I am moving soon from Germany (where my Mini hit it's maxmimum speed of about 120 MPH) to a location where only 87 Octane Unleaded fuel is available. What are some possible problems that I might encounter? For starters, is the MC's knock sensor going to be able to compensate to make up for the lack of Premium fuel? Also, if it makes any difference whatsoever, the location where I am heading is at sea-level, 80 degrees fahrenheit, and speed limits do not exceed 35 MPH. Should I be looking at octane enhancing fuel additives or not?

Thanks,
jafawce

 

Check outthis and this.

 

You give a lot of information about where you are going but you don't say whether the 87 Octane is a USA measurement or European. If it is European then you will probably be OK, if it is USA then some Octane boost may help. How long you will be using this fuel will also enter into the decision, a few tanks - no problem.

 

My mistake...the fuel is US 87 Octane. As far as a time frame, I'm looking at a minimum of one year, so just burning through a couple of tanks of fuel isn't a workable solution (I suppose I could always buy a bicycle and fire up the Mini once a week to show off in!).



_________________
2002 Cosmos Black Metallic Cooper w/Beige Leather, Premium Package, DSC, Spoiler, 16" Wheels w/Run-Flats.

 

It depends on where you are. There are many claims that in Colorado, because of the high average altitude, you don't need as high of octane as at lower altitudes. In Colorado we get 87, 89 and 91 octane (in some places it's actually 86, 88 and 90). In California that goes as high as 89, 91 and 93.

I can buy that on naturally aspirated engines but I have my doubts about force fed engines such as on the MCS. If you don't have a supercharger, and are relocating to a high altitude, you may be able to get by with 87 octane with no problems at all.

The way that works is if you are at sea level and have 15psi atmospheric pressure and have 10-1 compression, your cylinder pressure ends up around 150psi. At a higher altitude with (for example, I've not looked it up) 12psi atmospheric pressure you end up with 120psi cylinder pressures. You wouldn't need as much octane to prevent pre-ignition.

Since you are almost doubling the pressure being fed to the cylinders with the supercharger, I would recommend sticking with as high of octane as possible. In the example above, at sea level with 15psi, boost an additional 12psi with the supercharger so you are feeding the engine 27psi. Cylinder compression brings that up to 270psi. If you go to a higher altitude that only drops to 240psi, not nearly as large a difference.

I know there's more to the issue than that (with lower compression on many force fed engines, different timing, etc) but this simplification will help get things started.

Keith Hamburger

 

Interesting...So, that I am sure I follow what you're saying, the lower octane fuels (at sea level) would possibly not be all that bad (or, at least not AS bad) for a naturally-aspirated MC. But, that such fuels would not be that good for an MCS with its supercharger. Do I sort of get it? Or, was I reading too much into what you posted?

_________________
2002 Cosmos Black Metallic Cooper w/Beige Leather, Premium Package, DSC, Spoiler, 16" Wheels w/Run-Flats.

 

That would be my opinion but I'm not an automotive engineer, just a computer salesman with a physics degree.

Given the number differentials I tossed out I don't fully understand why the two models require the same octane. Some cars reduce cylinder compression drastically for superchargers or turbochargers compared to naturally aspirated systems but I don't know what MINI has done in that regard.

I find it hard to believe that anyplace in the US has stations that don't sell at least 89 octane at sea level. Where you're headed must be quite isolated with only one or two gas stations for miles around.

Keith Hamburger

 

>>Given the number differentials I tossed out I don't fully understand why the two models require the same octane. Some cars reduce cylinder compression drastically for superchargers or turbochargers compared to naturally aspirated systems but I don't know what MINI has done in that regard.
>>
>>Keith Hamburger

The mini Cooper is 10.6:1 compression ratio the S is reduced to 8.3:1 for all the reason that you spelled out above in your previous post. Much better than I could I might add.
Both need same higher octane because of the high compression in both engines. The engines will run on 87 but your going to get poorer fuel economy for sure. What the long (1 year) term effect will be I don't know. I have read in a couple of other threads that the car did not run as well and had poor performance.

 

I've wondered lately exactly what the combination of factors is that requires the HO fuel. (I understand the chamber shape, compression and spark advance all play a role here.)

The reason I started thinking about it is that I have a bike with a compression ratio north of 11:1 (~130 crank HP/liter) that runs well on 87 octane.

I wonder just what the margin of safety is inherent in the mini specs?

