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Octane

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2K views 24 replies 16 participants last post by  av_audi 
#1 ·
So I am someone who is almost always strapped for cash. University savings cant be touched so I have to have this question answered!

On my 2001.5 B5.5 should I be putting the full 91 Octane in on every fill? I hear its bad to mix octanes, but is it bad I am putting the lowest (87) in every time?

91 Is around 12 cents more expensive per litre than the 87 bringing it to 89 cents per litre today (Canadian)

Thanks in advance!
 
#9 ·
Just to add to this conversation. European Car March 2005 on stands now has a great article about fuels and actually compare a couple Octane boosters. As far as mixing your gas, it will simply give you an octane averaged tank. I am always using Xylene at the track and I am usually adding it to a 1/4 tank of 93 in Florida. It is amazing the difference in top end due to the computer not pulling timing back based on pinging.

As far as mileage. A tank of my Xylene mixture. Staying out of the boost. gives me right at 26 MPG. Same tank with just 93 comes in at 24. I have only done the Xylene tank test once on a road trip, but was shocked when I did the calculation. I bet the difference would be similar with 87 to 91. BUT, BOOST will blow all those numbers out of the water.

I have gotten as low as 10 MPG at road courses, pushing extremely hard. And average for a tank for me is 22 mpg, based on my love of Boost.
 
#12 · (Edited)
Here is a useful layman's guide to gasoline & octane topics: http://science.howstuffworks.com/gasoline6.htm.

Yes, you can mix gas with different "octanes" from the pump - what you get would be an average. In fact what you get at the pump is already a "mixture"; if you look at the posted octane, it is stated a "R+M/2". This means it is the average of the Research (R or RON) and Motor (M or MON) octane numbers of the gasoline (see link above) to get you Pump Octane Number (PON) which is what you see posted in on the pump. Depending on what is actually in the gas (again see link above - gas is not a homogenoous substance, but actually a mixture or blend of hydrocarbon compounds), the pump octane may actually vary.

The gas is mixed or blended at either the refinery (typical) or at the terminal (untypical but it happens). If its the refinery, they use either a knock engine or other methods (see here if you are REALLY interested) to analyse the RON before its shipped out. If they do "make" the gas at the terminals, there are usually no knock engines or octane analyzers to ensure they get the octane right - they splash mix and that's where the variations may creep in.

BTW, the xylene that VariantStg3 ^^^ uses is ~130 octane.

If you are chipped like me, 93 is the only option.
 
#14 ·
Feilo said:
... If you are chipped like me, 93 is the only option.
For those of us who live in California, 91 octane is the only option, because 93 is unavailable. When I bought the car, the dealer recommended 89, but I have been using 91 exclusively for quite awhile (stock 2001 150hp AUG 1.8T engine). Premium gas, platinum spark plugs, and synthetic oil are just part of the cost of ownership. Remember when VW made "economy cars"?
 
#21 ·
shuPASSAT said:
There's premium for diesel?
I believe VariantStg3 meant to say that diesel costs as much if not more than premium in most locales these days ...

Actual diesel's "potency" is measured in cetanes and I do believe the cetane # in the US is lower than the European diesel. IF that's the case, there is a premium grade of sorts for diesel. In the past, I have come across what is referred to as "high speed diesel".
 
#23 ·
Feilo said:
I believe VariantStg3 meant to say that diesel costs as much if not more than premium in most locales these days ...

Actual diesel's "potency" is measured in cetanes and I do believe the cetane # in the US is lower than the European diesel. IF that's the case, there is a premium grade of sorts for diesel. In the past, I have come across what is referred to as "high speed diesel".
That's is correct , Sir. I meant you'll be paying about the same but getting more mileage. So why not stick with the nicer more boostable 1.8t. As far as the 2.slow. I doubt you'd be spending less week to week because the mileage numbers don't take in to account your lead foot trying to keep up with all the boosted cars in your club. You've got the right car... :D
 
#25 ·
You should use premium for the following reasons:

1. The manual recommends at least 91 octane or higher
2. You get better mpgs with premium in our cars, which partially offsets the higher cost.
3. Most premium grades have higher levels of detergents than their lower octane versions. Chevron states so explicitely. So you engine will stay claener with it.
 
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