Is 100 Octane worth it? - Pontiac GTO Forum
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post #1 of 11 (permalink) Old 08-23-2004, 09:19 AM Thread Starter
 
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Is 100 Octane worth it?

Anybody have any hard data to show if 100 octane gas (and gas alone) is worth the $3.959/gallon?

I put some in this weekend, and I'm not sure if the seat-of-the-pants feel is just wishful thinking or not.

Larry
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post #2 of 11 (permalink) Old 08-23-2004, 10:21 AM
 
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If the knock sensors are not picking up any signs of detonation or preignition with your regular pump gas, then 100 ocatane fuel will slow you down. The higher the octane, the slower the burn rate. If the computer IS pulling out timing at WOT with pump gas, then you do stand to gain some performance from the extra timing that the octane will allow the system to use.
The lowest octane (fastest burn rate) fuel you can operate on WITHOUT detonation, will give you the greatest power potential.
It's not a concern on a late model injectd car, but on a carbureted car, especially here in CA, the oxygenated fuel requires more jet to achieve the same A/F ratio as non oxygenated fuel. So you start mixing some race gas and pump gas and you are changing the jetting and timing requirments both.
I know you said "hard data", but I can only share with you my experiences with a car that does not tune itself. I would typically run 1-2 tenths slower and loose 1-2 MPH by changing from 91 premium to a 50/50 91-110 mix with no other changes. Jetting and timing adjustments probably would have made a difference, but I chose not to at the time.
I am curious to hear other ideas and opinions on the subject.
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post #3 of 11 (permalink) Old 08-23-2004, 03:12 PM Thread Starter
 
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Is 100 Octane worth it

My "assumption" is that as the Grand National's did, the computer will advance the timing until it senses knock, then back down a notch. The higher octane, the more knock resistant, so the more advance the timing can get, and that alleges to be a good thing for acceleration.

My ex-wife put a tank of mid-grade in my Grand National once. Ugh! The difference between that and Premium was VERY noticable. I guess I was expecting an even bigger difference between 93 and 100 octane, but I don't think I felt that.

Of course, the mid-grade made the GN ping something horrible on even mild acceration, so maybe once you get to a "workable" octane, the difference does make as big a difference.
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post #4 of 11 (permalink) Old 09-15-2004, 09:00 AM
 
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That is how my SRT-4 is set up. The higher the octane the computer will bump up timing till it senses pre detonation. It runs on 93 great, so I don't feel a need to run 100 till I do more modding. BOOST HERE I COME!
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post #5 of 11 (permalink) Old 09-15-2004, 07:09 PM
 
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Larry M,
I can assure you the spark in your GN was not unlimited until knock was detected. There was base max spark advance. The GTO is the same. If it is not knocking on 92 octane, you will not get more spark advance with 100 octane.
The GN was a pressurized system in that the turbo was capable of adding more pressure than atmospheric. Such engines can and do have a very fine line between knock and no knock, even at part throttle. ( see the note below, but boosted engines almost always are knock limited. That is knock occures before MBT, under boost. That is not the case with non-boosted engines )And When your GN was knocking at even part throttle there was some spark removed for a longer period of time over the entire spark range, than the present systems. Therefore the 100 octane may have "seemed" to have more spark at WOT with greater octane but the fact is the 100 octane kept your spark curve closer to the maximun (in the ECM) at wide open throttle. The newer knock systems use a more sophisticated method from those in the GN. ECM are getting more memory all the time. Turbo's also have the drawback of adding heat as more and more of the air is compressed and injested back into the combustion chamber. 100 octane also helps that as the burn rate is slowed.
The combustion heat issue grows exponetially as boost is added for a turbo, but is more of a linear function for the LS1. Meaning 100 octane does not yeild the same gain in horsepower that it might have in the GN. When you consider the max. spark in the LS1 is optimized for 92 octane, no more spark is available with added octane for any increase in output.
One last thought. Many people assume the knock limit is max. power limit for spark advance. Depending on chamber design, camshaft, and many other design factors, there are many areas in the speed and load range where the Knock Limit Spark is indeed greater than MBT Spark....That is one reason ( there are several other good ones ) why engines are not allowed to merely run up to the knock limit of the engine.
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post #6 of 11 (permalink) Old 09-16-2004, 01:37 PM Thread Starter
 
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I'm not sure I followed every thing you said. Let me simplify and paraphase to see if I'm getting it:

I certainly agree that the boosted GN engine is going to "behave" differently than the LS1, and the designers no doubt had to account for that. BTW: I had a 1962 Olds Jetfire - also turbo charged so I remember that the biggest problem that boost causes is pre-detontation - aka knock.

