Now that gas is officially thru the roof, what are member's thoughts on running your Jag on mid-grade versus premium? I have a 2006 XJ-8 LWB. I have heard rumors, still researching, that here in PA, the mid-grade is actually a better blend with no ethnol added? I tried adding a hald tank of mid-grade on top of my half tank of premium and seem okay thus far?? Thoughts???

 

I've been running regular 87 with octane booster from an auto parts store. Seems to run fine. Sometimes I mix premium and regular 50/50 when I don't have octane booster. All modern cars have knock sensors that retard spark timing. Even if they are officially "premium only" it doesn't hurt to run lower octane, just reduces max HP. Exception may be high mileage cars in the summer, when they get more prone to pre-detonation.

 

This question comes up every week or so, have a look using the search feature. Octane boosters are 99.9% snake oil and do nothing. For the .1% that are real, it's cheaper to buy higher octane gas than add the bottles of booster. When they advertise boosting octane '1 point' it means going from 87 to 87.1, not to 88.

 

One has to look at cost per mile, not cost per gallon.

Right after I got my 2005 XJ8L (in 2010) I had the opportunity to make road trips to see our two sons, about a month apart. On each trip I was the sole driver, following the exact same routes, so the main variables would be temperature and maybe winds.

So I made one trip (678 miles in 4 legs) after having filled the car with nothing but 93 octane for a few weeks. I reset the trip computer before each of the 4 legs and logged the results. Each leg is 99% highway driving. Then during the next month I bought mid-grade 87 octane each time I gassed up. Then we made the same trip again, and again I noted the gas mileage.

What I found was that with the 93 octane I got 7 percent better mileage. And the price difference of the gas was 4 percent. So ever since I've only run premium.

My wife applied a different "scientific" method: She said, "The manual says Premium. So don't be such a cheapskate - buy premium!"

Ray

 

It was a few years ago (maybe a decade+) when we had Caltex Techron 100 and I was driving a Mitsubishi Spacewagon Turbo at that time (a JDM Spacewagon 7 seater which had an Evo III driveline in it) and the difference between 100 Octane and 98 Octane in terms of fuel consumption was noticeable (about 10% from my failing memory). Unfortunately they stopped selling the 100 after a few years as they said the average consumer wasn't willing to pay the difference, not realising that you get it back in the fuel consumption improvement. I've been using the highest possible (98 only sadly) octane rating that we have over here ever since.

 

Octane rating of a gas is a measure of it's resistance to detonation- NOTHING else. All gas/petrol contains the same amount of energy.

 

Mikey,

I'm sure you're right about all gas containing the same amount of energy.

But I would maintain that the difference in performance and/or mileage can be real, in certain circumstances. If an engine is designed to run on, say, 83 octane regular, then buying 93 octane would be a waste of money.

But, with a high-compression engine meant to run on 93, the reason it CAN run on 87 or lower is that the computer/anit-knock sensor system retards the ignition timing enough to prevent pre-ignition. This results in lower performance.

At least, I believe that's what is going on. I'm hoping folks more expert than I am will weigh in.

Cheers

Ray

 

In actuality, regular has more BTUs of energy than premium. The products added to premium to reduce the burn rate decrease its energy output. The thing that makes premium work to make more power is not the energy content, it is the fact that the ignition can be advanced without detonation. This is why using premium in a car designed for it will give better mileage than lower octane. It is true that it is generally cheaper to use premium in a car with knock sensors. When they retard th eigntion, the economy goes out the window.

 

This is true- except that our modern Jags do not seem to detonate under steady state highway conditions when using low octane gas.

BTW- high octane gas does not have a reduced burn rate- it simply has higher 'resistance' to spontaneous combustion. Burn rates (flame propagation velocity) are unrelated to octane ratings.

 

Mikey: "Octane boosters are 99.9% snake oil and do nothing. For the .1% that are real, it's cheaper to buy higher octane gas than add the bottles of booster."

Well, read the following: Fuel Rating - Octane Comparison - Dyno Tests Graphs - Hot Rod Magazine

In summary, the octane boosters available off-the-shelf are most effective with 87 octane regular gas. The do result in a modest maximum HP bump at optimal (advanced) timing.

There's another test in European Car that comes to a similar conclusion, but it's a longish article:
Octane Boosters Testing - Tech Review - European Car Magazine

Oldmots: "The thing that makes premium work to make more power is not the energy content, it is the fact that the ignition can be advanced without detonation. This is why using premium in a car designed for it will give better mileage than lower octane. It is true that it is generally cheaper to use premium in a car with knock sensors. When they retard th eigntion, the economy goes out the window."

True as I understand too. However, the difference occurs at peak (or elevated) power / throttle levels. With a high powered vehicle, how much time does the typical owner spend at full or near full throttle? Ok, maybe you XJR owners have heavy feet. Even so, that's only a factor for a relatively small amount of time, when accelerating away from lights and on-ramps. Otherwise, the engine is operating way below potential.

At high mileage you may have ring scoring or carbon deposits that affect cylinder pressures, which make pre-detonation more likely. It's really avoiding damaging pre-detonation that is the benefit of higher octane, not fuel mileage, as I understand it. That is, unless you're a hot-rodder looking for that extra 5-8 HP at full throttle.

 

How would you know that unless you somehow disabled the knock sensor? Modern cars won't knock unless you run gas of such low octane that the ECU can't retard the timing enough to stop it, and most cars won't run at all with the timing that far retarded.

