I don't have a reason anymore to press the throttle hard, so I don't worry about engine knock. If I did, I wouldn't be posting this reply.


I have several cars and bikes that the manual recommends premium fuel.
Having said all this, back in late 2014 when I traded in my 1997 Corvette C5 on a 2015 C7, I had 172,500 miles on the odometer.
Always used 87 octane, and on the highway (which accounted for about 65% of it's miles) I would get around 30mpg.
The engine had the same exhaust system, and was working fine. Passed the strict Massachusetts emission tests just fine.
Used about 1 quart of oil every 4000 miles.
Had original antifreeze in system.
Appraiser could not believe the car had some many miles on it.


Now when I fill up the 03 XK8 and the 2017 Corvette Grand Sport, I use mid range fuel at the pumps. Have not detected any problem.


Use high octane in Victory Vision bike because it does get 45mpg.

 

The octane-need depends on different terms and conditions
like revolution speed, temperature, combustion chamber volume and geometry,
compression ratio, air pressure, humidity, ignition timing, debris and many more.
To my mind not less than 90 (AON) minimum octane rating is indispensable.
90 AON (US) = 95 ROZ (Europe)

(Sorry english is not my first language)

 

It probably will not hurt anything, especially in a NA car. It is not the O2 sensors that will adjust there is no difference in 93 vs 87 octane that the O2 sensors would notice. It is the knock sensors will retard your timing and perhaps decrease your target AFR.

In a car built for premium you will have less power and less fuel economy due to retarded timing and possibly richer mixtures. These result in your engine being less efficient. You also may have poorer throttle response, but that depends on how much tip-in retard is already programmed in (would be more noticible in sport mode).

The reduced economy will probably not be enough to make up for the difference in fuel price but it is worth noting. You may be saving 40 cents a gallon but you will be using more gas so your "net" savings may only be 35 cents a gallon.

This all assumes the knock sensors do not fail. If they fail you will damage your engine over time. Also on a turbo or supercharged car there is far less margin and I would not run anything but premium. Again your knock sensors will probably catch it but if they are slow it could be damaged much more quickly with forced induction.

Regarding ethanol, by volume it does have less energy than gas but you lose no power because your O2 sensors will program a lower AFR to compensate. With E10 you will suffer slightly lower mileage but you will get the same power because you are pumping the same amount of air and converting the same mass of hydrocarbons and oxygen into H20 and CO2. You are just spraying slightly more fuel (volume) to do it.

That is my 2 cents.

 

The debate over regular vs. premium is probably influenced by the situation we had here in the Northeast USA in the late 80's and early 90's where even major brands were skimping on the detergents in the regular. Various consumer publications and regulatory agencies showed evidence of this. Mechanics recommended Premium for this reason and people associated Premium with better quality for this reason. It made a difference in my cars at the time.

Now there is no reason to worry about not getting the detergents that you need as the scandal caused the companies to clean up their act. Additionally a consortium of manufacturers and fuel companies agreed upon a better and consistent detergent and additive formulation to support the new and more efficient engine designs. The catch is that all of their grades must meet the same standard if they are to be part of the consortium. The way to see if a particular brand is part of this is to visit toptiergas.com. You will be surprised at how many brands are represented here. Technicians that I know personally have seen the internal difference in engines run on toptiergas as opposed to non conforming product regardless of octane rating. This detergent and additive standard is what most of the major manufacturers spec for. I personally noticed that it was no longer necessary to run a fuel system cleaner every so often. This was discovered by accident as I had one car that was always filled with a conforming gas and one that always was not. Carbon buildup difference was notable as were performance difference. After using a fuel system cleaner and switching fuel on the "dirtier" car; the improvement became apparent.

My Jaguar technician's opinion on the octane debate is that yes the engine management system on a normally aspirated engine will protect you, however he is concerned that the setback of the timing might long term create a lower combustion temperature or different burn rate than the engine was designed for leading to carbonization issues. He has seen evidence of his theory across his customer base. His opinion is that you are less likely to have problems using regular octane of you do a lot of long trips at consistent highway speed as this ensures full operating temperature and sustained higher RPM.

This is my first, so please don't throw too many rocks. Can we have an oil debate now, please?

 

Your technician has very little knowledge of engine function it seems. All gas/petrol burns at the same velocity (burn rate) and temperature regardless of octane rating. How efficiently that combustion event propels the vehicle forward is a different matter.

With respect to additives and Top Tier, a recent study by AAA demonstrates that there's precious little difference between those fuels and the classic 'mom and pop store' off brands some people love to hate.

BP went one step further and admitted that it would take up to 5,000 miles of driving exclusively with their fuel to make a difference.

 

@ Mikey

Regarding combustion temperature, i disagree with you with the following example. Late 1970's GM engines have a vacuum switch controlled by coolant temperature. Under conditions of an overheat vacuum would open the EGR valve regardless of throttle position and fully advance engine timing as these had a cooling effect on the engine. This was pre-computer management. Consequently, ignition timing can affect combustion temperature. With all of the tech now, they can probably do amazing things, maybe in spite of us. Also, I am an analytical chemist with a strong personal interest in thermodynamics. There is a slight difference in burn rate between the fuels themselves due to the slightly different chemistry, just not usually enough to make up for the higher cost of premium octane. It is funny, but you do get slightly better mileage due to the somewhat different composition of the higher octane fuel, just not enough to care or make up for the cost in the average regular fuel requiring engine.

Regarding BP's statement, yes it would take that long, however in the process you might not be making your fuel system and cylinders any dirtier. If it can fix a problem over time, it is not making it worse over time, the real reason. Plus, I used to drive over 15K miles a year, if not much more. This makes a difference here.