Jeff

 

A big factor in the requirements for higher octane has to do with the timing. The ignition fires at some point before top-dead-center (TDC) so that the flame wave has time to propogate as the piston moves through its cycle to the top. Pre-ignition is caused (in a new engine) by diesel combustion, the fuel-air mixture will ignite due to compression at some point. (For those that don't know, that is how a diesel engine works, no spark to ignite the mixture, just high enough compression that it ignites itself.) Higher octane fuel is harder to ignite and therefore reduces pre-ignition.

If the mixture ignites before the spark arrives, that is called pre-ignition and is what causes knocking. In modern cars the engine has sensors to detect knocking and will adjust the timing to compensate for it. By changing the valve timing and igniting the fuel-air mixture earlier than was intended you can eliminate knocking without using higher octane fuel, but at a cost of performance.

In the case of your motorcycle I would expect the reason it doesn't use high octane fuel is that at the highest point of its HP curve the piston is moving so fast that the ignition can ignite the fuel quite early in the cycle and the flame wave front will not completely consume the fuel until the piston reaches TDC. Where a car engine running at 6000RPM might need to ignite the mixture at 10 degrees BTDC (before TDC), your motorcycle engine running at 10,000RPM might throw a spark at 20 degrees BTDC.

Yes, chamber shape, piston diameter and a lot more work into that by affecting the amount of time requred for the flame wave to move through the entire mixture but the basic principles are the same.

It used to be said that higher octane fuel will not improve performance but that was before cars had all these sophisticated sensors and computers. On an older car, as long as your engine isn't knocking, no additional octane will help. On today's cars, just because your engine isn't knocking doesn't mean that you're getting the most performance possible. Higher octane can change the way the computer operates the engine to provide more power.

That is up to a certain limit, hopefully the manufacturer has given us appropriate fuel requirements to get the most performance out of the vehicle. Beyond a certain point there can be no return for higher octane fuel. It would be great if someone could run the curves for varying octane fuel on a Dynometer and report to us what the very best octane rating is, but, then there is the problem of the ECU having to relearn every time and waiting for that to happen with different octane ratings will add inconsistency to the performance reports. I really don't know how we could find out for certain what the best octane for our cars would be.

Boy, I do like to go on, don't I.

Keith Hamburger

 

Thanks - you've cleared up some of the mystery. The lowest octane fuel that you can run on without preignition will net the highest power. The explanation I read (as I butcher it here) was something to the effect of lower-octane fuel burns more quickly and exert greater mechanical leverage as the piston travels through TDC.

Octane is, after all, resistance to burning (well, I'm oversimplifying.) The argument, though, was that there is a specific energy in gasoline, and that you obtained greater power by releasing it all at once vs. a slower burn.

Several people I've talked to have done dyno comparisons to confirm this, but it's been so long the details and magnitude of the difference are lost in the cobwebs of my brain.

Jeff

ps: This argument goes out the window when you start talking about oxygenated racing fuels, which have a greater potential energy.

 

>>Thanks - you've cleared up some of the mystery. The lowest octane fuel that you can run on without preignition will net the highest power. The explanation I read (as I butcher it here) was something to the effect of lower-octane fuel burns more quickly and exert greater mechanical leverage as the piston travels through TDC.
>>
>>Octane is, after all, resistance to burning (well, I'm oversimplifying.) The argument, though, was that there is a specific energy in gasoline, and that you obtained greater power by releasing it all at once vs. a slower burn.

Well, not quite. Lower octane fuel is easier to ignite so it will pre-ignite at lower pressures. As the piston travels to the top of the cycle, the pressure in the cylinder increases. If a given octane would preignite at 15 degrees bTDC, a higher octane might preignite at 10 degrees bTDC. What you want to really happen is for the fuel to ignite when you give it spark, not before.

What it comes down to is that you want the fuel to burn under control, not on its own. Higher octane (may) stop all pre-ignition (won't burn at all until it gets a spark) so that the engine can ignite it at the best time. If the fuel pre-ignites before the engine wants it to the engine will have to play games to take back control. Those games result in a loss of power.

Keith

 

>>Given the number differentials I tossed out I don't fully understand why the two models require the same octane.

I don't understand it either. I had a Honda Civic, with a 1.6 liter motor, that produced a peak horsepower of 127 on 87 octane fuel. The mini , also with a 4 cylinder, 1.6 liter motor, makes 115 horsepower but requires premium fuel?
I've looked at the peak torque values, and the Mini does have a couple of pounds more torque at a few hundred rpm lower in the rpm band, but the differences do not seem large enough for the different requirements of octane...

 

jafawce,

I just read a post in the Owners Lounge from a guy in IL that has been running on 87 Octane for 6 months now. He claims no problems at all.

