Not to make this another MPG thread, it can be one of two things: 1) my exhaust, 2) I dont have "flats" or remotely close flat highways, they are hilly, especially up the "Mass Pike" (Route 90) through the Berkshires towards upstate New York. Those are the only two variables. I have never seen 28mpg, ever.

 

Do you have an OBD tool you could use to monitor the car's readings?

I just bought an elm327 for a mate from ebay for £13 (under $20), delivered.

 

I've rebuilt more than my share of motors over the years and can tell you that those engines that required higher octane fuels had significantly less coating on the valves....

9.1:1 is a high compression motor. Plus, when you consider the amount of pressure being added within the combustion chamber with the supercharger - well, pre-detonation will occur.

I've put in low octane in Vermont when I couldn't find anything else and my car ran like ****. I got terrible mileage and it took a couple tanks of the good stuff to get things running right.

And yes, I meant ignition.

 

I think I can explain that. At or near sea level in the US, octane will run 87 reg, 89 mid, 93 premium. At higher altitudes, posted octane values will decrease. Why? Think about it - the effective maximum cylinder pressure on the compression stroke at 6000 ft of elevation will be less than at sea level due to the lower atmospheric pressure available for the intake charge - at least for a normally aspirated engine. The only wrinkle that I see in this equation is that our supercharged engines are effectively operating at sea level anyway, due to the compression assist from said supercharger. I don't worry about it on my bike, as it is normally aspirated, but my Jag and Saab are both blown - so for them I use only premium at higher altitudes. At sea level I can mix premium and mid grade to maintain close to 91 octane - but if in doubt I just load the premium. What the hey...20 cents or less...big deal.

 

I have actually tested this in my Audi I used to have and it won't save you any money on a long trip. I ran regular one way and premium back the same way and actually got better gas mileage with the premium so the extra 3.00 you will save per fill up you are just gonna put it right back in the tank for the less miles you will get from that tank.

 

Thanks for sharing the story Vance.

Bob S.

 

Thanks for the rundown, I figured it had something to do with the altitude, but was unsure of the effect on vehicles. I put 85 in our 2010, Jeep Commander with 8k on it. Ran like CRAP. Dont know if it was the elevation, octane of just the vehicle. Possibly all of the above.

So your saying the same gas coming off of Exxon Valdez slated to be "regular" will be posted at 85 in Colorado and 87 in Boston. Doesn't it have to do with additives as well?

 

Interesting question. I've always thought the gas must be formulated to the lower octane level, but can't swear to that. Mere re-labeling doesn't make much sense to me. I was just trying to share my understanding of the physics involved - coming from my experience with aircraft engines in years past. The last tactical aircraft that I flew was the AV8A (first Harrier) - thrust calculations were part of flying that one, which kept you always conscious of temperature and elevation (density altitude). Reciprocating engines are similarly affected, with blowers (super chargers) only complicating the issue.

 

Got to agree with Bob S and disagree with my esteemed fellow countryman on this one. Octane rating has no connection with the level of cleaning agents in gasoline. The marketing BS (and Shell is the worst) would lead you to believe otherwise, but there is sufficient additives in any brand name gasoline to get the job done, short term or long. There is no technical reason for high compression/performance engines to require more cleaning additives than a 'regular' engine.

Regarding ignition timing, neither the supercharged or normally aspirated engine is anywhere near the point of pre-ignition at idle or light load (highway cruising on level ground). No engine is built to depend on knock sensors under those conditions.

Yes, the cars will suffer reduced performance with use of less than optimum octane levels but no damage will be incurred.

 

Using Regular Gas — Low Octane — Is Likely Costing You More Than If You Used Premium — High Octane Gas.

If you’re you trying to save money by using a lower octane gas than your owner’s manual calls for, I can almost guarantee that you will actually spend more money on gas (depending on the cost difference of the choices between 87 octane and 93 octane). Why? Because it’s a certainty that you will get terrible gas mileage. Isn’t that ironic? And horsepower? Forget it. But the worst part is that it will eventually harm your engine. Badly. And you won’t know about it until it’s too late.

Surprised? Sure. I’ll bet that you’ve heard that there is no reason to buy premium gas because the regular gas is just as good. That’s wrong! Then there’s the belief that the higher octane premium gas is just another rip-off by the oil companies. Wrong again!

First let’s look at what octane means. It’s not about horsepower or heat energy. All an octane measure of 87 or 89 or 92 or whatever, means is that the number signifies the average of two methods of testing a fuel’s resistance to detonation. They typically range from 87 octane for regular unleaded gas to 93 octane for premium fuel. If you read most of these signs you will see a notation that the octane has been calculated using the formula R+M/2. We could go into another page to explain that those are shorthand for the two tests performed and divided by 2 for an average between them.

Without going into all the scientific stuff, this means that a premium, higher octane fuel has a much higher resistance to detonation than does a regular fuel of 87 octane. You read it right. Resistance to detonation. So what’s this about detonation? The resistance to detonation is the primary quality of a good gasoline. This is desirable so the detonation inside the cylinder head can be ignited evenly; the spark plug ignites the flame front in the cylinder head as a mixture of gas and oxygen. The flame moves in a deliberate manner across the top of the piston in the cylinder head. And that’s how it’s supposed to work.

