Premium Gas?

[BigMike]

I asked someone in the oil business about mixing the octanes and told him about what I'd read in the thread here. This is his response...
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Hi Mike,

You are reading a half truth. When gasoline is manufactured, it is regular gasoline with an octane rating of 87. Then octane processing is the final step where the mid/plus and high/premium octane are created.

There are truly different grades of gasoline because the burn rate in relation to the engine is critical. It allows first for clean, maximum burning in the cylinder to minimize pollution. It allows the engine to get maximum horse power and torque. If a lower grade gasoline is used that burns faster, it can possible cause damage to the valves over time, not to mention screwing up mpg ratings.

Now concerning the federal fines . . . not all states test gasoline. I live in Texas and it is not done here. However, states that do will find the gasoline in the wrong tanks and fine someone. I have never researched federal guidelines because this is an issue that is usually handled at the state level. I guess I need to look into that.
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I'm back.

I was planning on alternating between 91 and 89 once my tank gets to half full. I'm not sure now.

 

[BigMike]

I think he's referring to the underground tanks at the stations, not your car.

 

[wnewport]

Leaded

It does mix. The octane rating is a rating based on the ratio of octanol to heptanol in the fuel. Octanol is a molecule that resists detonation/combustion, while heptanol does not resist detonation/combustion. More octanol = more detonation resistance = higher octane rating. Like mixes/dissolves in like, meaning the octanol/heptanol in 89 or 91 will mix together fine. And because they're a ratio, the average would be what you would get if you mixed it 50:50.

I actually remember some chemistry from my education, hope that helped.

As an aside, I remember from the leaded gas days that due to some strange chemistry thing if you mixed leaded and unleaded you got an octane higher than either of the originals. Can't remember why it was so though.

 

[bendrover]

Leaded gasoline didn't get mixed in the old days because then you would be producing a lessing of the octane value. Leaded gasoline had an ocatane value that was around the 105 mark. It was phased out after '71 because of the creation of catalytic converters - lead destroys the catalyst.

Today we use oxygenates like alcohols and ethers to maintain the high octane values. I think at one point benzene was an additive in the mid-1970's.

IN the OLD OLD days the lead was a good thing... the person you know with
the '56 vette for example needs to be using an additive to help the cylinders as in the old days lead would create a deposited cushion around the cylinders so the engine would run smooth and not eventually bang itself to hell. The "knock" they used to call it. And going back to the 20's iodine was added just for this purpose as the best thing.

Methyl tertiary butyl ether, or MTBE is probably the most common thing used today to keep the octane high while maintaining emissions. It's blending properties are quite outstanding as a fuel component. It has low residue and the mixture creates minimal deposits regardless of octane rating.

Personally, I think high octane requirments are really only necessary in very high end engines that are utilized for the purpose of speed. So really unless I really want a bit more kick from my engine I'll drive prem. but I'm okay where I'm at with the low end octane. (98000+ miles and everything under my hood sparkles - but I do regular flushes/cleanings.)

**I'm not a chemist, I just play one on TV.

 

[DonA]

I will drop my 2 cents because there seems to be a lot of confusion surrounding the need for 91 octane fuel in the 4.4L V8 LR3. The octane rating of fuel has already been covered quite well in earlier posts so I will cover the reason why LR put the sticker is on the fuel door.

I am by no means an expert but I have put quite a bit of time into studying the subject while I was into modifying engines. There are many other variables that engineer’s make a living from by studying this is a brief overview. I will do my best to explain…

Higher octane requirements of modern engines generally have little to due with fuel additives for cleaning. Yes premium gasoline may contain more detergents than regular, but it’s about octane, the measure of the fuels ability to resist detonation that is important here. The detonation resistance of a fuel is a key factor when extracting the maximum safe power from the given engine displacement.

In the competitive SUV & truck market, horse power has become a big marketing tool for vehicle manufactures. After all bigger is better, right?

One look at the engine spec page on Landrover.com tells me the 300 horsepower; 4.4 liter V8 in the LR3 has a 10.75:1 compression ratio. I don’t even have to look any further to know that generally a CR that high will create cylinder pressures great enough to need 91 octane or better to avoid detonation in some driving environments.


Detonation or engine knock, what ever you would like to call it occurs simply when fuel pre-ignites before the piston reaches scheduled spark ignition. This means that a powerful explosion is trying to expand a cylinder chamber that is shrinking in size, attempting to reverse the direction of the piston and the engine. When detonation occurs, the internal pneumatic forces can actually exceed 10x the normal forces acting upon a properly operating engine.

