4.8 93 vs 95 at Reef Altitudes

[dieselfan]

My logic says 93 due to our lower air pressure and normal aspiration. With a turbo car I'd say 95 but ours? In Jhb we lose around 22 - 30%. Although the octane rating is better with 95, the engine can't get the air required to take advantage or am I wrong?

I've been filling up with 93 so far and on the open road to Thabazimbi I filled up at my destination and used 24l working out to 14.2l/100km. Not bad this at 115-125 on GPS.

[Peter Connan]

Roy, the octane number is an indication of a fuel's resistance to pre-igniting (IE burning before the spark plug fires).

The higher the number, the better the resistance. Thus at low altitude, you should use higher octane. However, the need for pre-ignition resistance is also coupled to the engine's compression ratio, and the Patrols (even the 4.8) have relatively low compression ratio.

The cut-off point for the change from 93 to 95 octane here at the reef is around 10:1 compression ratio, but yours is only 9:1, so you should be able to use 93 with impunity here, although I would possibly use 95 at the coast.

The octane rating of petrol makes absolutely no difference to it's calorific value (IE how much energy burning it creates). However, modern engines with very high compression ratios can gain performance from using high octane fuel. Older engines using fuel of too low an octane rating just break.

The reason is that modern ignition management systems have knock sensors that can detect if the fuel is pre-initing, and then retard the timing as a consequence. This protects the engine from damage, but creates less power.

[dieselfan]

Thanks, Peter I just found this one my phone whats your view?

https://www.aa.co.za/about/press-room/p ... llyij.html
*******************************
13 September 2012: With last week’s spike in petrol, the pinch is once again felt most firmly by motorists and the pressure on consumers’ disposable income has increased further. “Now more than ever, economical fuel consumption is essential. While motorists should look at everything from the size of their vehicles to the way they drive them to try and reduce costs as much as possible, a key area to consider is the fuel they put into them,” says Gary Ronald, Head of Public Affairs at the Automobile Association (AA). However, it’s easier said than done as consumers faced with the choice between 95 or the cheaper 93 octane fuel may wonder what the implications are for their vehicles.

In a nutshell, when fuel burns it releases energy, and higher octane fuel theoretically means higher performance as it can withstand more compression before detonating. Therefore, the potential for better performance using higher octane fuel does exist, but the difference depends on a variety of factors.

Firstly, the design of the engine plays a role, with some engine designs responding with noticeably improved performance when the correct fuel is used. In fact, most vehicles are now designed to use a specific octane, and in some cases using any other fuel can affect the engine warranty.
Tip from the AA: Always refer to your vehicle’s handbook and use the recommended octane fuel. If your car can run on the lower octane, rather use that as it will save you quite a bit of cash.
It’s also important to note that performance is dependent on driving conditions.
Tip from the AA: At the coast, 95 higher octane fuel is best as the air pressure at these lower altitudes gives the best performance and economy. The higher the altitude the lower the air pressure, and the lower the need for a high octane fuel as there is no real performance gain. In this case, 93 low octane fuel is probably a better compromise in terms of cost and economy.
Another factor to consider is the model of your engine. More modern engines may run better on higher octane fuel, especially if they have turbo or superchargers. The difference will be more apparent for older vehicles on long- distance trips, where you’re likely to get more mileage for your money.
Tip from the AA: Unless your engine is turbo or supercharged, you should go for 93.
Lastly, different fuel suppliers use different additives in their fuel, meaning that one 95 may differ slightly from one supplier to the next.
Tip from the AA: Choose a fuel that offers high-quality fuel with performance-enhancing additives, for example BP Ultimate, allowing your car engine to perform at its peak.

“When filling up, it’s always good to weigh up your options. And while the cheaper option may work for your pocket, it could give you less mileage in the long run,” adds Ronald.
*******************************

[Peter Connan]

Sorry Roy, yes, I had it the wrong way around.

Whatever, I am sure you will not see a significant (any) improvement from using 95.

I will edit my original post so as not to confuse everybody.

[dieselfan]

Sorry Roy, yes, I had it the wrong way around.

Whatever, I am sure you will not see a significant (any) improvement from using 95.

I will edit my original post so as not to confuse everybody.

Well I'm now confused I'm going to do try my best to do 3 runs per fuel type with as comparable driving as possible. With my expected mileage this could take a while

I can't see how you were wrong? They hint that performance would be better on a newer engine even NA but not much? What you said made sense.

[dieselfan]

Also found this

" A rating of 90 does not mean that the petrol
contains just iso-octane and heptane in these proportions, but that
it has the same detonation resistance properties. Because some
fuels are more knock-resistant than iso-octane, the definition has
been extended to allow for octane numbers greater than 100.
Octane ratings are not indicators of the energy content of fuels.
(See section 4 of this page and heating value). It is only a measure
of the fuel's tendency to burn in a controlled manner, rather than
exploding in an uncontrolled manner. Where the octane number is
raised by blending in ethanol, energy content per volume is
reduced" and

" Most N/A cars regardless if it is a 2.0, V6 3.0 or I4 1.4 engine won't
see much difference between 95 octane VS 93 octane. But engines
designed for it like Audi RS4 will see substantial differences. Turbo
engines also benefit greatly. Those still have a limit on ignition timing
but they were tuned for 95 octane, when running on 93 octane actually
means the are running with decreased ignition timings VS normals cars
sold in SA which run with a 93 octane map and would run with an
increased ignition timing when you use 95 octane. A car tuned with a
93 octane map will probably not make full use of 95 octane because
you can usually go a bit higher than just 3 degrees when you move up
to 95 octane, depending on engine and the air/fuel mixture.
If you are running an engine at altitude or lower temperatures you
actually want fuel that ignites more easily. That is why the high veld
runs on 93 octane instead of 95 octane. The air has lower density here
and on a carburetor engine the car would have trouble starting with 95
octane when it is cold because the fuel just wouldn't ignite. Even a fuel
injected car would have trouble if it got cold enough (especially the
older fuel injected cars)."

[dieselfan]

Now the question is what mapping is my car setup for...

[Peter Connan]

Let me try from a different angle:

88, 91, 93, 95, 101: You name it'if it's petrol, it has basically the same calorific value. The amount of energy per litre is the same. But engines only use about 40% of that energy.

However, by increasing the compression ratio, more of that energy can be unleashed. The problem with this approach is that the fuel starts igniting before the spark plugs tell them to, and the flame spreads irregularly. When this happens, the engine can be damaged as the cylinder is under compression while the piston is still moving upwards.

Now the compression ratio is a fixed design element, and cannot be changed by the mapping. In SA we have three octane ratings, but generally only two in a specific area. Here at the reef we get 93 and 95, while at the coast you get 95 and 97 (I think). The turning point (IE the point where you need to start using the higher octane fuel) is a compression ratio of about 10:1. Compression ratios lower than that will show no benefit from using the higher of the two octane ratings available at any given filling station.

Thus if your engine is designed with a relatively low compression ratio (like the Patrol's 9:1) there can be no benefit from using the higher octane fuel, irrespective of mapping, because the mapping will run the optimum ignition timing and will only retard that if it senses a problem.

[Clem]

Peter does any of that advice change in respect of forced induction motors?

[Peter Connan]

Clem, yes definately.

The allowable compression ratio for low-octane fuel drops a lot lower (I think about 7:1).