Low octane fuel

[Zoom]

In most of Maine we can only buy 91 octane fuel. At the suggestion of my dealer, I've started using a fuel additive to boost the octane and I think it may be making a difference in the performance. At 5500 RMP the engine is much smoother as is the over all acceleration.

 

One $9.00 bottle will do three tanks. I think I'm going to stick with it.

 

Has anyone else experimented with an octane booster? Are there possible negative side effects that I should be considering?

 

Thanks

[RodB]

I think that you are spending about $3.00 more per tank than you should.

[bmweerman]

+ 1 on that.

 

I experimented with that too and I found no difference in performance.

 

The only reason to use higher octane fuel is to avoid detonation on higher compression engines. Boxers seem to do fine on 91.

 

I heard from a long time tech friend of mine that he saw a couple of RTP's seize on 85 octane, but 91 seems not a problem at all.

 

Edit...oops just realized I was posting in the hexhead not the oilhead forum. You don't say exactly what bike you have, but if it is a can-bus, they can compensate for much lower octane fuel due to their knock sensors.

 

I think your three bucks would be better spent on Gatorade to keep you hydrated

[T__]

bmweerman, actually on the hexhead you can lose some performance if low octane fuel is used as they have knock sensors that pull spark out if knocking is detected.. Depending on the altitude,, ambient temp,, humidity,, & load I would be willing to bet that in a lot of cases the hexhead pulls some spark out at high engine loading on 91 octane fuel.. Those things are fairly high compression..

 

Twisty

 

[smiller]

Count me among the skeptical that going beyond the manufacturer's recommended octane rating is really going to provide much if any improvement, even in an engine equipped with knock sensors.

[bmweerman]

bmweerman, actually on the hexhead you can lose some performance if low octane fuel is used as they have knock sensors that pull spark out if knocking is detected.. Depending on the altitude,, ambient temp,, humidity,, & load I would be willing to bet that in a lot of cases the hexhead pulls some spark out at high engine loading on 91 octane fuel.. Those things are fairly high compression..

 

Twisty

 

You think so on 91? Besides pulling spark don't they also advance the timing? Just asking.

[T__]

bmweerman, yes I think so.. Maybe not much but those 1200 hexheads are real high compression.. Now how much that actually hurts performance is up for discussion.. I would think that for normal riding in decent weather probably not much removed but at 120mph in hot dry weather I would think a fair amount of spark pulled out on 91 octane..

 

The knock sensor is only capable of telling the fueling computer that a knock is present (most are peizo type).. So all the knock sensor can do is allow spark removal,, any advance would be handled by the fueling computer..

 

Twisty

 

[smiller]

I believe that the GS-911 displays spark advance so I suppose one could max out the octane with a booster and makes some runs under difficult conditions, and then try the same thing on 91/92 octane pump gas and see if there's any difference.

 

And on a related topic I've always wondered... how do you really know that the promised octane rating is really delivered consistently at the pump? I know that regulators check service station pumps for accuracy regarding delivery volume (a very broad spot check, but at least it's something) but does anyone really test advertised octane, or is that pretty much an issue of trust the retailer and little more? Is anyone aware of any investigative reporting that has ever done any such tests?

[T__]

Seth, that is a very good question.. Especially with these single hose pumps.. I do know they do a random amount (gallon is a gallon metered) check in my area here as I have seen the testers at different stations over the years.. But to do a real octane test would require taking the fuel sample & running it in an octane test..

 

I keep thinking of years ago when I about 15 years old & worked at service station after school.. A truck would pull in late at night (not even the brand we sold) & a little cash would be handed to the truck driver by the station owner & that fuel would be dumped into our premium tanks.. I seriously doubt that stuff was always premium fuel..

 

Twisty

 

[smiller]

You cynic you.

 

This thread just brought up something I've always wondered about, in that there's usually not a clear-cut performance difference (especially in modern knock sensor-equipped vehicles) between one grade and another under most operating conditions. If a station simply sold mid-grade as premium, who would ever know?

