Another octane lesson needed
 

I thought that my R12S would detune itself if I put lesss than 94 in the tank. Apparently not. All the debates about fuel I've seen here have to do with RON equivalents etc. but have not specifically addressed what octane is optimum (despite the manufacturer's recommendation).

Surprise - Today my trusted mechanic suggested I use lower octane fuel to get better performance and better mileage (yes, both). I just about fell off my stool. Since the beginning of time I've been using Chevron 94 and, from the looks of things, a lot of fellow Pelicans are octane junkies too.

Could it be we've got it all wrong?

My last tank was a meager 91 octane because that was all that was available for miles around. I waited for the drop in performance as I rode away from the pump... and it never came. The bike's torque curve felt more immediate and the power band was smoother up to 7500RPM or so with no end in sight. It felt like I had magically unlocked another engine or removed some sort of governor while I was expecting exactly the opposite.

Later today, Mr. Fuel Expert (I have friends in the industry) said something about a better burn rate with 91 and that he, a veteran of many bikes and many miles, just completed a 1,500 mile trip with never more than 87 in the tank and great performance.

Further, he said that all fuels (in Canada) have the same cleaning components and the hype that the higher octane fuels clean "better" is a push to spend an extra dollar. Shhhh....

Comments from the other knowledgeable members among us?

While I don't know that I am more "knowledgable" than you on this subject, several of my clients build race engines, parts of engines or tune them. Over the years I have taken a lot of hi performance mods in "trade" for my tax and accounting services and this subject has come up several times. Without exception they have all said "run the lowest octane possible without preignition to get the most power and best economy out of your engine".

The trick, of course, is to figure out what the "lowest octane possible" actually is without damaging your engine.

Is it true that the R1200 series engines will retard timing if they sense preignition conditions via the O2 sensor ? I have heard that bandied about on several R1200 forums.

If so, that would seem to indicate that a lower than spec octane rating would add soem efficiency if the air temps were cool and the engine load not too high (i.e. no lugging). Again, I don't really know how to figure that out without damaging my engine if I'm wrong, the bike doesn't trim the timing, or the detonation happens anyway - so I always run max recommended octane. Here in CA that means only 91 (r+m/2). I have never had audible detonation on either my R1200ST or my R1200S even in 100+deg days and hard engine flogging but fear keeps me from trying 89 or 87.

I do know its a complete waste of time, money, RPG and power to use 91 in my Mazda CX-9 which specifically calls for 87.

JT

Here is a definition of octane rating:

The octane rating is a measure of the resistance of petrol and other fuels to autoignition in spark-ignition internal combustion engines.

The octane number of a fuel is measured in a test engine, and is defined by comparison with the mixture of 2,2,4-trimethylpentane (iso-octane) and heptane which would have the same anti-knocking capacity as the fuel under test: the percentage, by volume, of 2,2,4-trimethylpentane in that mixture is the octane number of the fuel. For example, petrol with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90.[1] This 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 higher than 100.

Octane rating does not relate to the energy content of the fuel (see 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 octane is raised by blending in ethanol, energy content per volume is reduced.

It is possible for a fuel to have a Research Octane Number (RON) greater than 100, because iso-octane is not the most knock-resistant substance available. Racing fuels, AvGas, liquefied petroleum gas (LPG), and alcohol fuels such as methanol or ethanol may have octane ratings of 110 or significantly higher — ethanol's RON is 129 (116 MON, 122 AKI).[citation needed] Typical "octane booster" gasoline additives include MTBE, ETBE, isooctane and toluene. Lead in the form of tetra-ethyl lead was once a common additive, but since the 1970s, its use in most of the industrialised world has been restricted, and its use is currently limited mostly to aviation gasoline.


The interesting thing is that additives used to raise octane rating, actually reduce energy density, so there is your explanation, but it is contingent on what is being used.

