If this has not been covered I offer the following: 1) Our BMWs contain "High Performance" engines which means our cars have a high compression ratio compared to regular engines. 2) The higher the octane, the more stable the fuel. (please feel free to correct me on any point) Effect: Lower octane fuel ignites under less compression than higher octane fuel. Our high compression engines will prematurely ignite lower octane fuel before the spark does (the sound of knocking). Our engine computers will then compensate for the premature fuel ignition by slightly altering the timing (of the spark I guess). Doing so will prevent knocking however your engine is now operating at less than peak performance AND you have introduced a timing variable that is adding additional stress to the engine (as in igniting the chamber before the cam for that cylinder reaches the end of it's stroke which I). The stress may or may not have any long term effect engine wear (I simply do not know). Also, you will receive a drop in MPG (to what extent varies due to several environmental factors). My advice to all is not to purchase a vehicle that you cannot afford to operate and maintain. Life happens to us all so one may have purchased a BMW and then life through them a financial curve ball. Beyond that exception, if the difference in cost between octanes in gasoline factors into one's monthly budget I question the rationale behind purchasing this vehicle vs a lower cost vehicle with far better fuel economy. Showroom floor today would price our cars at $60,000 to $80,000 (if the e39 were still being manufactured today). Trying to drive an executive class car on the ultra-cheap is an embarrassment. Don't be mistaken, I've made several bonehead decisions throughout my life (will make more I'm sure), purchasing a car I can't afford just happens to not have been one of them.
Sorry. I don't buy that. More is not better. I believe gasoline companies pretty much buy from the same place. Much like orange juice. The different between companies are their additives and quality control. Octane has nothing to do with it.
I'm finding it frustrating that this is turning out to be a really poor forum to have a technical discussion. Some of you people really need to get a room with yourselves. When I was road racing, WITH THE EXPENSIVE RACING CARS WHICH HAD REALLY EXPENSIVE "HIGH PERFORMANCE" ENGINES, WHICH I OWNED AND WHICH WERE TOWED WITH EQUALLY EXPENSIVE TOW VEHICLES, we played around all the time with different octane fuels. I found that in some of those cars, they actually made more power with lower octane fuels than with the higher octane ones. With the tow vehicles, we ramped up all the way to 93 octane even though the manufacturer said to run 87 to prevent pinging. (Lord knows, you never want to do something other than what the holy manufacturer tells you to do.) That said, I can afford the car and any freaken' fuel it needs. I didn't buy it because it has a BMW emblem on the hood. I just wanted to see if anyone had tried lower octane fuels to see what their experiences were. What I don't understand is people who think it's a status symbol to pay for higher octane fuels if the car doesn't need it. Octane basically controls how fast the flame front propagates through the cylinder. BMW has to tune engines for all types of climates and applications. If your not running in high temps and in the mountains, you might not need to run 91 to prevent knock. 89 might work just fine so why pay for 93? I'm going to use data, not opinions to determine what I'm going to run. If it turns out that the car gets better mileage and performance running 93 than I'll run that. If 89 or the recommended 91 is better, than I'll run that. If someplace around here besides Sunoco sold 91, I never would have even asked the question.
As I say, as long as it's Top Tier fuel, I would think 89-octane should be fine. The difference is the additives, of course. I like Chevron because of Techron. You people with direct injection won't get any benefit from it, but those of us with valves downstream of the fuel supply---downwind?---keep our valves cleaner with it.
When my 2006 M Roadster came up with a cold start stumble, my dealer was quick to blame cheap "American Gas" for the problem... Seems like the 10% alcohol content, at our Texas pumps, is not a good match for our hi-tech engines either. Never fed the M anything but 93 octane, so octane is not the only factor.
seemyad was on point. Higher octane fuel is more resistant to predetonation. It has nothing to do with how the flame front propagates through the combustion chamber. That has more to do with the chemical makeup of the fuel itself, not the addition of octane. Also, additive packages at a single gas station are typically the same across different grades of fuel. As mentioned previously, the octane ratings between North America and Europe are different. Mike Miller wrote about this in Bimmer. Short of a long explanation, BWM power plants are not designed with the fuel grades we have in the US. In retrospect, no, more octane doesnt mean more power. Octane simply means that when your mixture reaches a certain compression point it will not predetonate. Examples: If you put 93 in a 8.5:1 compression ratio cylinder it wont have any more noticable effect than putting in 87. That engine is designed to run with maximum efficiency at 87. Alternatively, if you have an engine with a compression ratio of 11.5:1 that is designed to run at maximum efficiency on 93, and instead you put in 87, you will get predetonation. That means, in this example, that prior to a cylinder reaching TDC, the mixture will detonate while the cylinder is still moving up. More clearly, your mixture is expanding while your cylinder is still attempting to compress. Ping. This is when those sensors start making a difference and retarding timing. These computer controlled systems now provide a wider margin of error by allowing changes in timing. Changing the timing changes the power output of the engine. Thus, it can be said that in cases of higher compression, higher octane produces more power (than lower octane gas in the same engine). But that is not the case of engines with lower compression ratios. An additional note: on older engines with deposit build up, higher octane fuels can help restore the power these engines once had. Deposit build up takes up space, thus artificially increasing compression. Higher compression requires higher octane for maximum efficiency. Wait, have I said that already? Keep in mind those are all examples. Most BMW's have high compression ratios. What does that mean? hm? Sometimes when an engine is turboed compression is lower by design. Because guess what the turbo does? We should keep that in mind now that BMW has gone to turbo route to adhere to CAFE standards. You can throw around your internet knowledge and spreadsheets and supposed experience with drag racers, jets, and rocket ships on the internet all day. Ultimately, higher compression engines need higher octane to maintain power and maximum use of the fuel for better mileage. You can use lower octane fuel if you wish and claim to maintain the same fuel economy and performance, but the physics and engineering are not going to agree with you.
