91 Octane Fuel

[Thaiboxer]

Hi Folks,

I have a 1981 Toyota hatchback that burns more fuel than the space shuttle. Thus far, I've used 95 octane fuel in it. I know many newer cars can run fine on the cheaper gas (91) but what about these older models? I would like to start using 91 to save a few baht here and there if the old tub can handle it. Any advice would be appreciated. It uses a carburator.

Edited August 29, 2005 by Thaiboxer

[Thetyim]

It will probably run OK providing it is not a high compression model.

The only adjustment you can make, that will help, is to retard the ignition slightly for 91 octane.

[Crossy]

Run it almost empty (have a can in the back in case you get too enthusiastic).

Stick a couple of gallons of 91 in the tank.

With the engine warm trundle down the road in top gear at about 30-40k's (assuming a manual transmission).

Accelerate hard without stamping on the gas, you don't want the accelerator pump in the carb. operating (still in top), if you hear a clatter or tinkling sound from the engine (pinking) you will need to retard the ignition just a tad (some older vehicles actually have a fine adjuster on the distributor), otherwise time for the toolkit.

Repeat the above until you get no pinking.

Do the same with a cold engine, you're aiming for the minimum amount of retard consistent with no pinking. Retarding the ignition will reduce your engine horespower slightly (not really noticeable) and if you overdo it you can cause overheating. Just keep an eye on things and don't let it run if it won't stop pinking, put 95 in.

Or, you could try 50-50 with 91 and 95 to save a few Baht.

BEWARE that older vehicles don't like Gasohol, the alcohol content attacks certain plasic components in the fuel system, won't hurt the engine, but you may have problems when the fuel pump valves have turned to jelly.

Edited August 29, 2005 by Crossy

[Thaiboxer]

Thanks for the replies! The engine in this car is not the original engine; I believe the original siezed up some years ago and was replaced with another used Toyota engine. Originally, the car had a 1.8 liter engine that was adapted to take natural gas (the valve is still under the hood). The engine in it now is a 2.0 and compared to today's cars, it's a fairly simple layout. However I don't know enough to make any adjustments so I may need to visit my mechanic.

[bakachan]

Thanks for the replies! The engine in this car is not the original engine; I believe the original siezed up some years ago and was replaced with another used Toyota engine. Originally, the car had a 1.8 liter engine that was adapted to take natural gas (the valve is still under the hood). The engine in it now is a 2.0 and compared to today's cars, it's a fairly simple layout. However I don't know enough to make any adjustments so I may need to visit my mechanic.

<{POST_SNAPBACK}>

put some 91 in and see if you hear a pinging sound when you stomp on the gas pedal.

if it pings you may be able to retard the timing a bit to stop the pinging. pinging is very hard on head gaskets. you definately want to prevent it at all costs.

[Thetyim]

pinging is very hard on head gaskets.

Head Gaskets ? Where did you get that from.

Gudgeon pins, con-rods and bigends will suffer

[Crossy]

pinging is very hard on head gaskets.

Head Gaskets ? Where did you get that from.

Gudgeon pins, con-rods and bigends will suffer

Oddly enough, the part that gets damaged most often (and most quickly) is the centre electrode insulator of the spark plug. Pinking (pinging) is caused by the fuel-air mixture detonating rather than the rapid burning that is normal, the detonation can crack the ceramic insulator, if a bit drops off all heck can break loose (quite literally).

The advice is sound, avoid pinking at all costs.

[Gazza]

Ref octane ratings.

My Harley manual says use 87 octane or higher. But when I've used 91 in the past, I get pinging. It mostly occured when riding two-up going up a hill.

95 octane, never a problem.

Are octane ratings different country to country?

[Crossy]

Ref octane ratings.

My Harley manual says use 87 octane or higher. But when I've used 91 in the past, I get pinging. It mostly occured when riding two-up going up a hill.

95 octane, never a problem.

Are octane ratings different country to country?

The short answer is YES, US octane rating is lower than the international octaine value, the long answer is here:-

The octane rating is the most important characteristic of gasoline (petrol) and other fuels used in spark-ignition internal combustion engines. It is a measure of how resistant the fuel is to premature detonation (knocking). It is measured relative to a mixture of 2,2,4-trimethylpentane (an isomer of octane) and n-heptane. So an 87-octane gasoline has the same knock resistance as a mixture of 87% isooctane and 13% n-heptane. A complementary standard for diesel fuel is the cetane number.

The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel through a specific test engine with a variable compression ratio under controlled conditions, and comparing these results with those for mixtures of isooctane and n-heptane.

There is another type of octane rating, called Motor Octane Number (MON), which is a better measure of how the fuel behaves when under load. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, a higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON. Normally fuel specifications require both a minimum RON and a minimum MON.

In most countries (including all of Europe and Australia) the "headline" octane that would be shown on the pump is the RON, but in the United States and some other countries the headline number is the average of the RON and the MON, sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2. Because of the 10 point difference noted above, this means that the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "normal" gasoline in the US and Canada, would be 91 in Europe.

It is possible for a fuel to have a RON greater than 100, because isooctane is not the most knock-resistant substance available. Racing fuels, straight ethanol, Avgas and LPG typically have octane ratings of 110 or significantly higher. Typical "octane booster" additives include tetra-ethyl lead and toluene.

Higher octane ratings correlate to higher activation energies. Activation energy is the amount of energy necessary to start a chemical reaction- since higher octane fuels have higher activation energies, it is less likely that a given compression ratio will cause knocking.

It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. Using a fuel with a higher octane lets an engine be run at a higher compression ratio without having problems with knock. Compression is directly related to power, so engines that require higher octane usually deliver more power. Some high-performance engines are designed to operate with a compression ratio associated with high octane numbers, and thus demand high-octane gasoline. It should be noted that the power output of an engine also depends on the energy content of its fuel, which bears no simple relationship to the octane rating. Some people believe that adding a higher octane fuel to their engine will increase its performance or lessen its fuel consumption; this is false—engines perform best when using fuel with the octane rating they were designed for.

The octane rating was developed by the chemist Russell Marker. The selection of n-heptane as the zero point of the scale was due to the availability of very high purity n-heptane, not mixed with other isomers of heptane or octane, distilled from the resin of the Jeffrey Pine. Other sources of heptane produced from crude oil contain a mixture of different isomers with greatly differing ratings, which would not give a precise zero point.

So now you know

Edited August 31, 2005 by Crossy

[Gazza]

Thanks Crossy

[paulfr]

Crossy

Thanks for the education.

Very interesting.

[Thaiboxer]

Thanks for the info crossy! For years I never fully understood what octane was all about. If I have my mechanic retard the ignition slightly for the 91 octane fuel, would I need to switch it back if pinging occurs and I need to continue with 95 octane?

Thanks again for the replies

[Crossy]

Thanks for the info crossy! For years I never fully understood what octane was all about. If I have my mechanic retard the ignition slightly for the 91 octane fuel, would I need to switch it back if pinging occurs and I need to continue with 95 octane?

Thanks again for the replies

Like I said earlier, it's an iterative process, retard it a bit, try it, maybe a bit more.

If you do decide that 91 still causes problems, it's the work of a moment to put it back.

DC

[AsiaWolfie]

by the way, what term for the gasoline do the THAIS use when they go to the gasoline station ? they say "GaosipHaa" and "GaosipEt" when ordering ? or are there other terms such as "Premium" or anything like that ? I always wondered....

[Gazza]

Khao Nung and Khao Haa.

Also for tyre pressure too. You dont say i.e. 'saam sip hok' it will be 'saam hok'.

Edited September 6, 2005 by Gazza