At the risk of sounding stupid or oversimplyfing, the higher the compression ratio the higher octane required (to prevent pre detonation). If an engine is designed for a lower octane, higher octane will not (necessarily) be of any benefit. Written another way, running higher octane than the engine is designed for will not increase performance. :confused:

If the octane is higher than the engine is intended for you're just messing up the timing and not maximizing combustion. Right?

Or, I'm all washed up. :(

That's how it was explained to me today. I'm trying to reconcile that with the manufacturers recommendation on my fuel cap and the denizens of this forum that speak in favor of higher octane fuels.

Ian

My understanding is higher octane fuel has a higher resistance to detonation. Also, it burns more slowly than lower octane fuel. Because of that, the ignition timing can be advanced more resulting in an explosion that is more in synch with the movement of the piston rather than too quick.

Roger will straighten us out shortly, I'm sure. I'm hoping for the gold star vs the dunce cap this go-round...

89 R+N blend with ethanol from Weigel does make weird noises (not as dramatic a real preignition though, just some loud "clicking", like detuned valvetrain). 89 from other brands does not, and 89 Weigel without ethanol didn't either. I switched back to non-ethanol 91 only since then. The few bucks saved don't worth the risk.

Higher octane fuels are better resistant to detonation caused by high compression. Unfortunately, it also burns slower than lower octane gas, so you need more ignition advance to get the same power output (basically getting the 'bang' at the right time). Now there lies the problem: if an engine is designed and setup for high octane fuel, putting in low octane fuel will make it more susceptible for detonation due to the high compression, and the increased ignition advance will make it even more prone to detonation.

Fortunately, all newer BMW's like the R12S have a knock sensor, which will retard ignition if it detects detonation. If the octane rating is low so that the knock sensor adjusts timing due to fuel octane, but high enough to prevent detonation due to high compression, like 91 should be in most cases, there will be no performance loss. Performance loss will only occur if the advance has to be retarded beyond the optimal for the fuel burn to avoid detonation. Mind you, there should also not be any performance gain.

What is oilhead cylinder compression rated at 11.3:1, right? It puts it into category of performance engines. Anything above 11:1 is. Even though R11s lacks knock sensor, cylinder compression will dectate what octane to use.
I've tested all graes/octanes of fuel. I will not use AKI 87 (USA). Although I havn't noticed reduction in performance, I've noticed increased pinging and occasional knock (under load using wrong gear). AKI 89 gives fair performance with absense of above symptoms. Pinging and occasional knock have only happened when running at or slightly above normal opertional temps using AKI 87. I've also tried Candian AKI 94. Haven noticed inrease in perfromance, but the engine ran louder and hotter thus consuming slightly more oil than usual.
So, in my subjective oppinon I'd stick with 91-92 octane fuels for optimum engine perfomance, avoiding fuel station which add ethanol (ARCO i particular).
BMW rcommends 95 RON (EU) which translates into 91-92 AKI US grade.

Oilhead R11S is recommended to use 98RON, not 95RON. At least according to the sticker on my fuel cap. It will knock and ping on 95RON when in the mountains, so I don't use it.

Hexheads, have (I think) a recommendation for 95RON, but the K series have a 98RON recommendation.

EU usually has higher octane fuels than US hence the difference in stickers between ur beemer and mine ... mine states 95ROZ/85MOZ ... SmileWavy

There are a ton of misconceptions on this thread:

1. Euro fuels are not higher octane, but just have a higher octane number than US fuels because they use the Research Octane Number (RON) method of octane measurement. US fuels use "pump octane" which is an average of the RON and the MON (motor octane number). Since MON is typically 10 pts lower than RON for the same fuel, a 95 RON fuel will typically be 85 MON or a 90 pump octane. That's why BMW specs 95 RON in Europe and 90 or 91 pump octane in the states - both spec essentially the same fuel.

2. Octane appetite in an engine is dependent on many factors, not just CR. Air-cooling and big bores raise the required octane as much as a high CR, that's the reason an air-cooled big-twin Harley with a 9.0:1 CR will ping like crazy on a hot day running 91 fuel, while a water-cooled R6 with a 13:0:1 CR doesn't ping at all running 86 fuel.