Yes, I know. This is what happens when you read your owner's manual and then decide to ask a simple question on an on line forum because you're curious. I'm currently trying a tankful of 89, 89 & 93 mixed and 93 back to back to see if the in car mileage meter shows any difference. The 89 & 93 mix came in at 25.8 mpg but it was hovering at 26.1 right until the last half block before the station. I'm currently on the 89 and it's running at between 26.4 mpg and 26.1 mpg but I have about a quarter tank left. I'll update again when I get the final figures. Also, I've been doing more reading to try to understand the physics involved more clearly. I was wrong when I thought that octane affected flame front speed. two30grain, you provided a really good explanation. Thanks !
Although, when it comes to slowing the flame front, additives like propylene oxide are useful in racing engines. . . . Wait---what? What was the topic again?
Tell you what, when it comes down to octane, pick the car that meets the price you want to pay at the pump. I get excited when I find a station that carries 94 octane. I don't know, it makes me feel like I'm doing something good for the vehicle that I chose to drive. Do you buy a $1,000 suit & $10.00 tie? C'mon man...
After reading for a while I think the OP's question is an intellectual exercise rather than a desire to live on the cheap. I withdraw my earlier comment.
Something to consider: While the new 2013 BMW F30 328i says on the filler to use "Premium" fuel, the minimum octane recommended on the same label is actually 89 AKI. That is a change from previous models. I did not check the M5 to see what it said, but I will. Where I live I use Kroger gas (yes, the grocery store) and have for years in my E34 525i 5-sp with no problems. Sam's Club when that's more convenient. I noticed the same tanker trucks filling up the Shell tanks as the Kroger tanks and our Shell station even offers a discount to Kroger card holders. Hmm. Shell Super is what we use in our dealership vehicles. My car has over 200,000 miles on it with just one spark-plug change and no injection or valve issues.
You are exactly right this was an intellectual exercise for me. I just wanted to see what the car would do, if anything, when running on different octane levels. I just finished the third tank full and so here are the results: 1/2 89 and 1/2 93 = 25.8mpg (Hovered around 26.1mpg until about the last 1/2 block) 89 = 26.4mpg (Hovered around 26.1mpg until about the last mile) 93 = 25.8mpg (Dipped down to this figure from 26.1mpg the day before and stayed there) So, what we are really talking about here was a rough difference of about 0.3mpg between various types. I think this is more attributable to differences in the drive cycles, temperatures, etc than to the type of gasoline. So overall, I think running different octanes under these conditions didn't really seem to make much of a difference because I don't think that the timing retardation happens all that often (under these conditions). That said, I'm going to start running straight 93 as I did notice a very slight and subjective difference when accelerating. The car just seemed slightly more responsive on the higher octane. That is, if the vehicle did trip over into a retarded timing situation, it was probably only when I was hard on it passing trucks as no steep grades, high temperatures, etc. were involved in the drive cycles. Overall, it was an interesting exercise to try even if I should have never posted about it.
Au contraire: I think the discussion was probably enlightening for people too shy to ask questions...
Ditto - I think it's been interesting, and has been worthwhile. It never hurts to ask anything, even if the discussion doesn't go as expected.
I, for one, am glad you posted. I've also been thinking about experimenting with 93 in my coupe since that's what it's "supposed" to drink. Problem is that I haven't found 93 in the Bay Area. (Admittedly, I haven't tried all that hard.) I was tempted to put in a 50/50 mix of 89 & 96 from the pumps at Sears Point (while visiting Wine Country Motorsport), right up until I saw the $7.59/gal price tag for the 96. Ouch!
I promised the roadster that if we ever get to the track pumps, I'll make sure the tank is empty, and I'll treat it to a few gallons of High-Test. . . .
This is something I have found funny while traveling around the country here in the midwest our 89 fuel is cheaper then our 87 and way cheaper then our 91. Hence the reason I switched to 89 in my 325i my M5 however gets 91 and when I can find it 93 which is seldom. Garrison has a great point about the BMW's thought process behind there programming for fuel mapping. BMW knows that in the US there a different grades of fuel and qualities depending on where there customers are living. I have some of my customers that live in rule areas that don't always get the best fuel but there cars still run decent no pinging or misfires but do of course suffer lower fuel mileage and power. I mean for instance if I suspect one of my customers has been filling up with bad fuel I can hook the car up to the scan tool and see there past 30 tanks of fuel and see what octane they have been using. The engine management system is smart enough to actually tell that you have just filled up and it needs to figure out what kind of fuel is in the tank after that then it selects the appropriate fuel characteristic map and ignition timing for the fuel in the tank. We run into problems with fuel when the mechanical system components of the car can not adjust for the type of fuel thus causing detonation, misfires. This is where I agree with Satch about the S engines needing the higher octane because these engine were designed to be performance machines with very tight mechanical tolerances both physically and virtually with fuel injection timing, VANOS operation. Wow that was a lot longer then I thought sorry about that.
No need to apologize for lengthy posts, stuff you'd say in 18.5 seconds face-to-face takes awhile to write out. There are any number of tome-like posts from prior times, it might even be remotely possible that _I've_ written a few.
