what is octane

[u mad?]

Its a d16

[lolmclol]

Run 98 easier and cleaner for ur engine in my opinion. Low octane fuel doesnt handle as much compression until it spontaneously ignites, when this happens it causes engine knock. To combat this your ecu needs to delay the timing (with low octane fuel) and with high octane it has to advance the timing as the fuel is igniting higher on the compression

It is not good to run e85 on a car that isn't made for it. U have to change fuel lines, fuel pump, basically whole fuel system

This is what I have learnt from tafe last week
But feel free to correct me if I'm wrong

Hope this helps bud

should have said from the JDM preists in temple in Japan ... no one will believe you now.

[Indie]

I have seen no noticeable increase in mileage using 95 in my DC4, so I stick with 91. I would expect similar results from the EK1. Either way, it certainly doesn't need to be run on anything higher.

[JDM DC2R]

running 91 is fine. Just not E10. I run 95 in my crv only because i cant find 91 when i need it thats not e10. Also my dc2R is tuned to 98.

[GU357]

first of all octane, if anyone has done chemistry/ organic chemistry would know its a hyrdo-carbon of 8 carbons (and 18 hydrogens).
when petroleum is distilled it is put through a big ass furnace, the smaller chain compounds like ethane and methane (1 carbon and 2 carbon with 4 and 6 hydrogen) rise to the top because they are lighter.
somewhere below that octane is also collected.
and even further down diesel is collected.


so as u can see in the image above the gasier lighter stuff will rise and condensate on the umbrella looking things, then if that area is still too hot that means it is an even lighter compound and it will continue to rise further before it can condensate and run off.

petrol especially Octane is further below because it has 8 carbon instead of 1 or 2 (1 or 2 rise pretty much to the top).
below octane your would find diesel fuel.
and etc.

the temperature at which they condensate, how much pressure they require to combust, and how much energy will be produced by combustion is all determined by the amount of Carbons in the chain.

It's the reason Butane (4 carbon 10 hyrdogen) in your cigarette lighter can burn without compression its because its so light that it doesnt require it, you could collect it and hold it under virtually no pressure ignite it and feel a small explosion, where as if u were to use 98ron you could hold it under greater pressure ignite it and feel a much bigger explosion which leads us to why sports cars use 98...

Since more carbon is more potential energy, and to make the most of that potential it needs to be compressed more.
hence most "sport" cars have higher compression engines, if someone where to put 91ron into it the fuel could pre-ignite because it has reached its maximum compression earlier then 98ron would.
and by going off earlier it can cause extensive damage to pistons, rods, valves etc.

Also diesels as mentioned before are collected in higher temperature area of the distillery meaning it has more carbons, this is why diesel engines are high compression and you should never (EVER!!) put petrol in one.

it wont hurt your car to run 98 in the end, but it might hurt your pocket, a lil.
95 is probably whats rated on the car. 91 was always said to be "Lawn mower only".

[GU357]

also the reason petrol burns is because the fumes that are coming off are actually the carbon chains breaking down, and so its the ethane, methane, propane, and butane (1,2,3,4 Carbon chains) thats floating off and burning. the liquid itself does not actually burn, unless compressed.

also a kool trick but, you could fill up a drum with diesel and throw a match on top and use the fumes coming off to burn and create heat quite safely without danger of spontaneous combustion.
petrol burns a lil quick for this.

[Rayle]

Firstly, I believe the OP was asking about the octane number/rating, and not what the chemical octane actually is. Adding to my previous post, to determine the octane rating, they find the point at which the petrol will knock in a test-engine. Then they use a mixture of octane and heptane to produce knocking at the same point by controlling the ratio of octane/heptane. The percentage of octane in the final mixture of octane and heptane that knocks at the same point as the petrol is the octane rating.

Since more carbon is more potential energy, and to make the most of that potential it needs to be compressed more.
hence most "sport" cars have higher compression engines, if someone where to put 91ron into it the fuel could pre-ignite because it has reached its maximum compression earlier then 98ron would.
and by going off earlier it can cause extensive damage to pistons, rods, valves etc.