[smiller]

And back on topic, here's an interesting article. What I take from it is if (and only if!) an ECU generates enough advance to take full advantage then higher octane can result in more power, but even in the extreme cases tested in the article the difference was a matter of a few horsepower. Important in a competition environment but not likely to be very noticeable on the street.

[T__]

You cynic you.

 

This thread just brought up something I've always wondered about, in that there's usually not a clear-cut performance difference (especially in modern knock sensor-equipped vehicles) between one grade and another under most operating conditions. If a station simply sold mid-grade as premium, who would ever know?

 

Seth, you are correct on knock sensor vehicles,, probably no way to detect the difference.. Maybe a slightly lower MPG but that would probably still be within the variables of everyday driving..

That is one reason the auto companies use their own supply of very specific test fuel with known octane & known vapor pressure.. The only way to keep the testing realistic..

 

I guess the good news is most major stations in my area must give you close to what you pay for as my Ducati knocks like crazy on mid grade & regular but only cackles a little on premium & as a rule it is fairly good on premium.. I only buy from 1st tier stations though..

 

Your GS-911 should show the spark pulled out by the knock sensors but not sure how you can ride it with the GS-911 operational & linked to a viewing device..

 

Twisty

 

[smiller]

Your GS-911 should show the spark pulled out by the knock sensors but not sure how you can ride it with the GS-911 operational & linked to a viewing device..

The GS-911 software can log data so all you would have to do is toss a laptop in your top case and ride, and analyze the data at your convenience. Or if you have a Bluetooth-equipped GS-911 and a compatible cellphone you can watch it in real time.

 

In addition to spark timing the GS-911 can also display a voltage value from the knock sensors, but the readings don't make much sense to me and I'm not sure how to interpret them.

[T__]

And back on topic, here's an interesting article. What I take from it is if (and only if!) an ECU generates enough advance to take full advantage then higher octane can result in more power, but even in the extreme cases tested in the article the difference was a matter of a few horsepower. Important in a competition environment but not likely to be very noticeable on the street.

 

Seth, that article is maybe a little misleading as the testing was done on a water cooled 10.4:1 engine.. I have been able to run most modern 10:1 engines on regular quite easily.. That is a far cry from the higher compression BMW hexhead..

 

I do agree with you that for most riding using a lower octane would probably leave the rider with no loss of power or impede engine operation a noticeable amount.. In fact pulling spark out would probably lose some peak horse power but might make the engine a little smoother due to less spark advance & softer power pulses..

 

My guess would be a more noticeable difference in real hot weather or riding loaded up a long steep grade where the power stays high for a long duration.. I would think the real difference would show up in obtainable top speed in hot weather.. The engine loading would be very high for long periods with the engine making max power..

I would like to test that as I regularly run my hexhead out to max speed but it would be very difficult to find back to back hot weather in my area with a gas tank full of different fuels.. Maybe if the difference is enough I might see it even on different days.. As it stands now even a little head wind can make a notable difference in top speed as well as what riding clothes I wear..

 

 

Twisty

 

[bmweerman]

And back on topic, here's an interesting article. What I take from it is if (and only if!) an ECU generates enough advance to take full advantage then higher octane can result in more power, but even in the extreme cases tested in the article the difference was a matter of a few horsepower. Important in a competition environment but not likely to be very noticeable on the street.

 

That was a very confusing article, and one damn ugly dyno graph. Higher octane does not, in and of itself, generate any more bang than lower octane. It is simply a rating of the fuels resistance to detonation. More spark advance would give the less combustible fuel more time to fully burn I guess therefore generating a bit more power.

 

Just a side note.. officially there is no recognized measurement for fuel over 100 octane.

 

Quoting from the API here.

Gasolines rated over 100 Octane are ones which still exhibit 100 octane properties after the compression and/or ignition timing is increased and still exhibit 100 octane properties.

 

Since there is no "official" way to measure over 100 and one created a mixture exceeded that magic number I guess you could claim that your fuel was 1000 octane and their would be no way to confirm or deny it.

