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Old 08-27-2004, 02:41 AM   #21
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Quote:
Originally Posted by willwren
I'm up for this debate, but mainly because I'd like to know. I was under the impression that carbon buildup would be more common, as well. Strange magic goes on in the cylinders when the engine is running. Hell, I've got a MESS of fouled plugs right now because of another crappy alternator, causing a weak spark. The RATE of ignition has an enormous effect on cylinder/plug cleanliness.

So let'* talk about what that higher octane provides. It ignites at a HIGHER temperature. This is of benefit to us with Blowers, as we don't PRE-detonate. Now the question is, does it burn quicker? If it burns quicker, we get a better burn, leaving less crud in the cylinders.

If it DOESN'T burn quicker, we may have just lost the argument.
Yeah I love learning myself, thats why I wanna debate. Maybe someone knows something about it I don't, maybe Iknow something they don't, then we can both walk away with something...

I wanna start out with a couple definitions. You're the second person I've seen say pre-detonate. There are two terms relating to what you're referring to. The first is pre-ignition. The second is detonation. There is no pre-detonation, that would mean before detonation. Its like saying pre-takeashower. The difference between the two is when it happens.

Pre-ignition (as you may guess) happens before the spark plug ignites the mixture..hence PRE-ignition. This is caused by several things including sharp edges in the combustion chamber, plugs way too hot, and excessive carbon buildup. Its also the most deadly as not even forged pistons can last very long (1second tops). This is because the explosion is happening as the piston is on its way up to compress the mixture. This is when you see holes burned in the center of pistons. This is because its the weakest part of the piston and its just not 'easy' to compress an explosion.


Detonation, on the other hand, is a lesser known phenomenon where the mixture actually ignites twice in two separate areas. Generally just above the piston and of course, at the spark plug. When these two flamefronts collide they rattle the entire engine with their force. This is the noise you hear. Its not actually the two colliding but the resonating sound of the engine block and heads rattling. Crazy huh?

Now that thats out of the way we can go on to the octane.

First off no, higher octane actually burns slower and in a more controlled manner. The slower burn allows for more advance as it won't fight with the piston on the compression stroke as much. Every engine fights the explosion with the last bit of compression because of the time needed to get the actual explosion going. The explosion is not instant and some amount of time is needed to initiate the explosion before it accelerates rapidly towards the piston. This happens at 0.1" from the initial spark. This is why larger gap is better as the ideal gap would be 0.1". This isn't possible with current technology because the power needed would cause whats called cavitation where the spark is actually powerful enough to 'punch a hole' in the mixture and not ignite anything. They're working on a few that are capable of getting closer though and I've heard rumors of one of them available now.

That link I posted above, on the second page, addresses the carbon buildup misconception as do several other pages so I won't get into it here.


Edit: misread, thought you meant it burns hotter not it allows more heat before igniting. Myyy baad. We now return you to your regularly scheduled debate...

The actual energy differences between high and low octane is insignificant. Race gas actually has less energy per given weight than pump gas..crazy I know let it sink in ....The difference is how easy it is to ignite and how quickly that ignition spreads. Thats...about...it.

Anyway thats basically why higher octane allows for further advance. I hope that was clear enough.
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Old 08-27-2004, 02:56 AM   #22
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Quote:
Originally Posted by Rogue
Lower octane will increase knock in boosted cars. Because the lower octane gas burns at lower temps the airfuel charge is ignited from more than one source (the plug) and the charge is not consumed cleanly and thus is less efficient. This leaves unburned fumes (as well as exhaust gas reinserted from egr which does have carbon) and will buildup over time. Add to this fact that most of our cars are boost stacking even from the factory and realize that air in the cylinder is not being ejected fast enough anyways you have more of a propensity for inefficent combustion.

Knock WILL hurt the engine and over time you will burn/chip/break a piston.
Detonation generally burns out the ring landings, not usually something to destroy a piston entirely. It leaves that to pre-ignition. Anyway...

