The compression ratio of the MC is 10.5 to 1. The MCS has a lower compression ratio, I think it's 8.5 to 1. However, the supercharger forces more air into the cylinders resulting in higher cylinder pressures than in the MC. I don't think the automatic timing change that occurs when using different octane gas can affect the compression ratio, (which is determined by the physical design ot the pistons and cylinder head), but it can change the instant at which the spark plug fires the gas/air mix, which can prevent knocking by exploding the mixture earlier in the compression stroke, preventing pre-ignition due to compression only. Exploding the mixture early can cause a slight power reduction compared to optimum timing.

Actually I don't have a clue but I read back the above paragraph and kind of liked it.

The assumption that something called "premium" is better than something called "regular" causes billions of dollars to be wasted by motorist in cars than gain nothing from the higher octane fuel.

But, if it makes you feel better, go for it. I really don't know for certain if the MINI benefits from using higher octane on not....it might, but 90% of today's cars don't.
That's a fact your friendly oil company would rather you didn't know.

 

I just follow what it says in the manual and on the inside of the gas door; "premium"

 

Umm... I believe you are mistaken my friend. Even Honda four cylinders(deffinately not in a high state of tune) benefit from the added octane. 10 extra horsepower can be acceived in the Accord if you compare regular gas versus premium. This was sited from this months Car and Driver's question and answer section.

Also my MCS runs about one mph faster at the drag strip on sunnoco gt100. I'm sure at a certain point higher octane would not yield higher horsepower... But with regular versus premium there is horsepower to be had.

However if you feel that you want to make a stand against the oil company, for such high prices on premium fuels, for all means go for it.

 

This was a good question and I was wondering that myself. Personally, I have only used Premium. Therefore I've no idea what if any changes in performance would actually occur if I were to simply put in regular. But what I do know is that it cost me over 30.00 just to fill up my little Mini with Premium. Just my 2 cents...If it's do-able dollar wise it might just be more advantages maintance wise longterm to use the better grade of gas.

 

I could be wrong about the MINI and how much it benefits or doesn't benefit from high octane gas. What I'm saying is without some kind of controlled test I don't know for sure.

I'll stick with my statement that 90% of todays cars will get no benefit from using premium.

As I said in a previous post, using premium at least during the warranty period makes sense because it's possible a dealer could use the gas as a reason to deny warranty work, whether ir not it had anything to do with the problem. Although that seems unlikely, it could happen. This is true even though the dealers often put regular in the cars on their own lots.

 

There is a chance that even with Hi test you get lower than advertised octane. A friend who used to have several stations in the gas business says that it is a regular practice to empty out trucks with low octane into high octane tanks so that the drivers can go home etc. Obviously there is more to the story but my friend got out of the business because he thought it was too unscrupulous. Some gas companies are better than others. If you notice your car runs better on one brand over another he said it is not necessarily the gas but the integrirty of the gas company in selling you what your paying for. This man has years of experience tuning/racing at all levels so I trust him.

It's not that I wasn't interested but I just buy the best that's available and don't worry about it.
just something to add to the thread

 

i have put nothing but premium in my pepe and have threatened my husband when he almost put regular fuel in - mine gets excellent gas mileage and i'm certain that premium is the reason

i've checked the gas mileage difference (between tank of premium and regular) in other cars i've owned and premium is definately worth the extra money in the long run - i wouldn't suggest trying this in your mini though just stick with premium from now on

 

i have a question... what does the cooper manual recommend? does it say to use premium or regular? if it says premium, then any arguments are invalid. you need to use what is recommended to get the best performance out of your car. but, if it says to use regular, you aren't going to gain anything with premium. you're probably just experiencing the placebo affect. you believe it is better and so feel like you are getting better performance/mileage even though you most likely aren't.



http://www.straightdope.com/columns/041008.html

 

My friend did this for a project, and he scientifically proved that it was better to use premium octane gas. He proved it was more powerful and more effiecient (meaning more MPG) than regular. The difference in MPG is greater than that of the cost in fuel.

 

Only for cars that require it.

Back to the OP. Why did you bother buying a MCS if you are cocnered about the extra 5% in gas costs? Wouldn't a MC have made a better choice for you since I routinely used to pull close to 40MPG with it compared to the 25-28MPG in my MCS.

 

I have a 2003 MINI Cooper and it clearly states in my manual to use a minimum of 91 octane. This is definitley higher than regular (87) or the higher rated regulars (89). I generally use *Shell v-power gas and in my area (St. Louis) it has a rating of 93. On ocassion I have used other premium gas that were rated at 92. I have never had any problems associated with gas requirements with my vehicle, and with my combination of highway and city driving I get 30mpg, and that is okay with me.:smile:

*I have a Shell credit card and save 5 cents on every dollar of gas I purchase. Doesn't sound like much, but it does add up.

 

I run my regular Cooper on a mixture of water, pine needles and saltine crackers. It gets poor mileage and the exhaust smells odd, but it beats buying gasoline.