 

So when they are talking about fuels that are 114+ octane I puzzle over where those numbers come from. Any thoughts?

 

[T__]

Your GS-911 should show the spark pulled out by the knock sensors but not sure how you can ride it with the GS-911 operational & linked to a viewing device..

The GS-911 software can log data so all you would have to do is toss a laptop in your top case and ride, and analyze the data at your convenience. Or if you have a Bluetooth-equipped GS-911 and a compatible cellphone you can watch it in real time.

 

In addition to spark timing the GS-911 can also display a voltage value from the knock sensors, but the readings don't make much sense to me and I'm not sure how to interpret them.

 

 

Seth, depending on what type knock sensor BMW uses-- piezoelectric sensor, or a mass type sensor, it is not only voltage but can be frequency related also..

 

OK, glad you brought this up.. On the GS-911 cell phone thing.. How good does that work for you? I presume you can monitor computer output on the phone but can’t input much through the phone to the GS-911? Are you happy with your phone connection to the GS-911?

 

I am about to get a blue tooth GS-911 so that is easy.. I am also due for a new cell phone & not sure what I need there GS-911 wise or if there is a difference between cell phones on blue tooth connectivity?

 

Twisty

 

[smiller]

Well you'll note I said that you can monitor data on your Bluetooth phone, but I can't since I don't have the Bluetooth version...

 

But to answer your questions (from info from the GS-911 website), you can view realtime data and fault codes on a Bluetooth phone but not reset codes or perform any of the fancier advanced features such as stepper calibration, etc. If your phone is not on their list of approved devices you can test compatibility via a java application that is available for download. With regard to real-world performance of the Bluetooth link, I'd have to let someone with more experience comment on that.

[Zoom]

Well at least there is some debate.

I feel that I notice the most difference under hard acceleration two-up.

I haven’t checked the gas mileage.

 

[T__]

Zoom, one of the problems with doing ride off’s on different types of gasoline is you can’t really do the ride off’s on the same day in the same weather & ambient conditions..

 

There are just so many variables between days & conditions & it takes so long to run through a tank of gasoline.. The other problem is what is the BTU content of the gasoline you start with? You just never know if the base gasoline (before additive) is the same heat content as the last time you filled up (especially in today’s alcohol laced gasoline)..

 

I guess if you feel better using the octane additive (some are just alcohol) & you feel it runs better then you are just adding $3.00 worth of Dr. feel good.. Buy one less beer at bike night & put the beer money in the gas tank & feel better longer..

 

 

Twisty

 

[smiller]

Seth, that article is maybe a little misleading as the testing was done on a water cooled 10.4:1 engine.. I have been able to run most modern 10:1 engines on regular quite easily.. That is a far cry from the higher compression BMW hexhead..

 

I do agree with you that for most riding using a lower octane would probably leave the rider with no loss of power or impede engine operation a noticeable amount.. In fact pulling spark out would probably lose some peak horse power but might make the engine a little smoother due to less spark advance & softer power pulses..

Well so far I can't tell any difference whatsoever (power or mileage) in my K-GT (13:1 CR, but water-cooled) between 89 and 91 octane (haven't had the guts to try 87 yet) and typical days here have been above 100F all summer. Nor would I really expect to since any difference that may exist would probably be very slight under normal commute duty, even though my commute is relatively high speed. I would like to do some testing with the GS-911 to see if there is any difference in spark advance under typical operating conditions but as you note it's pretty easy to chase your tail in that kind of attempt as it can be pretty difficult to control all of the variables.

 

My guess would be a more noticeable difference in real hot weather or riding loaded up a long steep grade where the power stays high for a long duration.. I would think the real difference would show up in obtainable top speed in hot weather.. The engine loading would be very high for long periods with the engine making max power..

I would like to test that as I regularly run my hexhead out to max speed but it would be very difficult to find back to back hot weather in my area with a gas tank full of different fuels..

It's difficult to get a new K-bike into a condition where it's making anywhere near max. (or even high) power for any length of time. I agree that top speed in hot weather is where the difference is going to show, but that difference might be 159 mph vs. only 155 mph. I can deal with that.