Detonation occurs with increases on the load of an engine, not just full throttle/max performance. That's not just stop signs and on ramps, but hills, speed limit changes, etc. I have a 1969 Z/28 Camaro, and when I rebuilt the engine I had the pistons milled to bring the compression down from 12:1 to 10.5:1 because I knew I'd have trouble running on less than 93 octane. Around here 93 is getting scarce - most "premium" is now 91 - and on 91 octane the engine knocks like crazy. I have to drive it like there's an egg on the throttle and I still get detonation every time I touch the gas. The answer, of course, is to manually back the timing off, and of course performance goes down.
There was a debate for many years about whether premium increased your gas mileage, and before ECU control the answer was no. The extra octane would let you take advantage of things that would increase the power and mileage, like advancing the timing, more compression, etc, but by itself octane does nothing. Now the debate seems to be if lower octane hurts mileage. Every car is different and the only way to tell is to try it. You won't hurt your car to try it, but the answer is probably no. I tested my V12 BMW once, although it wasn't my original intention. I let the low fuel light come on because I was trying to get across a state line to where the gas tax was 15 cents lower. When I made it, with the range showing "0", they were out of premium and I had to run regular. My mileage on the next tank of interstate driving went from 26 to 23, roughly an 11% difference. Regular was $3.98, premium would have been $4.18, roughly a 5% difference. In my case premium was the better deal.

 

I've been pointed to that article many times before- and it helps my case far more than it hurts it:

"If some is good, then more must be better, right? Doubling the dose of octane booster to 4 ounces in the 2-gallon fuel cell (like putting two 16-ounce bottles in a 20-gallon tank), and leaving the timing set at 38, we gained 1.5 hp. While power wasn’t improved significantly, the 5,700-rpm horsepower peak and smooth torque and horsepower curves indicated continued protection against abnormal combustion. Octane booster works, but double-dosing an engine like ours wasn’t worth the added expense."

 

You're right- but of dozens or hundreds of people who have weighed in on the subject, the vast majority report no noticeable loss of power or reduced fuel economy. This infers that the knock sensors are not being activated at all or to such a small degree that the retarded timing has minimal effect.

I'm well aware of the compromises on old school engines, I have an older Corvette than runs into the same issues as your Camaro.

 

Sorry, that's backwards. Pre-ignition is just like it says - the fuel charge ignites before the spark plug ignites it - "pre" ignition. This destroys engines. Normally the fuel is ignited before top dead center so that the building pressure peaks as the piston starts back down. If you ignited the fuel at TDC, the pressure wave would lag behind the piston and you'd get no power. As the engine spins faster you can ignite the fuel charge sooner, as the flame propagation rate stays constant while piston speeds increase. You want it timed so the fuel charge gives a maximum push to the piston

Pre-ignition is like advancing the timing too far. The peak pressure is reached before the piston starts back down. This is very damaging.

Detonation, or spark knock, occurs when the fuel ignites in more than one spot. The knock is the sound of the flame fronts colliding. It's usually not damaging in the short term, but the rattle is very noticeable.

 

That was my point - it depends on the car, and the only way to find out is to try it. On my BMW it made a noticeable difference. On my wife's Solstice the mid grade makes no difference, while the regular grade drops the mileage a bit. I haven't checked my XJR.

 

I look at it from the perspective that if I spend $300 a month on fuel, and premium is 10% more expensive, if I convert to regular I'll spend $273 a month for a savngs of $27 or $324 a year. Switching to mid-grade is even less of a savings. I'd rather run on premium and get full performance. Not significant enough savings for me.

 

You've essentially agreed with me virtually word for word- please re-read the article. They've got them backwards. Please read:

Engine knocking - Wikipedia, the free encyclopedia

for a clear definition of detonation and

Engine knocking - Wikipedia, the free encyclopedia

for pre-ignition

Basically:

pre-ignition:

normal combustion, but prior to the plug firing, caused by hot spots

detonation:

explosive, abnormal combustion after the spark plug fires, too low an octane fuel

 

Sorry, must have read you incorrectly. The first condition described is detonation, and the second is pre-ignition. I thought you were saying that that was incorrect.

 

Here is my personal experience with lesser octane fuel in my 04 XJ: I not long ago attempted a tank of 89 octane. Not many miles into the tank I noticed the car running oddly and it worsened on "hot starts". It then worsened to the point to where it threw a check engine light and a P1316 code showing misfires on 4 cylinders.

After a trip to my local Jag mechanic and a can of Lucas fuel treatment, the problem was attributed to the fuel. We killed the codes and he reccomended using only Shell 93 octane from that day forward. After a return to premium unleaded I have had no further issues.

Lesson learned on my end.

 

Expensive as your 'gas' seems to be, it is nothing to what we pay in England - my latest fill up was £1.39 a litre, or £6.31 a UK gallon. For a US gallon this would be £5.24 or $8.29, over double your average price. Virtually all of this difference is tax.

So how do we all afford it ? The answer is we don't ! Basically, I no longer use the Jaguar to run around town, it's now a weekend toy for highway and byway trips out. Actually this makes using it even more pleasurable than it was !!

We have two gas (petrol) grades here, one at 95 and the other at 97, (some suppliers 98 or 99) all using RON octane rating. The US gas suppliers use AKI which is about 4 to 5 less for the same fuel, so our 95 equals your 90 or 91, and our 97 is your 92 or 93. At least this is what Wikipedia say !!

Our UK Jaguar diesel users have to pay much more for their fuel at around £1.47 a litre, but the cars then do many more miles in compensation.