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post #16 of 82 (permalink) Old 06-17-2009, 07:43 PM
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post #17 of 82 (permalink) Old 11-26-2009, 12:43 AM
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Ummm I'm new here, but I was wondering what does it mean that it has "a potential knock problem?" The reason is that my car is making some weird noise lately and it sounds like it's from the back. Any advice? Sorry I'm a noobie :D
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post #18 of 82 (permalink) Old 12-13-2009, 06:16 AM
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Originally Posted by eek View Post
Ummm I'm new here, but I was wondering what does it mean that it has "a potential knock problem?" The reason is that my car is making some weird noise lately and it sounds like it's from the back. Any advice? Sorry I'm a noobie :D
knocking happens when the combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The peak of the combustion process no longer occurs at the optimum moment and the shock wave creates the characteristic metallic knocking sound, and cylinder pressure increases dramatically. So when you use a lower octane gas, knocking happens...but because modern cars like ours have sensors that detect mixture levels the ECU reprograms to prevent knocking. So by using lower octane gas, your not doing any harm to the engine but it wont perform at its optimum peak, even head techs at Acura say they alternate between low and high octane gas, which is the best way to go...your weird noise is probably something in your trunk, check your spare tire and stuff, if its not, then have your car looked at

06 IS350 - C/TR202
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post #19 of 82 (permalink) Old 12-14-2009, 01:26 PM
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knocking happens when the combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The peak of the combustion process no longer occurs at the optimum moment and the shock wave creates the characteristic metallic knocking sound, and cylinder pressure increases dramatically. So when you use a lower octane gas, knocking happens...
So far so good...

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Originally Posted by vinconist View Post
but because modern cars like ours have sensors that detect mixture levels the ECU reprograms to prevent knocking. So by using lower octane gas, your not doing any harm to the engine but it wont perform at its optimum peak, even head techs at Acura say they alternate between low and high octane gas, which is the best way to go...your weird noise is probably something in your trunk, check your spare tire and stuff, if its not, then have your car looked at

And here it falls apart.

Your ECU has no idea what "mixture" of fuel the car has in it.

knock sensors don't detect fuel octane...if they did they'd be named octane sensors.

They detect knock

So you've already damaged your engine (very, very slightly) when they detect knock happening.

Because they can sense it so quickly, and report it to the ECU, the ECU can pull timing from the engine to stop it, also quickly, hence why you do only very very slight damage.


What happens next though?

The ECU is still programed for the timing/octane it came from the factory for.

So over time it'll add that timing back, figuring the knock was an abberation.

Then the car will knock again, doing very very tiny amounts of harm, again.

Then the sensors will notice, tell the ECU, and the ECU will pull timing again.

Repeat ad naseum.

So you probably COULD safely drive a car programmed for 91 octane on 87, and let the ECU and sensors keep doing this, for 50,000 miles.

but if you tore the motor down it'd look like it had 150,000 on it.

(Engines being as reliable as they are these days it might well still have lots of life left even then though).

But don't for a moment think you're not doing any harm running lower octane than your engine requires, even once.
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post #20 of 82 (permalink) Old 01-27-2010, 10:37 AM
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I always use 91 octane as that is the best grade of fuel we have around here. I do have a buddy that uses 87 octane on his car and whenever he shuts it off the car shakes a little bit. I am pretty sure this is because he uses crappy gas, but I was wondering if it could be any other type of problem.
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post #21 of 82 (permalink) Old 02-24-2010, 01:39 AM
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What if I use 93 octane on my '01 IS300? I swear by Sunoco petro and I hardly use any other gas station besides that solely because its one of the few, if any, that still has 93 octane. Will there be any negative side effects from using a even more refined fuel level?

Kudos,
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post #22 of 82 (permalink) Old 02-25-2010, 08:09 AM
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What if I use 93 octane on my '01 IS300? I swear by Sunoco petro and I hardly use any other gas station besides that solely because its one of the few, if any, that still has 93 octane. Will there be any negative side effects from using a even more refined fuel level?

Kudos,
Jay
Using a higher than required octane fuel will do nothing (compared to just using the required octane) other than maybe cost you more money.


The only thing octane means is resistance to knock. If your car doesn't knock on Octane X than running Octane X+ provides no benefit whatsoever.
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post #23 of 82 (permalink) Old 04-13-2010, 11:39 AM
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I still don't think I understand..
While I agree with most of the above, there are different levels of knock.

the knock sensor, as stated above safeguards the engine from knock by retarding the ignition and adding more fuel, this lowers fuel economy.
At the level the knock detector can hear I do not believe there is any damage being done, at the level where we can hear it yes there is potential damage, the first being melted or damaged piston crowns. Longer term piston land damage and maybe rings.