[Milestogo]

Let me add a couple of bits: After spending about 30 years in the fuels refining business and directly involved in the making of a zillion gallons of gasolines, there are still a few misconceptions floating around out there. Any gasoline that leaves a refiney is certifed to a given octane and those records are maintained forever. Review of those records and spot checks are performed by govenment agencies. The production of a gasoline that doesn't make the advertised octane can be a very costly item for the refiner. There are opportunities for dealers to mix lower grades into premium at the stations. There are some spot check of those, especially those that are branded by major oil companies. Since few of the majors own any of their stations, there is some room for fraud at the dealer level, but studies over the years have shown it to be very rare. The stakes are too high. There might be a bit more now in the horrible economy. By the way, most refiners blend to within 1/10 of a target octane, so when you buy 92 you get 92 or 92.1, not more, not less.

 

Using an fuel that has a higher octane rating than that required by the performance of the machine is a waste of money. Most BMWs that have a premium requirement run well on premium. The anti-knock instrumentation on newer engines does make some effect for lower octane fuel, but not much. They are used primarily for when the engine is under load and the insipient pinging starts. The only way to absolutely tell what octane works for a vehicle is under an individaul load test. These can be performed on a dyno (at a cost), or even easier, by the owner on a big long hill. If you hear more than an occasional "ping" try a higher octane fuel. If you don't hear (or feel) detonation, the fuel you have is OK and you might want to try a lower octane fuel to bracket your fuel.

 

Contrary to what I have read on this and other forums, there are lots of various compounds that boost octane. We used to use tetra-ethyl-lead, but that was banned, we moved to benzene/toluene/xylene and those compunds are being phased out. So we are left with a relitively narrow recipe to make a spec octane. As an aside, ethanol is an octane booster, so is butane, octane, and lots of other branched compounds. With new instrumentation, octanes in excess of 100 can be measured. These fuels are still available, but the cost would make you wilt and you thought $4 gasoline was expensive, try $30/gal or $50/gal.

 

All in all, it is best to use the octane that the manufacturer recommends. Some states don't sell the 92 any more and the 91 seems to run well in my machine. I have had instances where premium was not available and mid-grade or RUL ran fine as long as I didn't load the machine up to the point of knocking. I can get my machine to give an occasional knock on a long hill on I-5 at around 80 mph (indicated, but that type of riding is rare for me, so I'll stay with the normal pump premium. I usually gas up at all sorts of palces (do not patronize a specifi place) and never have had an indication of the dealer "upgrading" regular to Premium.

[Zoom]

Let me add a couple of bits:............

 

WOW!

 

That's one hell of a 1st post!

Thanks for taking the time.

Welcome to the forum.

[Dick_at_Lake_Tahoe_NV]

The Elevation where you ride makes the biggest difference in Octane Requirment. I have developed a "Rule of Thumb" based on experiences flying a private plane when we used to have 80/87 octane fuel. That rule is, you can lower your octane requirement by 1-point for every 1,200-1,500 feet of Elevation. Here at Lake Tahoe (6,500 feet) , I run 87 Octane in all of my vehicles and bikes--with no problem pinging at all. Pinging would be expected to occur most often under load (climbing a hill) or on the flatland at WOT--and at lower elevations. These are the conditions where the sensor system will retard the spark.

 

But for normal steady-throttle cruising, the spark will not be retarded so MPG does not decrease even with lower octane fuels. On a recent trip through Wyoming I noticed most gas stations have 85 Octane regular, and some that have 85 Regular and 87 Mid-grade--the 87 has 10% Ethanol added. I think that's how they get the 85 to 87 is by adding ethanol because ethanol has an octane rating of more than 100. By using 87 octane with 10% Ethanol you will get poorer MPG than using 85 Octane without ethanol--and at 7,000+ feet elevation, the 85 octane works just fine.

[smiller]

Let me add a couple of bits: ...

Thanks much, very interesting.