While I never put 100 octane in the GN (didn't even know it existed till a few weeks ago), it was considered "a fact" in GN circles that the more octane, the more the spark could advance, and thus the more HP. So many GN owners used octane boosters.

I sort of assumed the same would be true in any car, particularly one with a computer running the show (as opposed to mechanical devices, like jets on a carb that might need to be adjusted, or a distributor that needed to be twisted).

When you used the word "unlimited" spark advance, then I can see where the designers wouldn't give the computer that far to wander. Probably enough to take care of normal variations, perhaps centering on the 92 octane?

So in a nutshell: If I were having pre-detonation/knock problems with 92, THEN switching to 100 would get a performance boost, but as long as I'm not, then the $4/gal isn't going to make the car any faster.

Yes?

---Larry
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post #7 of 11 (permalink) Old 09-16-2004, 04:43 PM
 
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LarryM,
You've got the main idea. There is a max spark curve in the ECM software. That map is done with 92 Octane. If 100 Octane would allow more spark ( in theory for more power) that main spark map would still be a limitation, therefore no more HP would be available. ( Again assuming no knock )

This is more technical but there is no pre-detonation as a term as all spark before detonation is "pre". There is detonation and there is per-ignition.
Detonation is the uncontrolled combustion of the "end gasses" during the burn process. Detonation is what you can hear as knock. As the flame front burns the gas mixture in the chamber (as started by the spark plug ), if the octane is too low the heat and pressure of combustion will cause a mini-explosion in the combustion chamber of the unburned mixture called the end gases. This is heard as a ping of some degree and also loses power as the fuel cause a very high spike in combustion chamber pressure. Over time detonation can and will "peck away" at rings and pistons, sort of eroding the surfaces. Long term detonation is a bad thing for any engine.
Pre-ignition can be inducted from detonation. Pre-ignition is the lighting off of the combustable mixture in the chamber before a calibrated spark event. Detonation can cause "hot spots" the the chamber, such as carbo deposits to become hot enough to light the mixture very early. Normal combustion happens mostly after top dead center in an expanding cylinder area, ( even though the spark event is BTDC before top dead center ) even thought the partially unburned gases are being compressed as the mixture burns. During pre-ignition the piston is still moving upward under compression and the majority of the fuel is burned as the piston rises. It's like compressing a blow torch, with the same result. Pre-ignition does not last long before pistons are "holed". And pre-ignition cannot be heard.
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post #8 of 11 (permalink) Old 09-22-2004, 09:21 PM
 
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Not to get off of the subject here, but since there has been some good discussion I wanted to ask how everyone feels about Ethanol blended gas @ 92 octane?
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post #9 of 11 (permalink) Old 09-24-2004, 01:09 AM
 
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Quote:
Originally Posted by GTO TOO
LarryM,..... This is more technical but there is no pre-detonation as a term as all spark before detonation is "pre". There is detonation and there is per-ignition ....... Normal combustion happens mostly after top dead center in an expanding cylinder area, ( even though the spark event is BTDC before top dead center ) even thought the partially unburned gases are being compressed as the mixture burns. During pre-ignition the piston is still moving upward under compression and the majority of the fuel is burned as the piston rises. It's like compressing a blow torch, with the same result. Pre-ignition does not last long before pistons are "holed". And pre-ignition cannot be heard.
100%

Larry... Funny you bring the Grand National into the picture (you would bring in the exception to the rule situation). Being a forced induction engine their operation is based on slightly different principles. The air being forced in the engine by means of a turbo charger creates higher compression yeilds in the cylinder during boost. Also higher temportures... being that the air is plumbed through the exhaust and presurized. Both of these situations are contributors to pre-ignition problems, and to an extent can be remidied by using higher octane fuel.

However... as GTO TOO said... Your wasting your money if you chose to do the 100 octane thing with your current set up. I get the feeling that some day in the future... you will need the 100 octane. ... Then I'll meet you on the track.
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post #10 of 11 (permalink) Old 09-24-2004, 09:05 AM Thread Starter
 
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My thanks to George and GTO Too, while tempting, probably from over bearing product marketing ("My car must go faster because I buy this expensive fuel for it!") I understand that as it stands, there's no reason to double my fuel costs, since the engine is running perfectly fine on normal Premium.

thanks guys! --Larry
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