Saving my fingers from typing a rebuttal I'll just ask a question. Wouldn't, if the fuel wasen't burning 'cleanly', the o2 sensor pick this up and lean the mixture. If that was the case wouldn't you see better gas mileage with lower octane?
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Old 08-27-2004, 08:47 AM   #23
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Quote:
Originally Posted by DrJay
Wouldn't, if the fuel wasen't burning 'cleanly', the o2 sensor pick this up and lean the mixture. If that was the case wouldn't you see better gas mileage with lower octane?
In a perfect world yes, but as you stated before, because of timing pull, and lack of HP more fuel will be required to move the car at the same rate as a non-knocking boosted car running higher octane.

Also, race gas while harder to burn is often oxygenated so the fuel charge will have more power than standard high octane (93-94) gas.
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Old 08-27-2004, 09:40 AM   #24
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Quote:
Originally Posted by Rogue
Also, race gas while harder to burn is often oxygenated so the fuel charge will have more power than standard high octane (93-94) gas.
As much as I'd like to agree with you thats simply not true.
Go here: http://www.klotzlube.com/support/tec...on/octane.html
Read the second and third sentences.

I've found the same info on a number of web pages, thats just the first I could find at 5:30am Look further though.
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Old 08-27-2004, 10:48 AM   #25
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Quote:
A gasoline with a higher octane rating will produce less BTUš* when burned than a lower octane gasoline. Higher octane ratings allow racers to run higher compression ratios in their engines without detonation or pre-ignition to create more heat energy, which means more horsepower.
Nothing in these lines contradicts my statement.

In fact, our cars with learning computers will add and subtract timing based on among other things, knock detection. Without the protection of the computer, running lower octane gas in our boosted cars would be worse than it is. Also note that i'm sure this is not referring to boosted cars. We add a ton of oxygen into the air/fuel mix by compressing the air charge.

If race gas has added oxygen does it not act just like a supercharger? A supercharger increases the amount of air (and hence the oxygen content) of the air/fuel mix. This is what gives us the horsepower. Here is the spec of Sunoco'* Race Gas (100 octane)

http://www.racegas.com/fuelspecs/default.asp

Notice the added oxygen found in unleaded.

Taken from your same Klotz site

Quote:
Originally Posted by http://www.klotzlube.com/support/tech_information/fuel_mixture.html
Another variable that affects the performance and octane requirements of your racing engine is humidity and barometric pressure. Low humidity or dry air will lean out the mixture, requiring a higher octane gasoline to prevent detonation..
Notice in this octane chart from the klotz site that the recommended octane for our 8.5:1 compression engines is 93 octane?

http://www.klotzlube.com/support/tec...ion/ratio.html

Lower octane gas in a boosted vehicle (already adding more air and hence causing it to be leaner by default) may cause the car to go too lean and again you WILL blow a piston. Doesn't the 02 sensors and computer system overide this leaness and add more fuel? To a point yes, but at WOT the fuel trims are locked and the 02 readings are ignored and this is the point where low octane gas in a boosted car becomes damaging. The fact is there is only a set amount of timing degrees the pcm can pull (stock is 15 on an 00 pcm) and if your going WOT with 87 octane the computer can TRY to compensate to a point by pulling timing but as heat builds in the engine and cylinders get hot spots and detonation happens rapidly there is nothing the computer can do richen up the mixture and the end result is very bad.

While debating can be enlightening and fun, giving the expectation (especially to newbs) that anyone in an SSEi can run whatever octane they want is irresponsible and incorrect. There is a reason why it says run only premium gas in your manual and on most of our gas caps.
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Old 08-27-2004, 11:36 AM   #26
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Id throw in my .02 about the debate, but I figure this. I highly doubt GM engineers would recommend a fuel grade for no reason =). Compression has a big effect on which octane to use because the higher octane gases have less chances to pre-ignite, a theory everyone knows. Although a big deal of combustion and the efficieny has to do do with the design of the combustion chamber. I beleive it was jaguar who made an engine that had 11:1 compression and ran on 87 octane fuel. LT1 has 10.5:1 compression and runs on 87 octane, although with pcm work and higher octane you can obtain alot of stolen HP
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Old 08-27-2004, 01:31 PM   #27
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Quote:
Originally Posted by Rogue
Quote:
A gasoline with a higher octane rating will produce less BTUš* when burned than a lower octane gasoline. Higher octane ratings allow racers to run higher compression ratios in their engines without detonation or pre-ignition to create more heat energy, which means more horsepower.
Nothing in these lines contradicts my statement.