[Catbird]

I really have nothing earth-shattering to comment, just that I have used REGULAR 87 octane in my '05 RT since the first day I bought it new and it runs beautifully in all situations.

 

In my '96 RT, I had to run at least MID-GRADE 91 octane or it would ping (knock) terribly under hard acceleration. I would often run a blend of PREMIUM and MID-GRADE just to ensure all audible knock was eliminated.

[vizhip]

Great information... and welcome to the boards...

 

Regards -

-Bob

[Ken H.]

I really have nothing earth-shattering to comment, just that I have used REGULAR 87 octane in my '05 RT since the first day I bought it new and it runs beautifully in all situations.

+1 Yeah, me too in our '05 GS - no issues. Including a lot of hard 2-up way overloaded stuff.

[billS]

Interesting subject, here in Australia we have this crap they call unleaded fuel starts at 91oc then 95 premium and 98 premium. Makes me wonder how many octane standards there are around the world, my 07 RT book says use 98 prumin which I try to do and with a 12 to 1 compression ratio I’m not surprised.

 

The reason I call it crap is that as small engine mechanic I see the results of it going of in carbies, rotting fuel lines, sized two strokes for no obvious reason. Left in a tank for maybe 12 months it can smells like nothing you can explain and lingers in you airways and the air just make it worse. Our old leaded fuels used to last for years in a drum, now this stuff only seem to have a few months of life before it goes down hill. Another government balls up.

 

[Atomicman]

Just as a side note, a good friend of mine owned a BP station close to my house.

 

He only put Chevron with Techron in his own vehicles! He swore by Chevron's additive mix and Techron!

 

Following his lead, I have only used Chevron Premium 91 in my '09 RT!

 

 

[JayW]

Thanks for sharing your experience and insights, Milestogo. I found your comments very enlightening, but they also raised more questions. Were you privvy to the final destination the fuel you refined? Did you work for a specific oil company or could the end product end up in any of several or more brands of stations? Maybe the refining was tweaked and additives were mixed in based upon a specific "recipe" specified by the buyer?

 

I do not have an understanding of the process of how a particular brand of fuel is made and ends up at the pumps of that brand and not someone else's. Can you shed any light on this?

 

Thanks, and welcome!

 

Jay

[Milestogo]

Answer to a previous post: The octane posted on the pumps in the United States is actually the (R+M)/2 which is the average of the Reasearch ® and Motor (M) octanes. Other countries post only the Research octane on the pump. The Research Octane is always higher and measures the octane at a higher RPM and lower load than the motor octane which measures it at a lower RPM and higher load.

 

As to the distribution of fuels, all the refiners know pretty accurately where every delivery truck goes. On rare occasions when a bad fuel gets out into the system, refiners have had to go around and pump out all the offending gasoline from the various stations that obtained it. It is very rare becasue it is so costly to rectify.

 

Gasoline from all refineries in a region has to meet a specific set of gasoline specifications. There are more than a dozen characteristics that all gasolines in a region must meet. If there are many refineries in a region, then all must meet the same specifications. Gasolines today also have to have minimum levels of certain additives to be sold in the market. These are base additives such as detergents, because there are laws that govern these basic additives. Some companies (such as the Chevron with Techron mentioned elsewhere) have propritary additive packages that contain "secret" additives that they can advertise as better to differientiate their product from other gasolines in the marketplace.

 

Gasolines from a single refinery can and do end up in:

1)stations that may carry that oil company name

2)stations that carry another major oil companies name, and

3)stations that are generic gasoline stations or super-sellers like Costco, Sam's, or whatever.

 

If the gasoline from Brand X refinery is headed to Brand Y stations, then Brand Y's additive package is put into the individual truck at the distribution center after loading the Brand X gasoline. Gasolines that head to an unbranded station meet all specs and contain the minimum additive package.

