Here we go again-

Octane rating of a fuel has no connection, direct or indirect, with the type or amount of detergents contained in the fuel. There's little or no evidence that S-types need anything other than the basic additives contained in all brands of fuel. There's no evidence that Brand X gas is better then Brand Y etc.

Octane rating also has no connection to the 'quality' or 'purity' of a fuel. Gas is gas- it's all the same stuff that simply has varying degrees of additives blended into it just before delivery to the retailer.

As for octane requirements, Jaguar specifies the minimum octane level required for the home market in which the car was sold that will ensure rated performance, ie. detonation free operation. Should lower octane fuel be substituted, detonation may be encountered under certain operating conditions. This means that the 'knock sensors' will immediately retard ignition timing until such condition is eliminated.

In a practical sense, the actual conditions where detonation would be encountered (and the expensive additives contained in high octane fuel pay their way) are quite well defined, predictable, and actually make up only a small percentage of typical operation. Essentially, other than full throttle operation or when 'lugging' the engine in a manual transmission equipped car, high octane fuel is a waste of money.

I'm surprised that no one has developed a system whereby the car would run on low octane fuel from the main supply tank and be supplemented on an as-required basis with a high octane concentrate from a separate supply tank. I suppose the initial cost of doing this far exceeds the simple knock sensor system which is inexpensive to incorporate but causes the owner to pay for expensive high octane fuel throughout the life of the car.

Business opportunity anyone?

 

In earlier times I was chief chemist at 2 oil refineries owned by a recognized major. I know the specs of low-octane regular and high-octane premium gas, and I have seen the octane-testing engines at work in the lab.

Just for the record, the 2 types of test method for octane number are Research (RON) and Motor (MON). Each method measures the tendency of the fuel to pre-ignite (or knock) as combustion pressure increases, but with engine either under load or not under load.

Typically, there is a difference of 6-8 between RON and MON so, for example a premium gas might have a RON of 98 and a MON of 90. The 2 numbers are usually combined and reported as a Road Octane Index (ROI) or Average Knock Index (AKI) which, in this case, would be 94.

Apart from the fact that pre-ignition is an engine killer, low-octane regular gas was regarded as the poor relation of refinery products, with not-as-tight quality specs, where less expensive additives were used and a few barrels of off-spec refinery streams could safely be dumped during blending without going off-grade.

So, let's put it this way. If I was buying a used Jag, I would definitely prefer one where the owner had used a better-quality gas.

 

So wait, my magnetic fuel saver won't help me?

 

You got the Jag for free? In that case, perhaps consider the extra $5 per tank for premium to be your "car payment."

 

'Pre-ignition' is NOT the same thing as knock/detonation/pinging/pinking or any of the other common nicknames. The two phenomena are commonly confused but are in fact quite different and are initiated by very separate processes. Pre-ignition is most commonly caused by a spurious 'hot spot' in the combustion chamber and as the name suggests is the result of a 'hot spot' igniting the fuel air mixture prematurely rather than by the spark plug itself. Other than the element of being premature, the combustion event itself is relatively normal in nature.

Detonation on the other hand is an uncontrolled series of spontaneous 'explosions' of the fuel, frequently initiated at multiple sites in the combustion chamber after the plug has fired, rather than the desired subsonic deflagration or controlled burning initiated solely by the spark plug.

Octane rating of a fuel therefore has no effect on pre-ignition, other than a potential secondary effect of quelling extended detonation events that may lead to pre-ignition from overheated combustion chamber components.

 

Thought I'd stop by......

 

If you think they are unrelated, Mikey, that's cool. All I said was 'octane measures the tendency for pre-ignition or knock to occur'.

Here's a transcript from a knowledge base:

"Pre-ignition and engine knock both sharply increase combustion chamber temperatures. Consequently, either effect increases the likelihood of the other effect occurring, and both can produce similar effects from the operator's perspective, such as rough engine operation or loss of performance".

 

I just finished paying for my premium unleaded here at seven eleven and thought I would grab a brewsky for lunch, WOW ... please help, the price of beer just went up and I want to get the best beer for the price. Which one should I buy? Which one is the best??
Now I am also wondering if I should have even bought gas at this store, is it good to buying gas here? My Jag is down a quart of oil, Which one should I buy while I'm here?
Thanks guys. please hurry cause I can't wait all day.

 

Let me help you out here get a six pack of the good stuff to start. After that any other brew will do within reason

 

Ding, ding, ding, ding! We have a winner! The secret word of the day was OIL! Please drop by our broadcast studios within the next 20 minutes to pick up your prize and thanks again for listening to Radio Octane!

 

Think it's time for everyone to take a break and tickle an otter....

