zeicht

................is it??

rets

For what?

If you cannot have premium fuel as your owner manual has advised, it should not hurt your engine as long as it's unleaded when you're in an emergency.

If you bring your car to the track, your TL should be ok to eat their fuel out there, too.

I have 93/94 mixed sometimes...

need4spd

Actually the owners link FAQ page warns against using lower octane than 91 all time.

zeicht

what i mean is..... is it safe to have a mixture of say 91 and 93 octane fuels? what then will the engine read it to be?

bwillis

It's fine. Don't worry about it.

¿GotJazz?

What is the owner's manual suggestion for octane? Then highest I usually see around home is 91.

Z Factor

Mixing different octane's is fine as long as you don't lower the content to the point of going below what the manual says. If you get any knocking/lower performance, increase the level right away and don't drive the car like that.

Wanta04TL

I have always heard that you can run on the lowest octane rating that doesn't cause knocking/pinging regardless of what the manual says. I had also heard that the octane rating at gas stations is not all that reliable or consistent from one gas station to the next. I know I will be putting 93 in my TL (when I can manage to get one), but I don't want to be harming my wife's MDX but I try to save a few bucks by using a lower octane.

Norse396

Using lower octane even if knocking isn't heard doesn't mean the engine isn't backing timing off and making other adjustments which not only lower the MPG you'll get but can cause wear and tear long term.

I've had this argument with loads of people including my wife who feels that if Chevy says run 87 octane in it, it will be fine.

EVERY car I've tried 87crap octane in got worse mileage than with 89, I know the difference is only 2 points but to the engine it makes a difference.

Now on a car with 11:1 compression, the only outcome of using anything lower than 91 is negative. If you had a muscle car with 11:1 you would be using the highest octane you could find (usually 99) to insure against knock and ping and detonation, why treat your TL worse? I'm not saying use 99 as that really won't help much unless you could play with timing/fuel/air mixture settings, a minimum of 91 is a good rule in my mind.

DMZ

Hey everyone!!!!

Octanes don't mix!!! If you put 2 different grades in your tank, one or the other will be passing through the fuel system at any given moment.

LED Master

Jazz, the owners manual specifies 91 as the minimum.

Norse is right, the manual says use of lower octane for mutilple fill ups will cause additional wear

Chuck

DMZ - you're kidding...right?
A lot of newer stations use 85 and 93 in two tanks, and you "Select" the grade and the pump mixes it to that grade.
Sunoco does this.
Lower Octane results in less timing. Less timing results in Lower Fuel Economy. It will avg. out, and you'll save nothing.
Besides, around here, 93 is around 16 cents higher per gallon then 87. .16 cents x 17 gallons = $2.72 more for the entire tank.
I'm sorry, but if you have to "save" $2.72 per tank, you should not be buying a $35,000 car.

Teh Jatt

I haven't seen 93, I only see and use 91 for my TL

ndabunka

Differing octanes = bad idea (why anyway)? Today's modern cars can "sense" and adjust the knock points so you won't HEAR anything (down to a point) but the cars TUNED for higher octane actually REQUIRE that level of octane in order to get the target gas mileage. If your using less than 91 in this car...YOUR STUPID (IMHO)!

Chuck

No 93 in Cali....
I'm in Michigan. It's all 93 here.
Doesn't matter anyway, the car only needs 91.

Teh Jatt

LED Master

Chuck, you are 100% correct on the fuel mixing, and sunoco thing.

DMZ

No Chuck, I'm not kidding! I have a friend that owned and operated an Exxon station for 16 years. 'Octanes Don't Mix' was always his stance. He also said that Sunoco is the only exception as they specifically refine their gasoline to 'blend'.

One advantage to using Sunoco is that their crude doesn't come from the Middle East, and neither does Hess's. It's one way of showing the Saudi's :thefinger

rets

Very interesting. I thought mixed fuel as Sunoco does was quite right. Perhaps, Sunoco does something special to make up their oil from crude oil, too.

kef

All: This is from the Shell web site.
Probably the most familiar gasoline characteristic is antiknock quality or its octane rating. The octane rating is simply a measure of the gasoline’s resistance to knock. Knock is the sound made when the fuel/air mixture in the engine’s combustion chambers ignites by itself (auto-ignition), either before the spark occurs or in front of the flame already started by the spark plug. Advancing ignition timing increases the tendency to knock.

Not only is the noise annoying, but prolonged heavy knocking may also degrade engine performance and damage the engine. For these reasons, your engine’s octane requirement must be satisfied. The octane requirement defines what the engine demands from gasoline to keep from knocking.

In most cases, using a gasoline with more octane than the engine requires will not increase power or engine life. However, late-model vehicles that are equipped with knock-sensing devices may, in fact, perform better with higher octane gasolines. These devices retard ignition timing at the onset of knock, thus reducing fuel economy and power.