 

A gross adjustment of timing or fuel/air mixture will indeed change combustion temps but the minor timing adjustments required to avoid detonation have no significant effect on engine cleanliness or durability as your tech seems to think.

Referring to '70s vintage GM products, it was commonplace to retard the timing a few degrees if fuel of correct octane was not available or too expensive. There were no correlating operational issues associated with this.

Regarding the burn rate, it is not directly tied to octane rating. Some low octane fuel can and do burn faster than high octane, and vice versa.

Tech-Flame Speed Octane & Power.pdf

Octane is not how fast a fuel burns

 

Everybody knows that the type of fuel you use is dependent on the type of oil you use, right?

 

Yes, I agree that the octane formulation has a minor effect on burn rate as to test for this accurately you would have to control for compression, ignition, cylinder temperature, etc. However, timing does affect these variables and they in concert affect burn rate. There are many factors here that interplay.

There were issues with the GM engines of the time when people were setting back the timing that people noticed and did not appreciate. Hot weather performance was awful, passing acceleration (mid-range) was terrible, ruined rings. Many in my family switched to premium, set the engine to specification and were better for it. Yes, a lot of these engines were not that good to start, but bad can always be made worse.

My tech is relying upon years of experience and many disassembled engines correlated with known customer habits. My goal was to not ruin my car by seeking his advice. An example here is my 2001 Honda Prelude which absolutely must have premium as opposed to my CRV's which do not. I am not sure if the Jaguar engineers intended the Premium to be a better safe than sorry recommendation based upon issues with substandard fuels or actual design of a performance car maximizing variables that you do not want to go outside of due to the tightness of their design. For example many modern stock Corvettes of the past twenty years up to very recently do just fine on regular octane as there is enough wiggle room in the design for it not to matter unless you operate at the extremes. I do not know if there was some variable that Jaguar intended not to violate here. I am leaning towards that it does not and that regular octane of a quality fuel will be fine.

Regarding combustion temperatures, small timing changes can and will have a massive effect on burn rate and temperature. You are igniting the fuel at a different level of compression and cylinder volume and changing the propagation of the combustion. Would you believe that colleagues of mine would tell you that carbon buildup is probably caused by temperatures that are too high in the chamber as opposed to too low. Heat is low form or wasted energy. Better combustion is energy directed towards work, less heat (This is math; math is.). Carbon might be due to fuel scorching at high temperature instead of burning through a proper propagation during the power stroke, hence less efficient.

BTW: my Honda mechanic likes to drag race cars for fun. He can tell you that too little timing leads to a carbonized engine. These are v8 American iron blocks, not particularly high-tech.

Takeaway for me is that I do not want to ruin my car and my Jaguar tech sees some validity to the requirement like I saw in another brand that I am intimately familiar with. Until I know otherwise, cheap insurance.

 

My XK gets feed premium E10 except winter storage alcohol free premium; my wife,s XK8 gets mid grade (she only drives to grocery and the mall on side roads);my motor home gets regular E10 plus Sta Bil for winter storage; my Honda gets cheapest regular E10....

Incidentally, XK and Honda get about same gas mileage until I replaced the Dunlaps tires with Hankoks; lost 1-2 mpg!
Could not tell the difference between E10 and alcohol free gas mileage ........

So tires make the biggest difference IMHO! the Hankoks ride so smoothly that I don't care about loss of mpg!

Remember modern Jags have knock sensors to avoid problems with low octane gas!

Retired Boeing engineer, Adrian

 

Thanks Adrian,

Hankoks are awesome tires, have them on both my Hondas, made the biggest difference in the world. When I wear out the Michelin Pilots on the Jag I will see if the Hankoks are made for it.

My point is that the knock sensors, which I replaced, will set the timing back, yes. This will however affect another factor which could or could not affect the engine in undesirable ways depending upon how on the edge of performance and reliability the engineers designed.

I cannot PM yet, but you, my Jaguar and the previous owner might have a connection. When I figure out, get permitted, will send you a note.

 

The only thing that concerns me is that there "needs" to be knocking for the knock detectors to detect. I'd just rather not go there. As it is, even with 93 Octane, I swear that there are times when I do hear some brief detonation. I really hope I don't end up kicking myself for not replacing the knock detectors when I replaced the valley hoses (NOT an easy job on an XKR!).

 

A functioning knock detector will do it's job at a level far less than the threshold of human hearing. If you're hearing something going on, it's not detonation.

 

Thanks Mikey. Good (and comforting) to know.

Like many guys who've driven LBCs since they were kids, my ears are always "listening". Being a pilot for 30 years makes it even worse! If there's something to be heard - I'll hear it. Unfortunately, if there's nothing to be heard, I may "hear" that too - lol

 

My father was a pilot in Uncle Sam's Flying Club and he, too, was very sensitive to things heard and felt while driving...particularly any engine roughness.

Fortunately for me, since he brought his cars to me when they needed attention, he responded well to placebos. He'd complain of some minute hiccup or twitch....never felt by me.....and I'd tell him that I adjusted the timing or snugged-up a loose spark plug wire. He'd be happy with that for three, sometimes four months

Cheers
DD

 

Yup. I started out as a Navy Pilot and then an airline pilot. And I'll tell you what: when you're over the middle of an ocean, you hear (and "feel") EVERYTHING - lol.

Loved the placebo bit. But let's face it: don't ALL of our cars run better after they're waxed? Explain that! :-)

 

Jim,

After a fresh wax the sound waves slip past the car with far less turbulence so our cars sound quieter and stealthier leading us to believe that they are indeed performing better....

Does that do it for you?!

 

Hah!... And there you go. I "knew" I wasn't imagining it - lol

Perfect explanation, Jon

(Love this place!)

 

Every time I wash and wax mine it rains.

Explain that.

 

Easy. You're a jinx....