Highest fuel octane

[MyCorvette]

There are some gas stations selling 100 octane unleaded fuel. But I also know that I can get the high octane benefit only if the car is designed to run that fuel octane, or else power decrease will occur instead.

In the owner's manual, BMW recommend 91 octane for 135i. But at the same time from several dyno results, people obviously get more wheel hp when using 93 or 94 octane fuel.

My question is, what is the upper octane limit is good for the 135i engine? In another word: what is the highest octane that the N54/55 engines' ECU can take advantage of?

[MadMan77]

That will be a very difficult problem to work out unless someone has already done the tuning and found out where the octane threshold - if such a thing exists - is. You can get up to 104 (RM2 method) unleaded fuel from VP Racing Fuels (MS109) and people are seeing horsepower gains from it, but only with the proper tuning.

I know this doesn't really answer much but it I am sure someone with more experience can chime in.

[The1]

there is a limit, however, the cars ecu will try and adjust timing in the engine to match up to it after a while. Your car will run 94 very easily. anything above that is questionable as we don't know how much leeway the factory computer gives us.

all that will happen if you put a tank of that in is it will just burn through it super fast and you will run super rich. however, it will take a couple of tanks of higher octane for your computer to learn how to use it. So it may work in the long run, but that's only if the factory computer can adjust that far out of it's normal spectrum.

I'd recomend you run half a tank of 94, then put in a 1/4 tank of your 100 and see if you notice anything. It will bring your octane higher, but not all the way up. Just to test the waters before jumping in.

[Top Gon]

I would like to know. There's a gas station near my house that sells 100 octane unleaded and I wanted to see if it will make a difference.

[BMW86]

If your car is completely stock (ie. no tune) then just stick to 93AKI (98RON for the rest of the world). If you are running a tune or piggyback feeding your car 100RON octane will make a huge difference in performance. It also reduces the chance of knocking. You can run higher octane (ie. race gas) if you can afford it too. It may take some time for your car to completely adapt to the higher octane fuel to get all the benefits from it though. We have 100RON in every state but mine here in Australia! We've started getting E85 which is almost like race gas on pump but unfortunately our cars can't use it.

[Brandon26pdx]

Modern ECU's read and compute a variety of data to determine safe running condition for the engine. Fuel octane is a major variable, as is intake air temp. On a cold day in winter you might be able to get away with 91 or less and not suffer any loss of power, but on a hot day in summer even 94 might not be enough to convince the ecu it's safe to run the engine full blast, and you'd theoretically see some performance benefit from running fuel higher than 94 octane.

[pixelblue]

my buddy who is a professional racer told me regardless of your tune you will benefit from higher octane. however, being able to afford $8-10/gallon is another issue. so for daily driving just use 93 oct and if you track your car you can splurge and go for 100 oct

[john08135i]

I tried 104 and 110 with my SSTT..
it made a huge difference in the car.
I ran 12.7 in the 1/4 with sstt and 110 octane.

But it WILL eat up your cats, beware.

[wadny]

Originally Posted by maxnix

E85 has much less BTU energy than a gallon of gas. That's why Indy cars always use turbocharging.

Sorry to disagree, but current Indy cars are normally aspirated (no turbos) and have been for several years. They run ethanol with a little gas mixed in so the flames are visible.

[bryce]

Originally Posted by john08135i

I tried 104 and 110 with my SSTT..
it made a huge difference in the car.
I ran 12.7 in the 1/4 with sstt and 110 octane.

But it WILL eat up your cats, beware.

you were proobably running LOW LEAD 110 octane. it will mess up your cats if run for a while

there is NO side effects from 104 unleaded, however. even stock you'll see gains. i ran 5 gallons of 104 unleaded before, car was a beast! i'm tuned though w/ procede so the gains were substantial

[Brandon26pdx]

Leaded gas is a no-no for engines with cats and O2 sensors.

[RPM90]

Interesting that this thread came up.
I have a stock 135i. I was down to 1 gallon last week as I decided to try some 100 octane, unleaded "race" gas.