So the better the fuel’s resistance to spontaneous igniting the more controlled the burning of the gas and the subsequent expansion of the heated gases that pushes the piston down to turn the crankshaft. But when a fuel with a lower detonation is used where it shouldn’t be used (in an engine whose manufacturer says it shouldn’t be used) the whole process goes out the window.

Abnormal combustion is what it’s called. And that’s because the intense heat and pressure in the cylinder subjects the lower detonation fuel (the 87 octane fuel) to an environment that it wasn’t meant for. So pockets of the fuel can spontaneously self-ignite creating secondary flame flashes. When those meet in the out-of-control burning in the cylinder head, the engine gets a shock wave. This shock wave is the knocking sound we have all heard in an engine at some time or another, usually when accelerating or under a heavy load hill climbing. Sometimes it sounds like marbles rolling around in the engine. Other times it actually sounds like the piston rods are loose and rattling. Whatever it sounds like, it’s not good for your engine . . . really not good!

So now comes the part where technology almost outwits us again. In older cars – those that didn’t have computer controlled engine functions, we all knew that the knocking sound meant that either we were using el cheapo gas or the spark timing was wrong. That was easy enough to fix. Just buy better gas or have your mechanic re-set the timing. Some guys could do that themselves in their own garage or under a shade tree. Just use a timing light and synchronize it with the main belt running by turning the distributor to adjust the spark up or down. Old, old cars used to have a little lever on the steering column by which one could adjust the spark after the engine started. Kind of like adjusting a choke until it sounds right.

But now we have these ingenious high-tech engines that are monitored by an on-board computer and they are constantly making adjustments to correct or improve what the computer perceives as a problem. Here’s where we fool ourselves. Or rather where the engine fools us and we believe it. These little critters are equipped with knock sensors.

When they detect the typical bad detonation from using low octane fuel, the little devils automatically retard the spark timing or take other precautions to protect the engine. So, in reality, the sensors are de-tuning the engine to accept the fuel’s octane. The designers put this in to protect the engine from damage, but only for the short run. But with the spark timing compromised, you may not have a clue that it’s happening. But the engine’s performance is just shot. This is why it looses horsepower so badly, as I pointed out earlier.

In addition to robbing the engine of power, the electronic computerized de-tuning has put the engine into a mode that, by the nature of the low resistance, uncontrolled detonation, wastes the fuel’s energy to power the vehicle. So, if you think you’ve outsmarted the oil company, think again. You’re among their best customers because your vehicle’s miles per gallon are in the toilet. Said another way, you use more of their product than you need to.

The techno-wizardry of this is amazing. But the down-side is that the electronic computerized controls – the techno-wizardry — can mask the inferior detonation so effectively that you may never be aware that it’s happening. So you are merrily driving along thinking how clever you were to not fall for that premium gas trick at the service station. Meanwhile your car is running sluggishly and you are using a great deal more gas than you should be using.

 

Well your handle/user-id is very apt. You're certainly enthusiatic about your argument. That's a very nice and long explanation. I think I agree with quite a bit of it too. Just don't get so worked up about it. I only posed a hypothetical question and few people challenged "some" parts of your argument not the entire thing. Can we relax a bit now?

Bob S.

 

I didn't write that. Just copy and pasted it. I will leave it at that except to say again that lower octane fuels typically have fewer detergents in them and continued use of said gas in a high compression engine will lead to deposits. I'm done now.

 

What Enthusiast copy pasted is pretty well 100% accurate and true- except for a few critical things that are left out. IF an engine was on the verge of suffering detonation/pre-ignition/knocking/pinging AND the knock sensors were constantly required to retard timing, then yes it would be counter productive to use low octane gas. BUT as stated above, neither version of the 4.2L engine is anywhere near that threshold under light or constant throttle.

I guess we'll have to agree to disagree about the relative quantities of detergents, the relationship with octane levels and their actual need.

 

Yes, yes, yes over and out ...

 

Have a read of this:

http://www.cartalk.com/content/featu...ths.html#myth2

 

I run cheapo premium gas from BJ's wholesale club. Along with it, I run Lucas upper cylinder lubricant. It nets me about 1mpg more in city driving. The ethanol blend gasoline does not have the lubricants that the older gasoline had, so putting it in is a cost saving measure for me.

Based on what I have read here, I may decide to switch to a name brand gas. Any suggestions? Is something like QT or Racetrac a name brand? The reason I ask is that they are as ubiquitous as any other brand I can recall.

 

No gasoline ever had (or needed) 'lubricants' so I'm not sure what your intent is. Modern cars were designed and built with E10 in mind so there is no need to compensate with any snake oils.

 

Er... your source for that? And since when? Was E10 even around when the S-Type's engines were on the drawing boards? (Especially the 4.0 / 4.2, which were designed and built here in England.)

 

I think they're being reckless with their statements about running regular instead of premium though they do qualify the statement with "modern".
You ARE going to be missing out on the full performance unless the engine really doesn't need premium to perate at it's maximum potential. But most people don't understand what octane rating is all about and the term "premium" probably is more indicative of the pricing differential.

Bob S.

 

E10 has been available here for at least 15 years, nothing 'new' about it. A friend of mine worked on the Ford/Jaguar cooperative effort to bring the Lincoln LS and S-type project from design to production and can vouch for the fact that both cars were designed with ethanol fuels in mind.

Aside from ensuring that rubber components exposed to the fuel were compatible, no real design changes would have been required anyway.

It amazes me the amount of myth, misunderstanding and voodoo science that exists, but that's the benefit of these forums I guess.