My pathfinder has "Premium preferred" rating , and most on the Nissan forums run regular, as I do, and have no damage in hundreds of thousands of miles.
Does it make a difference at end of life of the engine ? maybe.

My daughters 86 Audi 4000CS had 450K on it, engine never touched, and was still running great. we brought from first owner who was a friend. Yes i had to twist their arm to sell it to me.
Maybe with modern engines its not a factor.
Why do engines fail ?
Number one reason ( from a Toyota dealer and shop for older Toyota's ie not new dealership) leaks causing engine to run dry or hot !!! Mainly Oil leaks.
Turbo it and then its a different story, spun bearings seem number one !!!

2004 IS300 MSM LSD Manual ..Hail damage written-off so now grand daughters car
2003 Pathfinder SE 4WD
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post #24 of 82 (permalink) Old 04-29-2010, 09:18 PM
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Just to Piggyback off JayMarino i do think that using 89 vs 91 octane may cause a knocking because i didnt always use 91 and when gas got outragous i started using 89 gas from base and my car did start knocking.. i didnt know that gas could have an effect like that. I would normally use a fuel boosting suplement but guess that didnt help too much.. HEy jaymarino do you know if the knocking will go away????
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post #25 of 82 (permalink) Old 04-30-2010, 07:32 PM
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I use 91 here in Toronto anything higher is a waste of money
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post #26 of 82 (permalink) Old 04-30-2010, 10:14 PM
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Quote:
Originally Posted by Rux View Post
Just to Piggyback off JayMarino i do think that using 89 vs 91 octane may cause a knocking because i didnt always use 91 and when gas got outragous i started using 89 gas from base and my car did start knocking.. i didnt know that gas could have an effect like that. I would normally use a fuel boosting suplement but guess that didnt help too much.. HEy jaymarino do you know if the knocking will go away????


The definition of "octane rating" is the rating of the fuels resistance to knocking.

If you want the engine to not knock, use fuel of the required octane. It's that simple.


(and the "octane booster" you see in stores is crap.... when it states it raises octane "8 points" you might wish to be aware each "point" is 0.1 octane. So it turned your 89 into 89.8.
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post #27 of 82 (permalink) Old 06-16-2010, 07:06 PM
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I noticed on a bottle of octane booster it said it raises it 10 pts which would raise it from 89-90.

So if i use the lowest octane available and add the octane booster, it will still do damage to the engine?

I was under the impression that if the engine doesnt knock using lower octane than premium, than it isnt damaging it? Although you would loose a little power..
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post #28 of 82 (permalink) Old 06-17-2010, 02:19 PM
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I noticed on a bottle of octane booster it said it raises it 10 pts which would raise it from 89-90.
Correct... 10 "points" would raise it 1 octane.

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Originally Posted by is300fire View Post
So if i use the lowest octane available and add the octane booster, it will still do damage to the engine?
Yeah, because it's only raising it from 87 to 88 (87 being the lowest octane generally available) and the engine is designed to run on 91.

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Originally Posted by is300fire View Post
I was under the impression that if the engine doesnt knock using lower octane than premium, than it isnt damaging it? Although you would loose a little power..

You should always run on the lowest octane with which the engine never knocks.

For an IS that is 91, because that's what it's programmed to use from the factory. That's what the compression and timing are intended for.

Less than that and it will knock at some point (probably under load, when it's warm, or some combination of conditions... probably not when it's just sitting there idling).

You might not HEAR it do so, and your knock sensors will then pull timing to stop it pretty quickly... then gradually add it back... then it'll knock again, and repeat the cycle... doing a very tiny bit of damage each time.

The pulling of timing is where you "lose power" (or mileage in many cases, more than negating the 10 cent/gallon savings)

On the other hand, running anything higher won't help you at all.
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post #29 of 82 (permalink) Old 06-18-2010, 04:33 PM
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ok so i mistakenly put 87 in my tank. i know if i use an octane booster that wouldn't really help raise the rating. what if i use up 1/4 of the tank and put in 100 octane. would that compensate at least through the rest of the tank? noob question i know...

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post #30 of 82 (permalink) Old 06-18-2010, 06:50 PM
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unleaded 100 octane ?
add octane booster allowed for unleaded fuel, you should be able to raise it up.

2004 IS300 MSM LSD Manual ..Hail damage written-off so now grand daughters car
2003 Pathfinder SE 4WD
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