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Old 09-08-2004, 11:52 AM   #71
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Quote:
Originally Posted by Buzz38
Sure thing Doc. For a knock sensor to work it has to see detonation. Since you stated 400 times already that detonation is bad I will say we all agree with you on the point. Not until after the sensor sees detonation does it goto work so there is the first part of the problem. Once it sees it it signals the pcm and the pcm retards the timing. Octane is the easiest way for the owner to avoid detonation. By choosing to run the lower octane you are actually asking the engine to detonate and hoping it will survive by the pcm pulling enough timing. The next problem is what exactly does retarding the timing do? A little retarding will cause some power loss, but retarding the engine is like lugging it. It creates heat which in turn creates sludge, deposits, and can even lead to more knocking which means more timing pulled and so on. The fact of the matter is the knock sensor and retarding of the timing is only meant to happen for short periods of time to save the engine from instant destruction. The is nothing the system can do to protect itself from the long term abuse by someone too damn cheap to run the fuel that the manufacturer recommends or requires. Does this mean you are guaranteed to destroy your engine running low octane fuel? Nope, just like rotating the cylinder with one round in the chamber doesn't guarantee your brains on the wall. The question is how many times are you willing to try it? If you still don't buy it I will ask Dustin Whipple for his explanation. Maybe someone who designs and manufacturers blowers will have more clout in your eyes the ours. Until then.....
And just to back this up, I was at the track last Thursday with a clubgp buddy with a 12.5 second GP. I ended up racing him on the last run and he beat me by about .3 tenths (good reactino on my part). Come to find out the next day he chunked a piston becuase he went lean. His log showed the computer pulled every bit of timing it could (15 degrees). Did it help? nope... Low octane = lean, too lean = bad in a boosted car.

I've seen what happens when you have a lot of knock. That is why sooooo many people do whatever they can to get rid of EVERY little bit of KR they see while modifing our engines. A few blips here and there on a hot day will be saved by the knock sensors yes. Running 87 octane and relying on the PCM and Knock sensors to save you from a constant 8, 10, 15 degrees of knock is absurd, ludicrious, and flat out wrong.

Its that simple. I'm done with this.
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Old 09-08-2004, 12:23 PM   #72
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i must say everyone this is the best topic i have ever read here this should be in the hall of fame and saved as a sticky or somethin
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Old 09-08-2004, 12:31 PM   #73
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I think the ROOT disagreement here is whether or not running lower octane fuel will DAMAGE (or rather CAN damage) an L67. Short of that argument, it'* just a thread of clarifying the mechanics of combustion and nailing down the terminology.

So in light of the DAMAGE question, I'd ask DrJay this:
(I love this part..... and yes, this is a good debate)

Are you willing to do a long-term 87-octane test in your car to see if damage will occur or not?

I'm not. And this needs to be firmly stated. On the CHANCE that it might cause damage, we cannot suggest or condone the use of any octane lower than 91 octane in an L67-powered car. It'* a liability concern, and we're ultimately responsible for it. Until we PROVE in an L67 long-term that 87 octane is ok, we're not in a position to say that it is.
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Old 09-08-2004, 12:43 PM   #74
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There need'* to be tests done with a modified car and a stock one, to have an accurate diagnostic. 1 constant would be the lower octane, and the two variables would be Bill'* car and then just a regular SC Bonne from the same year. Im sure two different numbers would appear, and clarify something up.
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Old 09-08-2004, 12:50 PM   #75
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Somehow I don't think you will get Bill to run 87 in his car.
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Old 09-08-2004, 12:52 PM   #76
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Quote:
Originally Posted by Rogue
Somehow I don't think you will get Bill to run 87 in his car.
It dont even have to be his, just a modded Bonne of some sort, with the same engine as the stock one.
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Old 09-08-2004, 12:53 PM   #77
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DrJay'* car is out for the test. I was talking with him last night (continuing this friendly debate on Yahoo Messenger). His car is ported and cammed. Totally different issue. He should be getting less KR than me on equal footing as far as octane and conditions.