Each engine has its own changing octane requirement. This requirement depends on engine design, operating speed and load, mechanical condition, and amount of engine deposits. Ambient air conditions also affect octane requirement. For example, higher altitude and humidity decrease octane requirement, while hotter air temperature increases octane requirement.

rets

Thanks, kef, very useful tips, some of them are really helpful to me.


Prolonged heavy knocking may also degrade engine performance and damage the engine.

Late-model vehicles that are equipped with knock-sensing devices may, in fact, perform better with higher octane gasolines.

The octane requirement defines what the engine demands from gasoline to keep from knocking.

Higher altitude and humidity decrease octane requirement, while hotter air temperature increases octane requirement.

zeicht

thats what i mean...what would the engine sense then if u got a mixture of different octanes...say 93 and 89...does it become 91???

Norse396

I believe that mixing lets say 91 octane fuel with 93 at a ratio of 50% each would raise the octane, not by very much mind you but when we race we sometimes mix 104 with 93 depending on the timing we are dialing into the engine (not TL).

Below is a commentary on fuel written by a guy at a VW site. It's very well written I think and may help explain octane for some of you. I have edited out the VW parts and used what I think is pertinent to TL owners.

The octane number of a gasoline is NOT a measure of it's hottness or coolness in the burning process, and it is NOT a measure of how 'powerful' it is. The octane number is simply a measure of how good the gasoline is at resisting detonation (knocking/pinging).

The internal combustion engine is - in simple terms - a gas pump (that's "gas" as in vapour, not "gas" as in gasoline). The higher the gas pressure inside the cylinder, the more 'push' there is on the pistons, and this means the higher the power output will be.

We create this pressure by heating a cylinder full of air; and we do THIS by adding a small amount of gasoline to the air and igniting it with a spark.

The engineers aim to get the highest possible cylinder pressure without creating uncontrolled burning of the gasoline.

Detonation (pinging/knocking) occurs after the fuel is ignited by the spark plug, but before the flame front has finished racing across the cylinder to burn all the fuel/air mixture. Don't confuse it with pre-ignition, which occurs when the fuel is ignited before the spark occurs.

So why does detonation occur? It relates to the nature of gasoline. Gasoline is a mixture of different hydrocarbon molecules, and some of these molecules decompose more easily than others when heated under pressure.

So when we ignite the fuel/air mixture with a spark, the flame front starts moving across the cylinder, burning the mixture of air and gasoline vapour as it goes. This increases the temperature (and therefore the pressure) of the remaining fuel/air mixture rapidly. The remaining fuel starts to decompose before the flame front reaches it. If this decomposition produces 'auto-ignition' compounds (those which will start burning without a spark), you end up with an uncontrolled over-rapid burning of the remaining fuel/air mixture, which sets up an opposing pressure wave in the cylinder. This uncontrolled burning and the opposing pressure wave produces the characteristic clicking/pinging sound of detonation, and results in the piston getting a 'hammer blow' instead of a steady push.

(You can test for detonation/pinging quite easily. Get the engine up to normal running temperature, put the car in 4th gear at 30 mph floor the throttle (high load/low rpm). If you hear a harsh uneven clicking sound from the engine - that's detonation. You need to either adjust the timing and/or use a higher octane fuel).

These hammer blows can quickly destroy the engine.

Higher octane fuels are better at controlling the decomposition into auto-ignition compounds than lower octane fuels. They do this in several ways - by interferring with, and reducing the actual decomposition of the fuel, or by chemically reacting with the decomposing gasoline so less auto-ignition compounds are formed.

There are three main sources of heat inside the cylinder which contribute to the decomposition of the fuel:-

1. The residual heat in the heads, cylinders and pistons. An air-cooled engine runs hotter than it's water cooled cousins, so more residual heat is present, compared to a water cooled engine.

2. The heat produced by the ignition of the fuel itself. This depends on the nature of the fuel, and on the fuel/air mixture - rich mixtures are "pre cooled" from the evapouration of the extra fuel, so burn a little cooler, lean mixtures burn hotter.

3. The heat of compression before the spark. Compression of a gas raises the temperature of the gas. We want the temperature and pressure to rise, because the higher the compression, the higher the pressure rise after the fuel is burned - giving us more power.
The first two items are largely fixed - not easily adjusted. Altering No1 would need extensive alteration to the engine design. Altering No2 would require a change in the fuel formulation, which is not even in the hands of the engine designer.
No3 - the heat of compression (compression ratio), is easy to adjust in the design of an engine, so this is the one used to match an engine with the fuel it will be using.

Continued in next post.....

Norse396

Continued...

