High octane vs Standard Unleaded

[openspaceman]

Less ethanol, higher octane, should work better in high performance engines, both s & 4 stroke.

 

Why? Ethanol has a higher octane rating than any petrol.

 

Generally the higher the octane rating the lower the calorific value. That's why a JAP 500 grasstrack bike had to have 40% bigger main jet.

 

Thats what I thought all those years ago with the Shell 101 octane . Don't see why it should be any different today ?

 

Engines are better designed nowadays, so a cooking 4 stroke motorbike engine then would typically have a compression ratio of 9.5:1 and a carburettor that was always near enough right. Hence a pre disposition to pinking (detonation of stagnant fuel:air mix in advance of the flame front).

 

Better control of fuel:air mix, better mixing better turbulence (squish) in the combustion chamber means a 9.5:1 engine can run on 95 octane rating. My wife's MG only gets 95bhp out of 1800cc, pinks on anything less than 99 octane rating and is outperformed by my pug 206 diesel.

 

Generally compression pressures are lower in 2 strokes as the compression doesn't properly start until the transfer ports are closed, so 2 strokes traditionally work on lower octane fuels.

[GardenKit]

High octane fuels do not, as commonly believed, contain more energy. in fact it is the opposite.

 

The octane rating is a measure of the fuels resistance to ignite. It is needed in high compression engines to prevent pre-ignition (pinking) where the high compression raises the temp and therfore the fuel burns to early in the cycle.

 

So in fact, if you put it in a lower compression engine such as those we use in our trade it will result in late ignition and loss of power and also cooler running, which can cause extra carbon buildup.

 

So its a total waste of money to use it and you will lose performance.

 

The only benefit is that it has less ethanol , therefore absorbing less water and staying fresh longer.

[Dan Earp]

Get aspen ! Lol

[openspaceman]

High octane fuels do not, as commonly believed, contain more energy. in fact it is the opposite.

 

We agree

 

The octane rating is a measure of the fuels resistance to ignite. It is needed in high compression engines to prevent pre-ignition (pinking) where the high compression raises the temp and therfore the fuel burns to early in the cycle.

 

I disagree, it's a comparison of it's resistance to auto-ignite, the spark still ignites it and the flame front then runs through the mixture. A low octane rated fuel will reach it's autoignition point from the heat caused by the high pressure developed in front of the flame. This detonation causes a sudden rise in pressure which can damage the engine. The ideal is for the flame front to progress quickly through the mix and then the hot gas pushes the piston down. The opposite is true of diesel which has a low octane rating and a high cetane rating, this means it burns at a fairly low temperature but as the diesel is sprayed in over a few milliseconds the pressure build up is controlled by the rate at which the fuel droplets are burned. Oddly Rudolph's aim was to counter the shock that spark ignition engines suffered with poor fuels by introducing the fuel gradually rather than premixed.

 

So in fact, if you put it in a lower compression engine such as those we use in our trade it will result in late ignition and loss of power and also cooler running, which can cause extra carbon buildup.

 

I disagree, the flame speed will be the same, it could possibly burn cooler (as you say lower calorific value, but this could be down to the weaker mix).

 

So its a total waste of money to use it and you will lose performance.

 

 

Agree

 

Actually high leaded fuels did have an advantage for 2 strokes if they overheated or the petroil mix was lacking oil, they ran a bit longer before they seized.

[Eddy_t]

I was having a day long discussion with my colleagues today about putting High octane fuel through our 4 stroke machines but also using the high octane fuel for the 2 stroke mixes.

 

All our petrol gear is carb fed but i have noticed that since switching to the high octane fuel, everything starts easier and idles better.

 

My question is other than the price difference, what are the pros and cons of using either high octane or standard unleaded?

 

The higher the octane rating, the more stable the fuel is, the fuel doesn't vapourise or auto combust as easily, allowing more stable running in extremely high compression and heat, the fuel contains more large-chained hydrocarbons, as it is harder to burn, it requires a hotter spark, and unless you're running over 200psi comp, produces less power due to more unburned fuel, a lower octane rated fuel burns more readily, giving more power, but in higher compression or temperatures ignites prematurely causing knocking or misfires

 

Ironically, a fuel with high octane content has a very low octane rating (one), as octane is more commonly found in propane or butane, as the chain is very small, petrol is made of 20 carbons or more, to raise the octane of petrol, they add longer hydrocarbons such as aviation fuel

 

People who have used diesel contaminated fuel may well find the RON in excess of 200, whilst mixing lighter fluid would drop it considerably, I can't remember the exact formula to work out a rating, but there really isn't much in the rating, no more than 30-50 degrees Celsius between 96 and 99 RON in terms of ignition point

 

Hope this helps explain it a bit

 

 

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[GardenKit]

We agree

 

 

 

I disagree, it's a comparison of it's resistance to auto-ignite, the spark still ignites it and the flame front then runs through the mixture. A low octane rated fuel will reach it's autoignition point from the heat caused by the high pressure developed in front of the flame. This detonation causes a sudden rise in pressure which can damage the engine. The ideal is for the flame front to progress quickly through the mix and then the hot gas pushes the piston down. The opposite is true of diesel which has a low octane rating and a high cetane rating, this means it burns at a fairly low temperature but as the diesel is sprayed in over a few milliseconds the pressure build up is controlled by the rate at which the fuel droplets are burned. Oddly Rudolph's aim was to counter the shock that spark ignition engines suffered with poor fuels by introducing the fuel gradually rather than premixed.

 

 

 

I disagree, the flame speed will be the same, it could possibly burn cooler (as you say lower calorific value, but this could be down to the weaker mix).

 

 

 

Agree

 

Actually high leaded fuels did have an advantage for 2 strokes if they overheated or the petroil mix was lacking oil, they ran a bit longer before they seized.

Pretty much the same then.

[mendiplogs]

I know momentum is 98 but dunno about the rest. We only use tesco momentum as our other petrol station is charging 163.9 for their top end fuel.

 

HI MATT £1.63.9 per lt weres that to then mate our is £1.45 per lt thanks jon

[mendiplogs]

Get aspen ! Lol

 

HI DAN what aspen per LT THEN mate plus 2 stoke oil stihl hp thanks jon

[GardenKit]

Why would you want to put Stihl HP with Aspen Jon?

[Eddy_t]

Why would you want to put Stihl HP with Aspen Jon?

 

Because he runs stihl, and stihl runs best on stihl oil apparently

 

 

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