Have you spoken to your dealer about the move? What do that have to say and recommend? Since it is a "world car" I'm sure that you are not the first person to ask this question. Premium is best, but whit if?

 

I doubt you're going to 'kill' your MINI, but it might not like it's old age as much..

Rocketboy_X

 

I'm real interested in khamburger's and friedduck's discussion. I have run my MCS on 93 octane at 500-600ft mean sea level (MSL) since it was new in November. I did not want the knock sensor to do anything to the performance. Through this thread I now find out that more energy might be available with a lower octane. OK I need a knock sensor light so that I know when it's interfering with my performance. Seriously, at my MSL, could I run 91, get more HP and save $$$?

 

Mine drinks only 94 .... just like my rum and cokes... 151 all the way... LOL

 

No, you won't get more energy out of lower octane. But you are wasting money if you use more octane than the engine needs. The book says 91 octane which implies that if you use that you will get maximum performance. Higher octane won't decrease performance but it won't help, either.

Unlike what was said (by someone else in response to one of my messages) earlier, higher octane doesn't burn slower or with less power, it simply is harder to ignite. If you apply a spark to it all gasoline has pretty much the same power and burns just fine.

If you want to spend more money with questionable benefit, that's your business but you're not giving up any performance.

Keith

 

Just wanted to pass this along...i once read in a very detailed report on fuel octane ratings...the bottom line was you wont get ANY performance increase with higher octane and it is a waste of money. Also,they said you may see some benifit from high octane in a full on high compression racing motor...otherwise,it's just hype and same thing for octane boost...just my .02

 

Sorry, missed the discussion for a couple of days. Khamburger is right. Once you reach the maximum octane that your engine needs, all else is wasted.

I forgot to mention something that's important. Engines that can take advantage of higher octane, and advance the timing automatically, will make more power with the higher octane. I don't know what the limits are, nor do I know if the correllary is true--that the mini will retard timing if it senses lower octane. (Wish I did, as it would answer the original question.)

Khamburger, I told you that I'd butcher the physics behind my statement and I probably have. The gist of it was that gasoline has a potential energy that's constant. Unless you're using oxygenated fuel or something, octane doesn't add power, just allows the use of higher compression or more advanced timing without detonation. (Octane is resistance to detonation as you rightly put it.)

For engines that don't need it, though, (like my Ducati SS), the lower octane will produce higher HP ratings (and in our conversation we were only talking about 2%difference.) My recollection of the discussion between the tuners/engineers was that this is because lower octane fuel burns more quickly, or the flame front advances more quickly, or something to that effect. If you want me to dig up the details I'll find out.

Jeff (not an expert, don't play one on TV.)


_________________
Jeff in ATL

 

Well here in PA/NJ we can't get 91 so I typically use 93 and sometimes 94. Is it possible that the ECU could sense this increased/decreased pressure and compensate for it? Also theory is nice but does anyone have any dyno tests of a comparison of a lower vs. a higher octane.

My basic understanding of fuel is that the MINI will run on 89 but you will definately have a noticable performance decrease. I believe Sunoco is the only company that has 94 octane... This is what my dealer filled my MINI up with (94 octane) as there was a Sunoco down the street. (I guess where they fill up all their MINIs)

Some people say that this 94 octane is just a gimic while others swear by it. Has anyone done any actual tests? (on higher vs lower octane fuel)
-Chris

 

>>Well here in PA/NJ we can't get 91 so I typically use 93 and sometimes 94. Is it possible that the ECU could sense this increased/decreased pressure and compensate for it? Also theory is nice but does anyone have any dyno tests of a comparison of a lower vs. a higher octane.

I think there might be a problem running dyno tests on different octanes. First, you would have to run the tank dry and put more fuel in at the different octane. And second, and possibly most significant, the car might not adjust to the new octane very quickly. You would probably have to do the dyno runs several days, if not weeks apart, and that's not considered good technique.

>>My basic understanding of fuel is that the MINI will run on 89 but you will definately have a noticable performance decrease. I believe Sunoco is the only company that has 94 octane... This is what my dealer filled my MINI up with (94 octane) as there was a Sunoco down the street. (I guess where they fill up all their MINIs)
>>
>>Some people say that this 94 octane is just a gimic while others swear by it. Has anyone done any actual tests? (on higher vs lower octane fuel)
>>-Chris

If it works for you and you have the money to spend, you're not hurting anything. You're unlikely to see any significant performance difference but the MINI does have a very high tech engine so you might be able to get something for your money.

Keith