:15 AM
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tunedbysti's Avatar
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Detonation...

I was talking with my mechanic lately about the subject of detonation, but he really wasn't too specific on what it actually was... he just said something about not using 87 oct. gas for my STi, to just use 91 and up... What IS detonation?
Old 05-10-2012 | 11:39 AM
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detonation happens when the fuel ignites before the piston reaches the top of the cylinder. Its bad because it pushes back at the piston instead of pushing it forward. It shows up as a "knocking" sound which is why cars are equipped with a "knock" sensor which will cause your car to pull timing to avoid further detonation.

Using the correct grade fuel for your car is important but in a pinch lower octane fuel will work but you will have reduced power as a result.
Old 05-10-2012 | 12:02 PM
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Close Marioshi but it sounds like you are describing pre-ignition.

Engine knocking - Wikipedia, the free encyclopedia

Knock or detonation is when the explosion of air and fuel is irregular yet still initiated by the spark plug.

Engine knocking - Wikipedia, the free encyclopedia

I am no expert though.
Old 05-10-2012 | 12:38 PM
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When I had my car tuned in Washington with 92 octane and then moved down here and started using 91 octane I started getting detonation. Basically, I would floor it in 3rd, 4th, or 5th gear and at around 3k rpms when the turbo would spool I would get a kind of crackling sound with a little hesitation and then it would go away. Didn't know what it was until I saw this dyno video DETONATION - YouTube --> that's what detonation sounds like.
Old 05-10-2012 | 01:23 PM
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Originally Posted by queeg9k
Close Marioshi but it sounds like you are describing pre-ignition.

Engine knocking - Wikipedia, the free encyclopedia

Knock or detonation is when the explosion of air and fuel is irregular yet still initiated by the spark plug.

Engine knocking - Wikipedia, the free encyclopedia

I am no expert though.
No expert here either and I typed really fast. The higher the octane fuel, the slower it burns so basically the 87 octane fuel combusts too fast. Honestly, our engines could be tuned to lower octane fuel and run just fine but they wouldnt be as powerful.

Bottom Line: Put the recommended octane in your car to avoid reduced power and/or damage to your engine. your mechanic is correct about this.

You are correct that my limited explanation did seem to describe pre-ignition once I went back and read it

Last edited by Marioshi; 05-10-2012 at 01:26 PM.
Old 05-10-2012 | 01:33 PM
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91 octane is fine as long as you're not running a tune meant for 93 or higher. You can't run low or midgrade octanes unless you absolutely have to because it's all that's available and you stay out of boost. Your stock ECU will retard timing when it starts detonating as a self-preservation method but will eventually start to advancing it again until you start knocking once more. If you keep using 87 you're gonna constantly run into the retard -> detonation -> retard -> detonation cycle and that can lead to you replacing the motor.

Last edited by Brfatal; 05-10-2012 at 06:30 PM.
Old 05-10-2012 | 01:59 PM
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+1 to making sure you put the correct octane in your car!
Old 05-10-2012 | 03:00 PM
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Down here in California all that is commonly available is 91. I try to only go to 76's that have one nozzle per grade of fuel so I do not get a little 87 with my 91. If I have to take from multi-grade nozzles I stay out of boost until I can top of with a gallon or two of 100. Some people think I am nuts for doing this.
Old 05-10-2012 | 03:21 PM
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I am new here so please dont take offense but you are nuts for doing that lol.
Old 05-11-2012 | 07:59 AM
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Thanks everyone for your help. I HAVE been running 92 since I got it, and I dont plan to go any lower unless I absolutely need to... I was just curious on what detonation actually meant. I really appreciate you guys answering my question.
Old 05-11-2012 | 08:26 AM
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I see that detonation is DEFINATELY not good, but I also have heard of people like running higher octane fuels like Citgo 110 leaded fuel... what would something like that do to your motor?
Old 05-11-2012 | 11:56 AM
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For you running 91, a little bit more power. I've read that our ECUs might advance enough for anything up to 95 octane to be beneficial. Anything after that will require tuning to take advantage of.

Last edited by Brfatal; 05-11-2012 at 04:47 PM. Reason: spelling
Old 05-11-2012 | 03:05 PM
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Ahh i see.
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