I put in a bit over half a tank. It was $6.50 per gallon, so not too expensive to give it a try.
Contrary to what others have said, modern ECU/ECM's don't need a tank full to adjust to fuel. Modern cars are continuously monitoring combustion and adjusting a/f ratios and timing. If that weren't the case, then a serious knock event would cause serious damage.
It doesn't because the system responds almost instantly and adjusts parameters to regain proper combustion.

I wanted to try it to see if I felt anything. I had a 2006 A4 2.0T with an APR 93 tune.
Running the same 100 in that engine resulted in a distinctly better running condition.
The power felt smoother and more linear. Idle was very smooth. Throttle was very responsive, and it felt like there was a bit more power.

In my 135i, I ran for about 40 miles before trying to assess what the 100 fuel might be doing, as I needed to run the 93 fuel out of the lines, and make sure that the 100 had time to mix in with the 1 gallon of 93 left in the tank.

One thing to note is that the temps have been fairly cool around here, below 50F mostly, and today it got up to around 65F.
So the air temp hasn't been an issue in terms of potential knock due to high ambient air temp.
Overall, the performance difference hasn't felt as significant as it did in the A4.
Granted the A4 did have a tune, which was reportedly tuned for 93/94 octane, and not for 100. So, there was modest but noticeable positive running effect.
In the stock 135i the main things I noticed with the 100 octane is that the idle has been smoother, and throttle response a bit crisper. Overall power feels about the same, but the power does feel a bit more linear.
But as I said, the temps have been on the cooler side, so a smoother power band could be attributed to the cooler temps. I do feel that the crisper throttle response might be attributed to the 100 octane fuel.

I'm down to about 2.5 gallons, so after tomorrow I'll fill up with regular 93 octane, and then see if anything noticeable happens after about 30-40 miles. I'm going to wait to fill up once I'm down to about 1 gallon of 100.
MPG has been about the same, though it's hard to tell with only about 1/2 a tank, cause I've been hard on the throttle for the past few days just to test things out.
Overall though the MPG has been about the same when comparing the daily commute.

Next spring when it get's warmer I'll get a full tank of 100 and see what happens to MPG.
$6.49 for a gallon of 100 wasn't too bad considering 92/93 goes for about $3.70+.
But, since it ain't doing anything worthwhile, in the final analysis the thing that has lessened is the amount of extra money in my pocket.

[babyboss]

any 1 use toulene/xylene/methanol mix to fuel ?

[plasar]

Sheesh, the amount of misinformation in this post is astounding...

http://en.m.wikipedia.org/wiki/Octane_rating

[kingnonis]

Originally Posted by babyboss

any 1 use toulene/xylene/methanol mix to fuel ?

When I had my '08 w/JB3 I used xylene once in a while mixed with 91 octane (max we have here). Ratio was one gallon of xylene to 6 gallons of 91 oct approx.

[Itanius]

Originally Posted by plasar

Sheesh, the amount of misinformation in this post is astounding...

Here's the correct link:
http://en.wikipedia.org/wiki/Octane_rating

But, be warned as that Wikipedia article has some issues and disputes aside from the standard wiki expectations. If your aim is to educate people, use a source reference and not a wiki article.

[blacknbean]

they sell 100 octane out of the pump here and it makes a huge difference! too bad its 6$ a gallon tho

[RPM90]

Originally Posted by plasar

Sheesh, the amount of misinformation in this post is astounding...

http://en.m.wikipedia.org/wiki/Octane_rating

What misinformation?

I don't see a post stating "high octane" fuel gives more power, outside of an engines ability to utilize it.

Higher octane is simply a higher resistance to detonation, which is beneficial for tuning engines, especially when advancing timing, or increasing compression ratio, or both.

In and of itself a higher octane fuel can not make an engine produce more power.

In a turbo engine, there is a higher chance of detonation due to the increase in combustion chamber pressure from the forced air.
The TT engine is designed to work just fine on 93PON (pump octane number is ron+mon/2) and still provide acceptable performance at 91PON.

