Premium or Regular Gas ?

(snip)

I agree with your logic and plan to test 87 PON mileage against the 93 I normally burn in my E46 - as soon we no longer have the "reformulated" winter gas. But I'm not really tempted by 89 PON as it's 10 cents more than

87, but 93 is only 10 cents more than 89 (in my area). In other words, by the pricing, "mid-grade" should be 90, not 89. Also, a new phenomena is "off brand" stations charging a differential of only 12 or 13 cents between 87 and 93, making the 93 even more of a bargain.

The manual for my BMW R1200RT motorcycle (12:1 compression) actually states that rated hp drops from 110 to 101 when the knock sensor retards the timing because of lower octane.

Wouldn't it be nice if EPA mileage figures specified the test octane on those cars which call for above regular grades?

Tom K.

Octane number has no effect at all on any of the things you mention. Octane does not affect fuel heating value (fuel energy content). Since this is so misunderstood let me explain what an octane number is/means.

First lets understand what knocking (preignition/detonation) is. Consider a cylinder that is charged with air and fuel at a proper mixture for combustion. The spark plug starts a fire in some area of the cylinder. A flame front starts out from the spark plug and it continues until the flame front crosses entire combustion volume and the fuel is consumed. This is the normal situation, where no detonation occurs. The pressure rise is steady and reasonable.

On the other hand combustion can occur in the following manner. A spark starts the fire, a flame front starts across the cylinder, raising the pressure and temperature everywhere in the cylinder. The mixture in some area of the cylinder sits there long enough that its "delay time" is exceeded. (Delay time is the time lag between when a fuel air mixture gets to the temperature/pressure conditions which would cause it to detonate, and when it actually goes off). To continue..Because of local conditions this volume of fuel air that has not been ignited by the flame front decides to go off all at once. This is called detonation or preignition and it is heard as knocking. The rate of pressure rise is instantaneous and most violent. If an engine is run in this condition for any sustained period it will be destroyed.

It was found years ago that if Octane, a liquid fuel was added to gasoline it would increase delay time and retard the the onset of detonation. Put another way Octane increases the "delay time" so that the flame can get to the area before the fuel detonates.

The Octane number of a fuel reflects the relative amount of Octane to gasoline which produces the same knock characteristic as the fuel being tested. This is tested in a research engine, under very controlled conditions. Octane is expensive and therefore additives were developed that increased fuel delay time just like octane does. In fact some additives are so effective they are more effective that 100% Octane, and this is why some gasolines have octane ratings over 100.

Provided an engine is not knocking, the octane rating of the fuel will not effect any of the things you mention. If the engine is knocking it should not be operated at those conditions and the question is moot. When an engine is knocking, power falls off, and fuel consumption increases. Cylinder pressures and temperatures skyrocket.

Gasoline alcohol mixtures can sometime exhibit high octane characteristics, however, because alcohol has a heating value lower than gasoline, high octane gasolines that use alcohol for octane control will have higher fuel consumption (lower miles/gallon) than straight gasoline.

HTH, EJ in NJ

snipped-for-privacy@veriz> >

I inadvertently said that the octane number reflects the relative ratio of Octane to gasoline. It is actually the ratio of Octane to Heptane. Octane has a long delay time, and Heptane has a short delay time.

EJ > Octane number has no effect at all on any of the things you mention.

And there was me thinking it was the *size* of their toys.

The Jaguar group, I presume?

Ximinez

Sorry to say my friend --- NO NO NO BMW E38 V8 '97.........

Jaguar cars are Ok when they are Jaguar and not FORD MONDEO reskinned. I will give that it's a nice car with all the usual Jag leather and wood but it is all looks.

Now when we move up a notch - the UK EDSEL is the "S" Type neither real retro (Dodge Charger/Barracuda) nor attractive styling. The V8 4.2 DOHC is a nice engine and better than the Real 6 pot Jaguar but the car is all wrong.

The KJ8 is still the one to buy but NOT the supercharged one - nice idea in Texas where gas is dirt cheap??? but not in the UK or Euroland.

I'll stick with a REAL engineered car BMW - Merc are far too like a TAXI and all the 'performance' models are usually V6 or like something out of a Detroit drug induced theme party - Star Wars????

Sir Hugh of Bognor

The difference between men and boys is the price of their toys. Intelligence is not knowing the answer but knowing where and how to find it!