It's all a balancing act, the octane number came about as a result of research carried out in the 1920s and 30s by Sir Harry Ricardo ("The Internal Combustion Engine" 1925, 1935, and other books) and Charles Kettering (he also developed the distributor and coil ignition system). Harry Ricardo developed the concept of a test engine, in which the compression ratio, valve timing and other factors could be altered whilst the engine was running. Kettering assigned Thomas Midgley to investigate the problem of knocking which was destroying his test engines. Midgely conducted a long search of additives which would help a fuel to resist knocking. Amongst the chemicals tried were Iodine, Aniline, Selenium Oxychloride, Methylclopentadienyl Manganese Tricarbonyl (MMT - currently used in Australia for Lead Replacement Petrol - known as LRP), and other Phosphorus, Sodium and Potassium compounds. Midgley even tried melted butter! Some compounds worked better than others as an anti-knock agent, but many had serious defects (Selenium oxychloride corroded the metals in the engine); and Midgley gradually focussed on organo-lead compounds, and eventually developed a combination of Tetra-ethyl Lead (TEL) with Ethylene Dibromide and Ethylene Dichoride acting as scavengers to prevent an excessive build up of lead oxides inside the engine.

During these tests, it was discovered that Iso-Octane had a very high natural knock resistance, but Heptane had a very poor knock resistance. Because these two compounds are very similar in other respects (similar boiling point and molecular weight), they made a useful comparison point for gasoline. So the octane number for any gasoline is a comparison with a mixture of Iso-Octane and Heptane. 91 Octane has the knock resistance equivalent of mixing 91% Iso-Octane with 9% Heptane.

The discovery in the late 1920s that organo-lead products enhanced the anti-detonating characteristics was a revolution in fuel design, as engines could be designed to operate at higher compressions for better efficiency. So gasolines became 'doped' with tetra-ethyl lead (and the associated bromide scavengers) to enhance their octane numbers.

Another useful feature of lead in gasoline is that the burned lead products (mainly lead oxide) coated the hot exhaust valve seating area, and prevented a problem called Valve Seat Recession (VSR) which results in the exhaust valve 'eating' it's way into the head. With the 'soft' cast iron heads of the day, this was a real bonus. Many older engines in use today which have cast iron heads will suffer VSR on unleaded fuels, and so additives or Lead Replacement Petrol (LRP) is often recommended for this type of engine.

Lead is being removed from fuels because it pollutes the atmosphere, and when ingested by animals, it builds up and causes health problems.

An additional feature of lead additives was that they provided a small but useful amount of lubrication to the valve stems. This was important in engines which had cast iron heads with the valve guides cut directly into the head metal. Most cars also required valve seals to prevent excess oil from dripping down the valve stems and causing a smokey exhaust, and so they had little engine oil available to lubricate the valve stems. TEL in the fuel provided a useful additional amount of lubrication - partially replacing the "missing" oil based lubrication.

Gasoline which is high in aromatic compounds like Benzine has a high 'natural' octane rating and so needs less additives to increase the octane rating. Unfortunately, these aromatic compounds are also those most responsible for atmospheric pollution, so these compounds are being reduced in gasoline in many countries. This creates another dilemma - how to increase the octane rating without lead additives, and with reduced aromatic compounds in the fuel.

A number of other chemical compounds called Oxygenates have been developed to enhance the natural octane number of gasolines. The most common one used is Methyl Tertiary Butyl Ether (MTBE). Other compounds include TAME, ETBE, Methyl Alcohol (Methanol) and Ethyl Alcohol (called Gasohol when mixed with gasoline). But MTBE and the other oxygenates contain 'used' oxygen, so cars using oxygenates fuels burn MORE fuel (because there is less 'fuel' in the fuel) and this increases pollution anyway (Source - "Cleaner Burning Gasoline" California EPA).

Octane numbers are measured in two ways. The 'research' method of measuring the octane number uses a constant speed (1500 rpm) engine in laboratory conditions. This is the RON - Research Octane Number. The other method is the MON - Motor Octane Number, which uses the same test engine, but has a harsher test regime more closely related to road conditions. So the MON is usually lower than the RON for the same fuel.

Often you may see the octane rating quoted as (R+M)/2. This means an average of the two methods is used to give the fuel a number. This number method is often called 'pump octane' or AKI (anti-knock index) in the US.

In conclusion, the octane rating is a measure of the fuel's ability to CONTROL the burning process (to prevent detonation); it is not a function of burning 'hotter' or 'colder'. And the higher the compression ratio (in the same engine), the higher the octane number needed.

Commentary from Rob Aussiebug1970@yahoo.com as it was written here
Edited for this forum by me.

Norse396

Ok, spoke with a few guys who know their stuff regarding fuel.
In a nut shell, if you mix half a gallon of 87 with half a gallon of 93, you'll get the average of the two: 90 octane.

Mixing gas does work however why bother, buy the good stuff and avoid the hassle.