 

Gasolines at a refinery are blended to specific "recipies" based on their processes that they have. Regardless of the different processes at a particular refinery the gasolines have to meet the specifications that all gasolines must meet. Different gasolines may have different compunds in them, but the gasoline must meet the specifications, such as research octane, motor octane, volitility, vapor pressure, distillation, gravity (or density), oxygenate requirements, and other specs as well. Once the gasoline meets these specs, then it can be sold by anyone that wants to buy it from the refinery. Chevron (as well as other majors) gasolines are sold all across the country, but Chevron does not have refineries in all regions of the US, nor does it ship gasolines all across the US. In regions that Chevron does not have refineries, they will purchase gasoline from other major refiners and independant refiners. This is the same for all companies.

[lvnvbiker]

Correct me if I am wrong, but I believe that our 87 octane regular fuel is nearly the same as the premium grades in Europe.

I had found a comparison someplace once and may be remembering it wrong, but I am pretty sure that they are close anyway. Aside from that octane is just a measure of resistance to detonation and has nothing to do with power production, and allthough I run the higher test fuel 99% of the time I have run regular when the premium was not available like at a fuel stop in rural areas with no noticable effects on performance.

[smiller]

Correct me if I am wrong, but I believe that our 87 octane regular fuel is nearly the same as the premium grades in Europe.

While the numbers may be different (RON vs. MON) our regular, mid-grade, and premium fuels are all roughly equivalent to the same tier products in Europe.

 

As has been noted a higher octane rating doesn't increase power in itself, rather the value lies in the fact that it may allow for more spark advance and that will increase efficiency and/or power... although just how much is highly dependent on engine design and operating conditions.

 

[JayW]

Very interesting, Milestogo. This answers some fuel-related questions that I have had for a long time, but could not get a credible answer for. I appreciate your detailed response.

 

Jay

[motoguy128]

Correct me if I am wrong, but I believe that our 87 octane regular fuel is nearly the same as the premium grades in Europe.

While the numbers may be different (RON vs. MON) our regular, mid-grade, and premium fuels are all roughly equivalent to the same tier products in Europe.

 

As has been noted a higher octane rating doesn't increase power in itself, rather the value lies in the fact that it may allow for more spark advance and that will increase efficiency and/or power... although just how much is highly dependent on engine design and operating conditions.

 

It should also be noted that compression ration in itself, does not indicate than and engine will require premium fuel. Thsi is a common misconception.

 

I believe the new Toyota Prius now has 13:1 compression... but still uses regular. Almost all passenger cars have at least 9.5:1 and more commonly 10:1 compression these days. A few decades ago, thsi was considered "high" compression and you woudld'nt thnk of running anything but premium. My first 600cc sportbike when stock, ran fine on 87 octane and was 12:1 compression. After I added a aftermarket exhaust and messed with the FI mapping and igntion timing...then it definitely needed premium.

 

I believe the improved design of pistons and combusiton chambers, better fuel mixing (swirl), more accurate FI and better knock detection have allowed higher compression engines to run on regular gas. Thsi si good because higher compression fundamentally will improve engine efficiency.

[Milestogo]

Our (in the US) 91-92 (R+M)/2 octane usually has a RON (Research octane number) in the 97-98 range. Countries in Europe that post a 97 or 98 RON are very close to our premium as motoguy says.

 

One of the biggest changes for lower compression engines accepting lower octane fuels is fuel injection. Gasoline designers have take a page from diesel designs and because of the atomization and mixing of injection, lower octanes have been able to be used versus carburated engines, although new carb designs have helped some also. Although higher compressions engines do produce more power and fuel effeciency, the main reason that engine manufacturers were forced to lower compression is that higher compression engines are larger polluters. The NOx out of a high compression engine is many times that of the same engine in low compression operation. Our polution laws have forced the change.

[TomfromMD]

Thanks to milestogo for his informative posts.

 

In the original (2005) 1200RT brochure, BMW actually specs 101 HP when running 93 RON (roughly US regular 87~88 octane) vs. 110 HP running the recommended 98 RON (92~93 US). The question is if this 8% reduction in peak power translates to 8% poorer fuel economy. If so, that would mean that a $.20/gal. premium for "premium" makes economic sense.

 

Either way, I run 93 (R+M)/2 (sans ethanol if I can find it).

