Tesco Super 99 or 97 Octane

[bobcat]

Originally Posted by Georgey dee

So it's the Timing that's altered no the fuel pressure.

yes, plus the ecu's in the 850 can vary fuel pressure anyway.

[Georgey dee]

The 200SXs must be so crude that fuel can make such a difference. I'm thinking of sticking a MBC on and don't want to melt my engine. I was assured that the ECU will look after the engine before damage is caused.

[pigapumbu]

Originally Posted by bobcat

the fundamental issue is the issue dosent run a leaner mixture!!!!!

with higher ron you can run a higher cylinder pressure and thus get more torque.

the reason i gave the example of antilag, is that when in use the engine will still be drawing in a lot of air and fuel, however the engine will be pretty much idling, and still with good afr's. if yo drove it like that you would have s$%t mpg - but that then goes against previous posts in this thread that link mpg solely to the maount of fuel injected.

the reason lower rons fuels make less power is that you can run to much timing advance as the cylinder pressure become so high the fuel\burn becomes unstable. with higher ron you can run more timing and thus higher cyl pressure and thus more torque and therefore more power.

however for those that want to believe its all down from leaning out the mixture - well i hope you never start mapping cars!!!

got to ask, how does the car know what fuel it is running?

This gets even more confusing. Maybe I need to ask, what do you mean by higher cylinder pressure and how is that achieved? Only by timing adjustment? No changes in fuel/air mixture?

The fact you say more RON means a higher cylinder pressure can be obtained by being more stable means, a higher RON is better for a car. I wouldn't want a lower RON in there.. or put it this way, if I get my car modified, surely the intention of the person modifying the car wants to use the best cylinder pressure (timing) and this means using a high octane fuel. Sure he can also run with 95 but what would that achieve? Unstable burning, less timing, less cylinder pressure and hence less power? Why would any tuner suggest to use 95?

How does a car know what petrol it uses? Er I dun know that. Maybe Knock sensors in the engine detect the first signs of engine knock and takes measures to prevent it such as adjusting the engine timing? Maybe you can tell us.

cheers.

[pyaap]

Originally Posted by davev70

thats all fine but it was not tested in a volvo

Sorry, I too am somewhat perplexed as to what you mean. If we were to apply the same logic to a similar scenario, are you then saying you would question the validity / efficacy of a drug your doctor may prescribe to you simply because you did not take part in any trials which tested it?

[st5ve]

Hi bobcat,

I think I understand what you are saying, the higher RON fuel has greater explosive power, so if per say with 95 ron fuel, the mixture is 1 part petrol to 15 parts air the energy released when ignition happens is less than 97 or 99 ron fuel. As more energy is released at combustion with higher RON fuel then compression would be higher when the fuel air mixture ignites? Therefore more torque and bhp or am I just got the wrong end of the stick again?

Originally Posted by bobcat

the fundamental issue is the issue dosent run a leaner mixture!!!!!

with higher ron you can run a higher cylinder pressure and thus get more torque.

the reason i gave the example of antilag, is that when in use the engine will still be drawing in a lot of air and fuel, however the engine will be pretty much idling, and still with good afr's. if yo drove it like that you would have s$%t mpg - but that then goes against previous posts in this thread that link mpg solely to the maount of fuel injected.

the reason lower rons fuels make less power is that you can run to much timing advance as the cylinder pressure become so high the fuel\burn becomes unstable. with higher ron you can run more timing and thus higher cyl pressure and thus more torque and therefore more power.

however for those that want to believe its all down from leaning out the mixture - well i hope you never start mapping cars!!!

got to ask, how does the car know what fuel it is running?

[bobcat]

Originally Posted by ttacornwall

Hi bobcat,

I think I understand what you are saying, the higher RON fuel has greater explosive power, so if per say with 95 ron fuel, the mixture is 1 part petrol to 15 parts air the energy released when ignition happens is less than 97 or 99 ron fuel. As more energy is released at combustion with higher RON fuel then compression would be higher when the fuel air mixture ignites? Therefore more torque and bhp or am I just got the wrong end of the stick again?

Not really, another common belief is you get more power as the higher ron fuel has more energy. This is not the commonly the case for fuels of the same type with different ron ratings.

the advantage with higher ron fuel is that it will still burn under greater stress (compression, temp, cylinder pressure from the burn) and not knock which is the fuel exploding.

so with higher ron fuel you can ignite the mixture when the pistion is nearer the top (air heavily compressed), and due to the pistion being nearer the top and the volume is the cylinder being lower you get higher cylinder pressure.

higher ron fuel is actually harder to ignite and burns slower.

if you did the same with lower ron fuel it may knock (explode) due to the higher cylinder pressure.

so as you can see to get the most torque you want to explode the fuel when you have the most compression to use all of the burn on the power stroke. but with a low ron fuel you cant do this as the high compression would just cause the fuel to explode (knock), so you retard the timing and ignite the fuel with less compression, this getting a burn instead instead of knock but not using all of the potential fro the fuel.

[Engineer]

Talking fuels never seems to meet a solution IMHO so here are a few thoughts, hope they help.