People who have a tune on their engines might benefit from a higher octane if the fuel they are using, along with the driving conditions, may be experiencing timing retard due to knock events. If that happens power will be reduced.

For example, 91PON is acceptable, and it's all you can get in some states.
However, the engine is tuned and capable of providing best performance with 93. If the driver is using 91 and is running hard on a hot day and in a hilly terrain, it's quite possible that he is not getting full power from the engine. In those conditions the ECU will try to maximize power up to it's full potential, but those conditions could lead to knock events that will lower power. Going to a 93PON will bring peak power back.

The "magic" of higher octane is it's greater ability to keep knock at bay for the given tune.
The reason I tried the 100 octane was simply an experiment to see if a very high octane would allow my engine to perform any better.
I was not expecting some more power because there was "more octanes".
Yes, some people think more octane means there is something "more" about that fuel that will magically give them more power even when their car is tuned to max performance at 89PON.
But, I don't see that anyone here promoted that idea.

If you were to do a search you'll find that this topic has been covered quite a bit with some very good accurate info.

[plasar]

Hi everyone, sorry, I have a crappy web connection where I am (South Africa), so my brief post was from iPhone.

Here's the misinformation list so far:

1. "...or else power decrease will occur instead." - Power decrease from higher octane fuel? Doubtful, never heard this before - please state your source and/or dyno results.

Octane is all about avoiding pre-detonation, and the catastrophic results if ongoing. There is nothing I've come across to say that there is a "slower burn" or less energy overall.

Per Wiki: "Higher octane ratings correlate to higher activation energies. Activation energy is the amount of energy necessary to start a chemical reaction. Since higher octane fuels have higher activation energies, it is less likely that a given compression will cause autoignition." Higher activation energy, yes, but once activated, burns like an explosion.

And again Wiki: "The power output of an engine depends in part on the energy density of its fuel, but similar fuels with different octane ratings have similar density." Ethanol has a lower energy density, but let's just talk octane in gas here.


2. "all that will happen if you put a tank of that in is it will just burn through it super fast and you will run super rich. however, it will take a couple of tanks of higher octane for your computer to learn how to use it." - Where to start: a tank of higher octane "burns super fast" (no), "run super rich" (no), "couple of tanks to learn" (no).


3. "It may take some time for your car to completely adapt to the higher octane fuel to get all the benefits from it though." - Please state your source. Since these calculations happen real time to prevent pre-detonation (with knock sensors), of course they should run real time to advance timing and increase compression to "take advantage" of higher octane.


4. "But it WILL eat up your cats, beware." - how does octane eat up your cats? (Leaded fuel does.)


RPM90, as usual, no misinfo in your post. But I think the above are pretty numerous and significant boners - the posts, not the posters.

[plasar]

Thanks for the better link Itanius.

I agree, primary sources would be best. The Wiki article on octane is pretty darn good though. States the basics quite succinctly. And I like that fact that it's "peer reviewed". Well, more "public reviewed".

[The1]

why are you quoting Wikipedia?

not that i disagree with anything you're saying other then the point about a few tanks to learn octane, but if you're quoting wikipedia, you may as well ask a 5 year old questions about aerospace engineering and take their word for it. It's just not a credible source.

now on to that other point, yes you are right, the engine reacts almost immediately to the changes in fuel. It's sort of a fail safe. However, it does take several tanks of gas to make the engines ECU adapt properly for best performance, not just the safety aspect.

it has a learning curve, first it protects itself, then it optimizes for that specific fuel. One is instance, the other takes some training.

[plasar]

Hmm, Wikipedia not a credible source... Well it's peer reviewed, accepts comments and differing opinions, and sources are quoted so you can follow up on the original research.

So please allow me to peer review your statement below:

"It's sort of a fail safe. However, it does take several tanks of gas to make the engines ECU adapt properly for best performance, not just the safety aspect.

it has a learning curve, first it protects itself, then it optimizes for that specific fuel. One is instance, the other takes some training."

Do you have any sources to back up your proposal for how BMW's engine management system reacts to high octane gasoline?

I am skeptical, but would welcome more details about how this works.