It'll affect him less than me.

And no, you won't get me to run it! 91 is as low as I go, and that'* only in CA when I can't find 92 And there'* usually a gallon of magic juice in the trunk to boost it with.
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Old 09-08-2004, 02:33 PM   #78
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Wow, this thread got carried away while I wasn't looking . I hope I have a chance to add my $0.02...some of it fact and some of it opinion and I'll differentiate the two.

Fact:
Knock happens first, which the Knock Sensors hear to which the PCM responds by retarding the timing.

Opinion:
This is a bad thing because knock happens first and then the timing is 'corrected'. Using Knock Sensors is a reative method to deal with a destructive problem, using higher octane fuel where required is a proactive method the dealing with knock.

Fact:
The PCM can only retard timing by 15 degrees maximum.

Opinion:
Part throttle conditions can sometimes have ignition advance settings of 35 degrees, can the PCM pull sufficient timing to eleviate any knock?

Fact:
The Series II L67 hypereutectic pistons are not strong. (Not sure about the Series I).

Opinion:
Under normal operating conditions the pistons are fine. When subjected to very high heat from lean conditions or extra load from pre-ignition or detonation, the weakness in the piston around the dish near the upper ring land is subject to fail. The upper ring land is very high up on the piston for emissions purposes but is a weak point from a high performance perspective.

I would expect that either coating the top of the piston with ceramic would help and that forged pistons would take more 'abuse' than stock pistons.

More Opinion:

For my money I would rather not be behind the 8 ball and rely on the knock sensors and PCM as my only means of dealing with knock. I would be proactive in using higher octane fuel and working to eliminate causes of knock before they happen.

Of course the other side of the coin is for engines that do not require higher octane fuels should not use them. When an engine is designed for 87 octane fuel, it likely does not have the capability to use extra ignition timing advance to take advantage of the fuel. In that case, higher octane fuel will burn slower and contribute to carbon build up and high emissions.

Cheers,
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Old 09-08-2004, 03:19 PM   #79
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since the lines have been drawn, and it'* been decided that this is about whether knock will harm your engine, not about whether it'* good or not (huh??), let'* just look at your first statement on the issue:

Quote:
Originally Posted by DrJay
Hehe I'll assume you meant detonation, as pre-ignition is a whole separate issue
From what I've learned these statements are half right half wrong. As luck would have it we are equipped with a knock sensor. This will pick up detonation and retard timing in the milliseconds. This happens that fast. If you put lower octane in you won't hurt your engine but you will lose some performance as the computer retards timing to combat knock. This problem will be bigger in higher mileage cars as the carbon buildup requires higher octane than when the engine was new so you'll lose that much more power. This isn't harmful to your engine though so if you need to put lower octane in things should be fine. If you do experience a very harsh running engine and detonation its the knock sensor/PCM'* fault really.
in order for a knock sensor to retard timing, it must first pick up detonation.

end of story.

really.

if you have detonation, it WILL shorten the life of your motor. milliseconds of detonation may not mean very much as far as the engine'* useful life, but it will still shorten it'* life. the problem, it would seem, is that there seems to be the mindset that a little bit of detonation isn't bad. sorry, but any detonation is bad for your motor. when the force of your detonation is in the wrong direction, it is putting additional stress on your engine internals. so unless you have a motor that maintains zero wear, regardless of load, you're causing pre-mature wear on your engine by having *any* knock to begin with.

it'* just a matter of how much it is. by the time the knock sensor retards your timing, there has been verrry little damage done. but that doesn't mean that there was no damage done.
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Old 09-08-2004, 03:46 PM   #80
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Guys, I would like to chime in here on the quality of the discussion. This is a topic that could have easily gotten into personal insults. I have seen it time and again on many different forums. I would just like to thank everyone and say I am very glad I found this forum. (I'll save up the personal insults for those other forums)
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