3. Energy densities of different grades of fuel at the pump do not typically vary much by octane. (This assumes they're all gas or at least all have the same percentage of ethanol if they have ethanol. Ethanol does have lower energy density by about a third, but I've never heard that the percentage of ethanol varies depending on what grade of fuel you buy, and in any event, it is only about 10% of the mix at worst. And please, please, there are other threads on this and other forums if you want to rant about ethanol.)

4. The idea that you will get more/less power out of a stock engine by running different octane (either higher or lower) generally doesn't hold water. Two exceptions are: 1) If the engine is octane-adapative, continually adjusting engine parameters on the fly to stay right on the edge of knock (which no motorcycle engine currently is); and 2) If you're under such demanding conditions (load, temps) that the engine knocks on the octane you're running. Under scenario #2, if the engine has knock sensors (like the R1200), then engine timing will be retarded to control the knock and that will reduce power. If the engine doesn't have knock sensors, then the knock itself will reduce power. In either case, running higher octane should help restore some of the lost power. But if the engine is not knocking, then you're not losing any power and running higher octane will do diddly-squat.

5. Octane doesn't materially affect the rate the fuel is burned in the engine. The reason you retard the ignition to control knock is to have the spark plug ignite the mixture before it is preignited by compression alone. That's the whole idea of a higher octane fuel - to allow more ignition advance before the fuel ignites on its own.

It's been said a thousand times before by a thousand engineers: USE WHATEVER OCTANE IS RECOMMENDED OR THAT KEEPS THE ENGINE FROM PINGING. Beyond this there is little or no benefit to going higher or lower. Octane is just the resistance to engine knocking - nothing more and nothing less.

- Mark

right, I'm sticking with my fuel lid advice 98 which I think is RON over here

What about having a bike with no cat, does that mean anything?

That's the bottom line. The only thing not mentioned in any of the posts above is that the ratings are based on fresh gasoline. The toulene in it, as well as other ingredients, used to boost it's rating evaporates over time. Faster in the 6.5 gallon plastic gas cans I keep in the garage and slower in the station storage tank but sometimes I buy gas in very remote areas where their tank isn't filled as often. So I always buy 93 "premium" and forget about the rest.

Yeah , the leftovers coming out the tail pipe smell better . :D

Bad Gasoline

Here is someone elses .02. YMMV

Philip

I'm not seeing the logic in this. My understanding is retarding the ignition is to delay the explosion so it doesn't finish too early for the position of the piston and cause a knock. I stand by my statement that higher octanes burn more slowly. Engines with knock sensors retard the timing to compensate for the faster-burning, lower octane gas and make less power as a result.

Its not just about knock, you also want the peak combustion pressure at the ideal crank angle to generate maximum torque. If the ignition timing does not match the fuel octane and combustion chamber requirements that does not happen.

The HEX head comes in three forms, R12S (12.5:1) which requires 98 RON, the R12RT and ST (12:1) which require 95 RON and the R12GS (11:1) which can use 91 RON.

Going one step up in octane is harmless and conducive to smooth running, however using a lower grade than recommended is typically not good. My R12S ran like crap on 95RON (98 RON is recommended), the engine vibrations where bad enough to put my hands to sleep.

Octane ratings and their relationship to the PSDTA factor:

Waiting until fuel tank is almost empty, invest in a gallon of the lowest grade fuel available.
Ride bike, short shift (eg: 4th gear at 30 MPH), crack throttle wide open, use ears to listen.
Repeat a few times. If it pings, it qualifies as an official PSDTA (Probably Shouldn't Do That Again).

go up a step in octane until pinging/knocking/detonation or PSDTA is no longer present.

(PSDTA is neither metric or english, RON or RAM, left or right. It requires no conversion charts, adapters or translation. PSDTA is applicable to almost everything from buying fuel with too low of an octane rating, to putting your face into a campfire).

Problem with that system is it dose not take into account knock sensors that retard ignition timing to eliminate knock, the engine damaging knock is addressed but performance suffers significantly when inadequate fuel is used.