95 Ron Vs 98 Ron: Are there any tangible benefits in filling up with a higher than recommended octan

It's not a higher quality fuel, it simply has an additive to reduce knocking/pinging, aka pre-ignition. However the Atkinson engine has a reliable way to adjust the valves & timing to prevent this so "higher quality" fuel is a waste of money.

 

As PriusCamper also touched on, octane and quality are two different things. A 95-octane fuel will self-ignite at a certain temperature and pressure, and a 98-octane fuel will do so at a higher temperature and pressure. That's all. And you could have a good or bad quality 95, and a good or bad quality 98.

The only kind of quality difference there might be between a 95 and a 98 from the same brand would be if they choose to put a better detergent blend into the more expensive one. And that's purely a marketing choice, independent of the octane.

 

Unlike my Corollas or Chevrolets in the Prius I've tried everything from the yellow gas all the way up to 107 octane racing fuel Yes it's available right here at the pump down the street from my house and none of those fuels made any difference in gas mileage not even the crappy yellow fuel which is what 25% you know a flex fuel or whatever they call this yellow. I know it's not recommended for the Prius so I don't use it generally I just was trying a bunch of different gases when I first got one of these prii. And I noticed it didn't make any difference unlike in my Corolla the higher octane gas makes a huge difference you can see it on the gas gauge and on the MPGs but not in the Prius.

 

Ignoring any possible additives or detergents (for the moment), IF a particular engine has no detectable detonation (spark knock) and therefore doesn't retard ignition timing while using the recommended fuel, THEN you will have no gain from running a higher octane (RON) fuel.



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It does and I have used RON 91 for a good period of time thinking I was saving money using the cheaper (per L) fuel. What I actually found was that RON 91 was more expensive per km than RON 95. This is because using RON 95 I get at least 150 km (and up to 200 km) per tank more range, so the reality is the cents per km are lower using RON 95. I also found that using RON 91 the car lacked pep and power. I found on RON 95 that the car was much more responsive and ran smoother.

In our neck of the woods at least, this is true. From my experimentation and experience that while RON 95 has benefits over RON 91, the same cannot be said for RON 98. There is absolutely no benefit to using RON 98 in a Prius. You are just throwing good money in the bin.

Exactly true and as stated in previous posts, it is not 'higher quality'. What it has is a retardant (octane) added to prevent pre-ignition in higher compression engines.

Personally, I doubt any such thing and would go on to say it is just a marketing spin to take money off you. What I notice in the advertising (at least on this side of the Tasman) is that they are careful to frame the RON 98 in a way that leads you to infer the other grades are somehow lesser but they careful to not say how that is so.

If you really feel that you need to clean your injectors or fuel system, just add a bottle of injector cleaner from the likes of Nulon, Penrite, or Repco from time to time. I'm not sure if you can still get Caltex Techron Concentrate Plus premium fuel additive in Oz, but that works too.

I have found from previous discussions with American owners that, like for like, these findings don't seem to translate to the American market with NA Prii.

 

There may be some confusion here about octane numbers. There are actually two different methods of measuring octane, Motor Octane Number, MON and Research Octane Number, RON.

The number on the gas pump is the simple arithmetic average of the two RON and MON numbers which can be called pump octane. RON is always higher than MON because of different test method severity.

Modern cars with computerized spark advance can operate with a broad range of octanes. Some cars will get more horsepower with higher octane but the mpg is not correlated with octane.

Use regular gas which at normal altitudes is 87 pump octane.

 

Adding to the confusion, different countries (and the OP is in Australia) have different laws for which number is to appear on pump labels. In the (R+M)/2 method used in the United States, 87 is the pump octane number for "regular" gasoline. Other countries may show other numbers on the pump.

Back in the mid-1990s, anyway, it was definitely true that some fuel brands in the US were putting better additive packages into their premium blends than their regular blends. I had a list that Consumer Reports had compiled of which brands used the same additive packages in their regular as in their premium, and I liked to make a point of buying those brands. (One of those brands that did offer the same package in regular blends was Phillips 66, which at the time was common but has since, regrettably, kind of disappeared from my area.)

That was before the advent of "Top Tier" labeling, and I wouldn't be able to say what the situation is today.

 

As noted above, the OP is in Australia and he did say in the OP:

While this is true in the US, NZ like Australia (and the UK) uses the RON number on the pump, not the (R+M)/2, also known as the anti-knock Index (AKI). Further, in the UK only fuel with a RON of 95 or above is available, unlike NZ and Oz which also have RON 91.

So participants in this discussion need to be aware of these things so as not to get confused.

in addition, as I mentioned in my previous post, it seems that what seems to be the norm in regard to fuel and AKI numbers in the USA does not translate very well to the antipodean environment so some care and thought should go into posts to this thread by our US friends.

To help the US audience follow along, the following table cross-references RON and AKI.

	RON	EQ US AKI
1	91	87
2	95	90
3	98	93-94

 

I would also argue climate affects which fuel to choose.

Many times you will get better economy on 91 in cold winter weather as compared to higher octane due to heating requirements and energy density of the suspect fuels

 

Interesting thought. I just might try that out. And when you say 91 are you meaning AKI 91 or RON 91?

 

87-US
91-Euro

In cold weather (20F or lower)
I have found while monitoring that vehicles that are dedicated to Premium fuel only tend to have minimal knock and performance loss using lower grade fuels as compared to premium in cold climates.
*My disclaimer is that I was not towing, or driving hard over long distances , although ginger driving over 100 miles didn’t seem to affect performance as things heated.

My favorite fuel in Dedicated premium fuel cars is 88e15 which likely doesn’t exist in the euro zone.

 

Dagnabit, it doesn't ever get that cold here. We think 15ºC (59ºF) is cold LOL. On a very cold winter's night, it might get to 4ºC (39ºF) where I live, and quite a ways further south it might get to -10ºC (14ºF) at worst in the depths of winter.

So I might not be doing any experiementing after all.

 

Yeah 59F is a nice summer evening here

 

It’s due to what the refiners blend in to hit their octane and vapor pressure specifications. For example in winter you want the vapor pressure to be higher so refiners put more light ends like isobutane into the blend but they have lower energy per unit volume. In the summer isobutane is not helpful.

In Wisconsin the gasoline often has better energy content than neighboring Illinois where more ethanol is used to raise octane and provide oxygen for pollution control.

 

On the flip side, I have instances where using high octane fuel in an engine designed for "regular" fuel has led to fuel fouling the spark plugs and no start (with loss of compression from "washing" the cylinders) in very cold temperatures. The engine didn't produce enough heat when cranking to properly ignite the high octane mix.