We add a ton of oxygen into the air/fuel mix by compressing the air charge.

If race gas has added oxygen does it not act just like a supercharger? A supercharger increases the amount of air (and hence the oxygen content) of the air/fuel mix. This is what gives us the horsepower. Here is the spec of Sunoco'* Race Gas (100 octane)

http://www.racegas.com/fuelspecs/default.asp

Notice the added oxygen found in unleaded.

Taken from your same Klotz site

Quote:
Originally Posted by http://www.klotzlube.com/support/tech_information/fuel_mixture.html
Another variable that affects the performance and octane requirements of your racing engine is humidity and barometric pressure. Low humidity or dry air will lean out the mixture, requiring a higher octane gasoline to prevent detonation..
Notice in this octane chart from the klotz site that the recommended octane for our 8.5:1 compression engines is 93 octane?

http://www.klotzlube.com/support/tec...ion/ratio.html

Lower octane gas in a boosted vehicle (already adding more air and hence causing it to be leaner by default) may cause the car to go too lean and again you WILL blow a piston. Doesn't the 02 sensors and computer system overide this leaness and add more fuel? To a point yes, but at WOT the fuel trims are locked and the 02 readings are ignored and this is the point where low octane gas in a boosted car becomes damaging. The fact is there is only a set amount of timing degrees the pcm can pull (stock is 15 on an 00 pcm) and if your going WOT with 87 octane the computer can TRY to compensate to a point by pulling timing but as heat builds in the engine and cylinders get hot spots and detonation happens rapidly there is nothing the computer can do richen up the mixture and the end result is very bad.

While debating can be enlightening and fun, giving the expectation (especially to newbs) that anyone in an SSEi can run whatever octane they want is irresponsible and incorrect. There is a reason why it says run only premium gas in your manual and on most of our gas caps.
Let me start at the bottom of your page. As you may have noticed lately I've been putting a few myths to rest, most notibly the whole 'backpressure' issue. I think letting people believe this is true is more irresponsible than bringing up facts to discount the MYTH.

Yes it does say to run premium, sure why not! They want you to have the maximum performance available for the engine and why not when its not comming out of their pockets. The manufacturers actually use a term called MBT (Maxiumum spark for Best Torque) and this is where the engine is set to run at. This is set high (advanced) for our engines which is why they actually recommend the higher octane. Its not NECESSARY but only for performance of the engine.

We don't add a ton of oxygen to the air/fuel mix by compressing it. I'm curious about what you mean by that statement as ours run slightly richer than a comparable n/a engine. Why do you think our BSFC is higher than n/a? Because we run richer! This helps combat detonation by cooling the chamber. So no, we don't run leaner.

Now obviously our cars have more air pressed into them by the supercharger but we use fuel to match it so its not like our engines add 'extra' oxygen by forcing air in. They run what, 460cfm? If you were to find an engine that naturally pulls 460cfm you'll notice that we both burn about the same amount of fuel....maybe I'm just not getting what you're saying though.

Detonation is not a damaging issue with our engines because we have a sensor to take care of it. If we didn't have any sensors at all on our engine then yeah sure, running 87 might cause some damage over time. But thats not the case!

If you have an older distributor type engine try it out! Put premium in it and advance it as far as you can. Listen for knock then back off 1-2 degrees. Then go put 87 in it and back it off 15degrees. $1000 says you don't have detonation. And you think our PCM'* can't effectively combat knock? Our engines aren't even running at the maximum advance possible, why do you think people like Jet can be in business? Almost NO engine runs at the maximum spark for the rated fuel. Why else do you think this is a common "first mod" for distributor based engines? So really when doing the experiment with premium you should back off probably more like 5-10degrees then continue. That would put it more like 20-25degrees from where it would knock. You'd have to run something more like 70octane to get it to detonate and overcome the PCM'* ability to compensate.