Also diesels as mentioned before are collected in higher temperature area of the distillery meaning it has more carbons, this is why diesel engines are high compression and you should never (EVER!!) put petrol in one.

Actually, I don't believe that's entirely correct.

You're right that octane posseses more energy than butane (-5.53 MJ/mol octane vs. -2.88 mJ/mol butane), but that's on a molar basis. Octane all weighs more per mol, so when you convert the standard enthalpy of combustion to a mass basis you get around -49.51 kJ/g butane vs. -48.4 kJ/g octane. Note that it would be better to use a volume basis since injectors work that way, but butane doesn't exist as a gas at the standard temperature and pressure.

Point is that butane actually possesses more energy per gram than octane does at 25ºC, and even so the energies are fairly similar.


Now, moving on to thermodynamics from organic chemistry...

Sports cars use higher compression engines because in the Otto cycle that the petrol engine uses, the efficiency of the cycle (which loosely represents the amount of usable work you get out instead of heat) dependent on the compression ratio. Thus if you run a higher compression ratio, you will get more work out of the engine than if you use the same fuel in a lower compression engine. Power is the rate of doing work, so if you do more work in the same time, you get more power out!

You're right about 91 RON/98 RON though. The higher octane fuel will pre-ignite at higher compression level than the lower octane fuel, which is why you must use high octane fuel in high compression engines.


Additionally, diesel engines run off different principles to petrol engines, using an entirely different cycle.

Basically, the TL;DR for people who don't care about the science behind it is:
1. High compression gets you more power and efficiency.
2. Low octane fuel will pre-ignite and knock at lower compression than high octane fuel.
3. Therefore use high octane fuel in high compression engines.

The EK1 engine has a compression ratio of what, low 9s:1? Not high compression, use 91.

[connorling]

98 oct costs 14c/L more than 91 oct.
EK1 has a fuel tank roughly 35L?

so you are only playing extra $4.9 per tank.

might as well use 98.

[aaronng]

Just run a full tank of 91, measure how far you drive and then when empty, fill it up with a full tank of 98. Take the number of litres required to fill and divide by the distance driven to give L/100km fuel consumption. Then drive around with 98 until almost empty and then fill up again to full tank. Calculate consumption numbers. If 98 gives you better fuel consumption that offsets the extra 14/16/18 cents per litre cost over 91, then use 98. If the fuel consumption does not offset the cost, go back to 91. Easy.

[sensei_]

Just run a full tank of 91, measure how far you drive and then when empty, fill it up with a full tank of 98. Take the number of litres required to fill and divide by the distance driven to give L/100km fuel consumption. Then drive around with 98 until almost empty and then fill up again to full tank. Calculate consumption numbers. If 98 gives you better fuel consumption that offsets the extra 14/16/18 cents per litre cost over 91, then use 98. If the fuel consumption does not offset the cost, go back to 91. Easy.

just wondering, to get a more accurate figure, would it be necessary to:

use 91 for 2 weeks, average KM/L.
switch to 95/98 for a couple of weeks (to flush out any old 91 petrol in the system), again take average KM/L and then work out if the extra cost < added mileage.

[curtis265]

98 oct costs 14c/L more than 91 oct.
EK1 has a fuel tank roughly 35L?

so you are only playing extra $4.9 per tank.

might as well use 98.

if you fill up weekly, $5/week will result in 2500/year

makes a difference

running a bit of injector cleaner every 5000k's is a bit cheaper if you're worried about cleanliness

I find no difference in fuel consumption

just wondering, to get a more accurate figure, would it be necessary to:

use 91 for 2 weeks, average KM/L.
switch to 95/98 for a couple of weeks (to flush out any old 91 petrol in the system), again take average KM/L and then work out if the extra cost < added mileage.

you'd also want to reset the ECU and let it adjust to the new kind of fuel as well

[Rayle]

if you fill up weekly, $5/week will result in 2500/year

makes a difference

$5/week is actually ~$250/year