Detonation is generally caused by excessive heat, excessive cylinder pressure, improper ignition timing, inadequate fuel octane or a combination of these. Of the previous, excessive heat is usually the culprit.

Another problem with detonation is when it occurs it creates even more heat that can snowball out of control. Left unchecked Detonation can melt pistons, burn valves, and destroy the engine in seconds (Not over time as some suggest).

To combat meltdown the engines computer does a few things, it pulls back spark timing, and dumps in more fuel to cool down the cylinder temperature. These measures are usually enough to put detonation in check. However if the computer continually must dump in more fuel to control detonation your gas mileage will be very poor. A rich fuel mixture will also lead to carbon build up over time as well as increased emissions.

Now if you fill-up with 87 octane are you at the verge of nuclear meltdown? Well, it really depends on the situation. Land Rover cannot control the environment you or I drive in, and therefore must tune the engine for the best compromise of fuel efficiency, performance, as well as having a safety buffer for extreme operating conditions.

We already know that excessive heat is a major cause of detonation so if I am climbing a steep hill with a full load of gear towing my 22’ bass boat on a day where the outside temp is 105°F. I think it’s safe to say we are putting a heavy load on the engine.

Heavy loads increase cylinder pressures. Higher pressures create heat, combined with the already hot outside air temp we have the perfect recipe for detonation. In fact I wouldn’t be surprised to have some minor detonation now and then even with 91 octane but well within the parameters the computer can control with timing and fuel delivery. However if I had 87 octane in the tank I might be in for some trouble.

On the other hand a trip to the bank on a 35°F day is much different. Light load, cooler outside air temp = less pressure, less heat. So I could use a lower octane fuel if its cooler out and I don’t intend put a load on the engine.

Personally I run 89 octane in the winter months here in NJ, I feel it’s probably more than safe for going back & forth to work when it’s 30°F - 40°F outside.

One could argue the cooler air is more dense and would increase pressure but so far I have not detected any detonation using 89 octane in the cooler months during normal driving conditions. As the weather warms though I will be back to 91 or better, just to be safe.

Your experience may vary; understand what you are doing before you go against the manufactures suggestion on a $50,000+ vehicle.

After all this typing I hope this helps some of you understand a little bit more about what’s going on.

 

[beemer]

I see many people don't know the difference between preignition and detonation as they mix them up as well high compression numbers which mean nothing! Static compression is what engine manufactures post and most believe, dynamic compression is the compression the motor sees with variables like intake valve closing position induction pluse at certain rpm's as well temperatures. Your BMEP numbers will tell the story.
.....=o&o>.....

 

[DonA]

Pre-ignition is the ignition of the fuel charge before the spark plug fires

Detonation is the resulting explosion

What does it matter? It's like me saying "she had a baby" and you saying "no, she was pregnant and then gave birth".

I was not trying to write an internal combustion engine theory book here. Like I said it’s a BASIC overview of why LR requires 91 octane for those who think its a clean / dirty thing.

Find me a production car that has a 10.5:1 or greater Static compression ratio that does not require 91 octane. I’d be interested in seeing it. Diesels don’t count.

 

[bombardier]

good analysis Don...as for me, i wouldn't want to screw around with an engine known to drop sleeves/liners and various other problems. besides, whats the different between 91 and low grade 10 cents? $2.50 a tank doens seem worth it to risk.

i use the highest grade usually 93 (of coarse i'm only running about 6k/year on my '03 so its not finacially killing me), but i plan on keeping my disco until it dies...and i'm just trying to do everything to ensure that isn't in the short term.

if we were dealing with mechanically bombproof trucks here, sure, throw 89 in the tank, but knowing these rigs are tempramental seems that you'd want to make sure you are throwing the best of everything in them and staying on top of service.

 

[john65b]

Hey all, I am an engineer that works in a petroleum refinery. I have had the somewhat pleasant experience of tearing down a few knock engines we have for quality control here in the refinery. These knock engines are exactly what you think, dedicated engines in a lab that are hooked up to premium and regular Unleaded fuel. They run non-stop for (I think ) 10,000 hours before they go through an overhaul, where they tear out the motors. The premium knock engines are always much cleaner than the others.

Now I put premium in my 02 Disco because I don't want to crap up my engine...and it seems to run a bit better at high mountain altitudes...

Just my 2 cents here...

 

[bombardier]

thanks john, good info...now assuming john isn't *********** us that seems like all the info i need to put the best fuel in the tank.