Like I said before, if you run low octane and you detonate you need to replace your sensor or PCM cause thats probably whats causing the problem. Other factors like excessive carbon deposits and failing sensors can contribute also. But we're not going to reduce the debate to "Oh well my engine has 210,000miles and every sensor is failing, can I run 87 octane??"

Higher octane is ONLY a detonation inhibitor, thats ALL! It does not burn hotter, it does not burn cleaner, it does not burn stronger, it does not create power it ONLY helps with detonation. I realize old habbits die hard but a myth is a myth no matter how common...You know bats aren't really blind?
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Old 08-27-2004, 01:34 PM   #28
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You're correct about them burning the same. But some Gasoline makers have added more addititives to higher grade gasoline, then ours. I haven't seen it in MN since i started filling up as a wee lad, but ii'm sure it'* still out there.

The engine can run any kind of Octane, it just has to be setup for it through timing.
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Old 08-27-2004, 04:07 PM   #29
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Quote:
Originally Posted by Rogue
Quote:
A gasoline with a higher octane rating will produce less BTUš* when burned than a lower octane gasoline. Higher octane ratings allow racers to run higher compression ratios in their engines without detonation or pre-ignition to create more heat energy, which means more horsepower.
Nothing in these lines contradicts my statement.
.
Hah as long as my post was I forgot to address this portoin of yours. I found something that better describes it. For the sake of searching I'll post the important part here and the link so you can see for yourself:

"It should be noted that because oxygenates contain oxygen that can
not provide energy, they will have significantly lower energy contents.
They are added to provide octane, not energy. For an engine that can be
optimised for oxygenates, more fuel is required to obtain the same power,
but they can burn slightly more efficiently, thus the power ratio is not
identical to the energy content ratio. They also require more energy to
vaporise."

Found on: http://www.uvi.edu/Physics/SCI3xxWeb...solineFAQ.html

Next up:

"Oxygenates are just preused hydrocarbons . They contain oxygen, which can not provide energy, but their structure provides a reasonable antiknock value, thus they are good substitutes for aromatics, and they may also reduce the smog-forming tendencies of the exhaust gases [15]. Most oxygenates used in gasolines are either alcohols ( Cx-O-H ) or ethers (Cx-O-Cy), and contain 1 to 6 carbons. Alcohols have been used in gasolines since the 1930s, and MTBE was first used in commercial gasolines in Italy in 1973, and was first used in the US by ARCO in 1979. The relative advantages of aromatics and oxygenates as environmentally-friendly and low toxicity octane-enhancers are still being researched."

Edit: Also:
"However, on vehicles with engine management systems, the fuel volume will be increased to bring the stoichiometry back to the preferred optimum setting. Oxygen in the fuel can not contribute energy, consequently the fuel has less energy content. For the same efficiency and power output, more fuel has to be burnt, and the slight improvements in combustion efficiency that oxygenates provide on some engines usually do not completely compensate for the oxygen."

Found on: http://www.repairfaq.org/filipg/AUTO...l#GASOLINE_015

I hope these two explain how oxygenates don't make you more power.


BTW! Don't let me kill this thread! Debate back if you have something to contribute!
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Old 08-27-2004, 05:21 PM   #30
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I agree with DrJay on this one. I have the original owners manual, and it states that 91 or above is recommended, but that 87 can be used, but it will affect acceleration. It also says that a light pinging when under heavy load is normal, but a heavy knocking is bad.
This is right out of the owners manual.

In my case, I bought my car with 10K miles and is up to 57K now, and used 89 for most of that. I don't drive it hard most of the time. Usually, on a long trip, I'll run 91. Recently I switched to 91 all the time to see if it would help my mileage. I'm still not sure it makes a difference in my driving, 100% city driving most of the time, very light acceleration. Considering 91 is 10% higher than 87, if I don't get 10% improvement, I'm wasting my money. I'm thinking of trying some 87 at my next fillup.

Now if I was into lead footing my car, I would certainly recommend premium. In the end, you should use what meets your requirements.
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