Hugh Gundersen snipped-for-privacy@h-gee.co.uk Bognor Regis, W.Sussex, England, UK

What a great and informative discussion of this critically important issue!

My two bits worth:

Re: using lower grade fuels. I found the following in an article on the Internet, for what it is worth:

"... there are fuel refineries across the country that will occasionally blend in bigger percentages of the inexpensive and very plentiful alcohol. It is said that some of these blends can contain as much as 15 to 20 % alcohol. Is this legal? ... probably not. However in a world of discount gas stations located in outlying areas, a don't ask don't tell policy is in place when the price is right. If an unknowing motorist buys a load of 15% -

20% alcohol fuel, his car starts a little harder and runs a little lazier. How can you know the true content? ... you can't. This underlines the need to buy fuel at a name brand retailer, or a station that you are familiar with.

"Another complication of alcohol is the high solvency. That is, it has a lesser lubricicity than the fuel it is being blended with. This higher solvency literally washes internal engine surfaces that require a presence of lubrication.

"The last significant complication with alcohol is that it attracts water. That is to say that it literally draws water out of the surrounding atmosphere into the fuel. This drawn-in water then enters the engine along with the fuel and oil. While the percentage of water content is low, it further contributes to the internal "washing" of surfaces needing lubrication. Blends that contain more alcohol can attract more water."

A final note: I use Chevron 94 Octane (the highest rating available in Canada) in my 2001 VDP. Chevron contains Techron, which has been formulated to remove potentially harmful deposits from injectors, intake valves, and combustion chambers. Currently it costs $1.11 a litre -- about 10 cents a litre more than 91 Octane and about 12 cents a litre more than 87 Octane.

Alan Strickland Victoria BC Canada

You might want to check the recent state law changes in your area. In my state, Maryland, ethanol is now virtually mandated (see link )and has replaced MTBE for the "reformulated" gas we get from November to March. From what I can tell, my mileage is a bit worse and the pumps are not so labeled!

Tom K.

I suppose that is another thing we have in the UK and Euroland - UK = British Standards and anything sold must meet those standards. Ok the standards can change but not at some lardarse's whim. Similar things on the continent of Europe and especially in Germany (DIN).

I do believe that Japan has some stringent standards too and the one that sticks in my mind is the one about Alloy Wheels on cars. At one time there were loads of accidents with cheaply made flash wheels breaking and causing deaths. Japan made it illegal to fit Alloy wheels to vehicles unless the car came with them from the maker. Ok - the world has moved on and Jap cars do have aftermarket wheels but only after some stringent tests and law making......... I don't think I would like to buy flash wheels from China as they appear to be about 20 years behind everybody else on standards.

Sir Hugh of Bognor

The difference between men and boys is the price of their toys. Intelligence is not knowing the answer but knowing where and how to find it!

Hugh Gundersen snipped-for-privacy@h-gee.co.uk Bognor Regis, W.Sussex, England, UK

You also think it is just a Ford. Just because it shares some components with a Mondeo does not mean it is the same car with a different skin.

If the manufacturer recommends a specific grade of fuel why not follow it? It is not as if Ford sells petrol, is it?

DAS

Wow, reviving a dead thread, are we.

I do think it is a Ford. It is a Ford. It shared more than components. It's the same car with prettier bodywork.

I don't think being like the Mondeo is necessarily a bad thing.

I lived in the UK from 1994 - 1997 and one of the cars I had was a leased Mondeo II 24v V6. I liked the car. It was quick, nimble and had good utilitarian features as well as being reasonably economical. I think being like the Mondeo is a plus, not a minus. The Mondeo was far and away superior to the Sterling/Rover I had the time I was there before in the early 90's.

When I turned it in it still had the wheel covers on the left hand side of the car too.

I never said it was a bad thing. Great cars. Considering one for my teenagers first car, since it is AWD. People are reading things into my posts that weren't there.

My only point was that it isn't an XK8 and the performance difference of regular vs. premium would be really trivial in this car.

Hi Richard,

My comments were aimed at the larger audience rather than you.

I think that if the engine internals (compression ratio, computer software etc.) are indeed the same, you are correct.

However, I believe that the only engine block is shared (in the 3.0 liter 227 hp engine) and the x-type has jaguar high compression heads, ignition, fuel system and computer management. The engine (my understanding) is built by ford on a separate assembly line. This may account for the discrepancy?

Been away....

DAS

For direct contact replace nospam with schmetterling

I love it when dead threads are revived Double Tap