 

YES!! full moon is coming,and you can sure see it here,today!

i drive a Chevy 2500HD 6.0L V8, and i can tell you for sure it has more HP on 93 premium than on 87 regular gasoline, seat of pants guess,is 10/12hp. runs and drives great other than trying to pass or pull away from traffic signal!

so question; does ECM retard ignition by signal from knock sensor, or using any other sensors, like O2 sensor information,, or both??

its a fuel hog no matter what fuel is in it!

Ron

 

also,(as mentioned here before), would adding somthing like Alcohol injection device be of any help , only used at over 75% throttle opening, where higher octane would be required?

of course there would be the cost of Alcohol,but most are adjustable for amount used.

i have tried E85 fuel and it just sucks it up before you can get home.

just had a brain flash!! maybe put E85 in Alcohol resiviuor, much cheaper than Straight alky in small tank.

anyone have input info on this subject??
Ron

 

Of relevance to controlling detonation, only the knock sensor is used. If your Chevy is prone to this condition, it's likely that the spark is consistently being retarded from optimum which results in the reduced power level you've noted. Many S-type owners have switched to mid or low octane gas and noticed no difference in operation.

 

Pure alcohol (ethanol or methanol ) has an AKI octane number of 99 or 100ish, so a fairly large volume would be required to boost regular 87 to premium 91 levels and to compensate for the reduced energy content, as compared to gasoline.

E85 has an AKI of 94-95 so an even larger volume would be required to overcome the detonation and energy loss.

Good idea though! Possibly methane would be a solution, 120 AKI!

 

OK, my 2 cents....

I trust users like Mikey because of their vast experience in this kind of stuff. Like he has said in general, you won't kill your motor running lower octane fuel.. BUT... Here is my experience. I only ran what was recommended for my 3.0 when I had it. I will admit that I tried lower octane once.. and though I thought it was down on performance..that was probably just a mind trick. What I did measure was a reduction in fuel economy. So, any savings gained with a lower per gallon cost was eliminated by an increase in consumption. So, back to premium I went. The other thing I noticed, since I used cheap fuel, is that a catalytic converter died in my car at about 60 to 65k miles. Very strange.. I think the cheap fuel with ethanol in it was the culprit. So, I changed to a "better" brand that claimed they use very little ethanol.

I kept doing that even when I switched cars to the 07 300 with the 5.7V8. I found out some more interesting stuff the more I got into this car. It, unlike the Jag, does not have Variable Valve Timing. It is stated in the manual that you need to run a minimum 89 octane (mid-grade) so I did. I then got a tuning device. Basically a OBD plug in device that reflashes the PCM. It has a few "canned" tunes in it. 91 octane, 93 octane, and 87 octane for those that want to run regular. It also has instructions on how to modify the 91 octane tune to run 89 octane. To do that, you have to retard 6 different timing settings in the device and save them to the car. What these tunes basically do is set up the timing maps to match the octane that you run so you don't trigger the knock sensors. They also have a data capture mode where you test if any knock sensors are triggered.

So, after that wordy babble, here is what I found on this car.... again, I never modify my driving style. Plenty of wide open throttle passes along with in town driving and open highway cruising at 75 to 80 mph. On the 87 tune the car felt doggy off the line, but was fine on the highway. Terrible MPG though. Set-up for 89... Better drivability but same MPG as 87. 91 octane, much better performance, best MPG. 93 octane tune, will chirp the tires off the line in first and second gear changes, pretty much the same MPG as the 91 octane.

I will now run the 91 octane tune in the winter since I don't want the car to be that aggressive in case of poor weather. In the other months I run the 93 octane tune. Again, with this car as well as the Jag, MPG seems to be affected by octane. So, just like in the Jag, any money I might save by running lower octane is wiped out by worse MPG.

I did a road trip in the 300 to Missouri in January, with the car loaded up with a passenger and an entire dining room set broken down and loaded in the car. On the trip out we returned an average MPG of 24 something, on the trip back without the cargo load, it got 25.8 MPG.... Not bad for a 2+ ton boat going 80..... My normal combined MPG sits at 21 to 22.. Less if I run more errands on the weekend due to in town driving.

let the flaming begin... LOL

 

awwwwwwwwwwwwwww

 

Actually methanol is used and works very well to boost pump gas to be used in performance applications. Most people can run the same boost on a meth inj kit and 91 as they do on 110 race fuel. Its used daily to run race gas boost on pump gas with very good success.

 

At $2. a gallon premium was 2.25 that's 12.5% difference.
Yesterday I filled my tank at $4.69 regular was only $4.59
That's only 2% more.

 

Generally (but not at cold start or WOT) it's trying to get low emissions so relies on the MAF and O2s to get the fuel right and it knows when to trigger spark (using LOAD, RPM, etc). Any knocks detected will cause it to back timing off and may flag codes. It may also go OL (open loop) in which case it will run rich.

Running rich reduces MPG and damages cats.

It will try not to run lean because that's an engine killer.