 

Tom

[TomfromMD]

Correct me if I am wrong, but I believe that our 87 octane regular fuel is nearly the same as the premium grades in Europe.

 

Don't you mean that U.S. 87 = the regular Euro grade of 91~92? European premium is the same as ours (at least in areas where 93 U.S. is available), it simply has an different octane number referent.

 

Tom

[smiller]

The question is if this 8% reduction in peak power translates to 8% poorer fuel economy.

That would be pretty doubtful. The additional octane would be much more useful in achieving peak power than cruise efficiency. Higher octane could potentially benefit the latter if the engine can use the additional spark advance, but it's not likely the magnitude of improvement would be the same in both cases.

 

There are a lot of variables so it's not an absolute, but I would be surprised if the octane difference between the higher and lower fuel grades would pay for itself in normal cruise conditions.

[Joe Frickin' Friday]

In the original (2005) 1200RT brochure, BMW actually specs 101 HP when running 93 RON (roughly US regular 87~88 octane) vs. 110 HP running the recommended 98 RON (92~93 US). The question is if this 8% reduction in peak power translates to 8% poorer fuel economy. If so, that would mean that a $.20/gal. premium for "premium" makes economic sense.

 

Not likely. Octane is less of an issue at cruise conditions, where the RT's engine is operating at a modest fraction of peak power, and therefore experiencing far lower peak pressures/temperatures. At cruise, I'll wager the engine is operating at (or close to) MBT spark timing (and therefore optimal efficiency for the given RPM/load), regardless of whether it's being fed high or low octane fuel.

[motoguy128]

I agree, the ability to create peak power in a engine, is a different set of conditions than the ability to create the a moderate amount of power at part throttle conditons. Since both octane levels contain the same amount of energy, I would not expect any difference.

 

High octane fuel takes advantage of more optimum spark timing at higher RPM's to make more power. So what likely happening, is an extra 8HP is created because igniton of hte mixture is able to occur closer to top dead center due to slightly different flame propogation from the higher octane. You're not changing how much air and fuel makes it into the engine, only the spark timing and how the mixtures ignites.

 

Consider 2 very similar engines... a naturally aspirated 170HP Subaru Boxer engine and the turbocharged variety. Now each is tuned different, but hte same displacement, # of cylinder and similar engine clayout. The difference, is that at say 5000RPM, the NA engine is making mayb 160HP and the turbocharged variety is making 240HP. It's not like the less powerful engine is wasting fuel. They are still combusting most all of the mixture. Instead, the turbo engine is simply getting more air (with fuel) into the cylinder before the valves close. It may also ignite it sooner, so thre's more time and distance for the piston to travel after combustion has started. Whcih one generates the most pwoer with the least amount of fuel gets a little tricky. near it's torque peak, the turbo engine might be more efficient, but at lwoer crusing RPM's, it's not.

 

 

So you might get a little better fuel economy wiht premium in your RT... when cruising near wide open throttle in 6th gear at 7000RPM around 130mph.

[Tech1]

I have an 05 RT. Most of the local pumps used to sell 93 premium, now it seems that many of them are selling 91. I noticed this because the RT has started pinging, just briefly, when I suddenly crack the throttle at slightly low rpm's, below 3000rpm. It can happen easily when going around town and slow city street turns where you wouldn't quite shift down to 1st. It is annoying. I found a station still selling 93 and it is better, but it is still there. I never had any updates done to the ecu under warranty, have any updates maybe tweaked the calibration?

[Marty Hill]

Wonder what your dealer says?

[aggieengineer]

The knock sensor and associated electronics should retard the ignition timing sufficiently to eliminate detonation. You might try to locate someone with a GS-911 to see if any fault codes are present. The dealer might not even charge just to read the codes, assuming you're near a BMW dealer.

 

91 octane fuel is becoming more common in the stations that still sell conventional gasoline. It's called B.O.B. gasoline - before oxygenate blending. After adding the typical 10% ethanol, the octane rating will be up to the normal 93 for premium at lower elevations.