 

hey prcoop, thanks for the info. if they recommend premium fuel, then you will definitely get a benefit from using it.

 

I agree. If you were so concerned about the cost of feul, you should have purchased the MINI Cooper.

Just suck it up or sell it!

I switched from something that drank regular fuel at the rate of two large tanks a week. Now, doing the same driving, I go thru 1 tank a week of premium. I have no complaints about the high cost of fuel

Sip, don't guzzle!

 

Ok, I'm curious. In what car(s) did he test, what were the parameters, and the specific conclusions? Since octane only relates to the resistance of early detonation, I'm at a loss as to why it would improve MPG. I can understand a power increase in modern cars that will adjust timing based on knock.

I put premium in my forced induction cars, and regular in the ones that call for it.

 

If you put the same 87 octane gas in all the pumps and gave it a different name the consensus would still be that the premium gave better mileage, performed better, etc., etc. This would be a test I'd like to see.

 

I'll have all the details by the end of the week.

 

Great - I'll look forward to it!

 

BTW, ask him to show how much cleaner burning 91 is compared to 87.

That should be interesting. :smile:

A few points. A fuel mixture burns, it does not explode. Secondly, the higher the octane rating, the slower the fuel burns. 87 octane burns faster and cleaner than higher octane. So you greens out there, shame on you for owning an MCS.

An engine designed to run on 87 will gain very little, if anything, by running higher octane. In fact, it will probably produce more dirty emissions. In the real world, it is a waste of money to buy the more expensive fuel.

An engine designed to run on 91 will lose performance on lower octane. As long as no engine damaging detonation occurs, there should be no harm in running a lower octane, just the loss of some power. Under normal driving conditions, you may not even notice the loss.

What does make a difference is the true grade of fuel. Premium brand fuels (Chevron, 76, etc.) usually contain more calories per gallon than do the cheap brands. Hence, better fuel economy with the premium brands. This used to be more true a few decades ago. And who really knows when talking California gasoline.

The typical retail price difference between the 3 common grades is in 10 cent increments. Sorry to tell you that the cost to the refinery is about 2 cents. High octane fuels = higher profits. Wasting money on 91 when the engine doesn't need it just makes the oil companies more money.

 

An engine that is designed to run on premium fuel needs premium fuel to run efficiently and safely. If the engine has knock sensors then lower octane fuel can be used in an emergency situation, at a cost to performance. Knock sensors are basically tuned microphones that pick up specific frequencies of engine knocks. They then send a signal to the ECU to retard the timing until the knocking stops. Everytime the engine knocks a harmful and powerful shockwave travels through the cylinder and the potential for damage exists. So if an engine is always run with low octane fuel the engine will knock multiple times a day, which over the lifetime of an engine is a bad thing. Also with the timing retarded the performance and fuel economy will suffer offsetting any cost savings of cheaper fuel.


Dave

 

Doh!!! You beat me to it.....yes, naturally aspirated cars will have a higher compression ratio than supercharged cars....good call.


Use premium if you have a supercharger...knock retard will kill your performance and you'll get less enjoyment from your MINI S if you continue to use regular, or switch between the two (which I'm still trying to figure out why you were told to do so)

Dave

 

I agree with that statement, but I've gotta add that way over 90% of today's cars are not supercharged Coopers!

 

That is not entirely correct. Yes...normally apirated cars can have a higher compression ratio, but the real reason the MCS has a lower ratio is because the supercharger effectively increases your ratio when it is blowing. Turbines like superchargers and turbos "Force" air into the engine, effectively compressing the air through the impellers and then into the engine. The amount of PSI "Boost" pre-compresses the air so the cylinder doesn't have to.

The lower compression ratio is really required. I'm not entirely clear on specific numbers, but if you figure that the boost from the SC takes that 8.5 and makes it 12.5 effectively, pushing that into a 10.5 compression ratio will boost that to at least 14.5, mostly likely more since your space is even less. It will probably make you go faster, but the engine will also die a lot more quickly.

Cars with higher compression ratios usually require 91+ fuel to avoid pre-detonation which has been covered here ad nauseum. Since you're compressing more you have a higher chance of pre-detonation which throws off the timing. The knock you hear, is the spark plug firing with nothing to detonate. Higher octane fuel burns slower so it protects against pre-detonation.

 

Yes, I hear you..you're right, and well, right again, in that you were actually echoing what I said as well. But the knock I was referring to was not the "pinging" you get when the plugs fire and there's nothing to detonate....in alot of cases, knock retard consists of many different degrees of knock, from actual knock to false KR, such as the noises a transmission can make that your knock sensors on the block will pick up on and retard timing...it wasn't really your engine or pre detonation, but rather something as simple as the downpipe torquing and hitting against something else metal, thus causing "false knock", or a gear grinding in the tranny on a quick downshift, and causing false knock picked up by the sensors....

But alas, the point being, premium gas SHOULD be run in a forced induction car like the MINI S, it just makes no sense to try and save dough at the risk of poor peformance.

Dave.

 

I look at it like this... Does the mini deserve premium? Yes it does... Simple as that