Different engines respond differently to the fuels used in them so saying these weren’t Volvo engines is a valid point. For maximum power you should use a fuel that burns at the quickest possible rate without detonation and which initiates the fuel burning the latest but still produces peak cylinder pressure just after TDC. When the ignition timing is correct power output will then be highest for that rpm and throttle position.

However if you have a hot running combustion chamber due to the design etc. then you will need to use a higher octane fuel (which burns slower than a lower octane fuel) than for an engine which has a cooler running combustion chamber. The hotter engine will also need more ignition advance all the way through the rev range in order to allow the fuel to burn correctly and so deliver peak cylinder pressure without detonation. Note: Higher compression ratios don’t always mean higher combustion temps if the engine cooling is efficient enough to dissipate the heat generated.

If you use the higher octane fuel in the engine which does not need it and which has less ignition advance through the rev range then this engine will loose power as the fuel will still be trying to complete its combustion way to late after TDC. Of course if the ECU can compensate for different fuels by adjusting the ignition to suit, then power output won’t normally vary in a normally aspirated engine, provided nothing else has changed.

If you now apply this to turbo engines the more flow you have going into the combustion chamber the hotter it will become when compressed by the piston, you will then have to either cool the air / fuel charge before / after compression or both or use a higher octane fuel which burns slower and therefore more ignition advance will be needed. Another thing which can affect how a particular engine performs and also its combustion temperatures is the fuel blend which varies from one Fuel Company to another.

Cheers

[pigapumbu]

A little bit of help here...

RON
Think of RON as the ability of the fuel to resist compression detonation as opposed to the ability of the fuel to burn also known as, combustion.

Octane rating has no direct impact on the burn of the air/fuel mixture in the combustion chamber. Detonation is a different type of combustion and this is to be avoided in spark ignited gasoline engines. Octane rating is a measure of detonation resistance, not burn characteristics.


FUEL STRENGTH
A simple explanation is that carbon-carbon bonds contain more energy than carbon-hydrogen bonds. Hence a fuel with a greater number of carbon bonds will carry more energy regardless of the octane rating.

Note that to a certain extent a fuel with a higher carbon ratio will be more dense than a fuel with a lower carbon ratio. Thus it is possible to formulate high octane fuels that carry less energy per litre than lower octane fuels. This is certainly true of ethanol blend fuels. and we all know what Tesco uses to obtain its high RON. (Duh.. now I know why Vpower has a marked difference than Tesco99)

The power output of an engine depends on the energy content of its fuel, and this bears no simple relationship to the octane rating. A common myth amongst petrol consumers is that adding a higher octane fuel to a vehicle's engine will increase its performance and/or lessen its fuel consumption.

So with the above simple explanation, using high octane fuel for an engine makes a difference only when the engine is producing its maximum power. This will occur when the intake manifold has no air restriction and is running at minimum vacuum. something like flooring the throttle. Its then when a higher RON comes to play. And going back a few posts, I think its important when a car is modified, a higher RON is advised to be used. If not,... well let the brave ones put 95RON or less and floor it all the time. I'm sure the standard ECU can cope. ...or not!!!

cheers

[bobcat]

Originally Posted by pigapumbu

A little bit of help here...

A common myth amongst petrol consumers is that adding a higher octane fuel to a vehicle's engine will increase its performance and/or lessen its fuel consumption.

did you just copy and paste that from a wiki etc?

Well again thats more wrong info in the context of this thread*!!

imagine you had a fuel with a very low ron rating ie, you had to ignite the mixture when the pistion was nearing the bottom of the power stroke (for lower cylinder pressure) to get a burn rather than an explosion (knock), how much torque do you think that engine would make pigapumbu?

the greater the ron the nearer to tdc you can ignite the fuel and thus the more torque you make as your able to harness more of the mixtures energy. You also have to bare in mind that nearer tdc you will have higher pressure in the cylinder, and igniting the mixture there will create higher pressures than if you were to fire further down. so you can get a case were the fuel starts to burns and then as the pressure increases it then goes from a burn to an explosion. This is how you can have varying values of knock.


* however the above can be said to be true if the timing wasnt altered, as the higher ron fuel takes longer to burn and could still be burning on the exhuast stroke. so again its a case of using the correct grade of fuel.


But the std ecu is more than capable of varying its timing for greater efficiency, as well as other little tricks like fuel enrichment for pre-ign.

however if that above statement was copied from a wiki etc then it most likely in relation to engines with fixing timing and in those cases running higher octane fuel would actually be worse for the car assumming its running ok to start with.

[st5ve]

Well there does seem do be a diffrence of opinion on this topic, it is very confusing, for the layman anyway.

bobcat: from my very very limited understanding, for ignition to happen anywhere other than TDC then there would be extra stresses put on the gudgon pin and con rod. When you talk about the timing changing then I assume the ECU changes the firing point on the stroke very slightly?

I will say that all the posts are very usefull and I am sure there are many forum members who will be reading this thread with interest and learning from it, please don't turn it into a personnal war of words with each other, like some of the chipping posts.