My sweet wife, who is a bit leery about getting our first Jag (used) because of all the money we're having to put into it, was visibly not pleased when she found out that our standard 2009 XF needs 91 octane fuel which probably means premium. Around here, premium is 93 and standard is 87. Will 87 be OK?

 

NO!..Read the OM, if you have one..But it specifically says 91 octane or higher and IF and only IF you have to buy 87 Octane, to just get a little or enough to get you to a station that has 91 octane.

I wish I could get 93 octane around here.

 

Ummmm, here's what the owners manual really says:

Octane requirements
Premium unleaded gasoline with a CLC or AK
octane rating of 91 or higher should be used.
Note: Federal law requires that gasoline octane
ratings be posted on the pumps. The Cost of
Living Council (CLC) octane rating or Anti
Knock Index (AKI) octane rating shown is an
average of the Research Octane Number (RON)
and Motor Octane Number (MON).
Using unleaded fuel with an octane rating lower
than that recommended, can cause persistent
heavy engine knock (a metallic rapping noise)
If severe, this can lead to engine damage.

Note: Mid or regular grade gasoline with a CLC
or AKI octane rating of not lower than 87 may
also be used, but performance and fuel
economy will be reduced.

If heavy engine knock is detected when using
the recommended octane rated fuel, or if
steady speed on level roads, contact your
Dealer for advice.
Note: An occasional, light, engine knock while
accelerating or climbing hills is acceptable."

Bolding for emphasis is mine. If money is tight and Jag's prediction of reduced performance and fuel economy is correct when operated on 87, trying blending 87 and 93 in different ratios till the problem goes away. My guess, if the XF is like it's S-type predecessor, operation on 87 in average commuting and pleasure driving will be just fine

 

Would you run Low grade gas in your LS2 SSR? If It's a Direct injected Jag engine you need to run the highest octane readily available. While the manual says pinging or slight pre ignition is okay, I've seen it break rings, and hole pistons.

 

The 4.2L engine in his 2009 is not direct injected. Nevertheless, at least 91 octane (US rating) should be run at all times unless there is no choice, then at least 87 for brief periods only. Long term use of 87 can burn through O2 sensors and a number of worse problems.

 

Don't be willing to save a few dollars a fill up to potentially risk super expensive long term engine damage. Tell your wife it's cheap insurance to run premium fuel.

 

Preignition has nothing to do with octane rating of gasoline. Severe, prolonged detonation is required to do the sort of damage you describe. There is no evidence of this happening on any 4.2L engine even on those operated long term on 87 octane fuel.

 

The vast majority of vehicles use 87 octane fuel. Why don't those vehicles burn through O2 sensors? What causes it?

 

The correct term is Pre-detonation and it has everything to do with octane rating.

Pre-detonation happens when during the compression cycle, the fuel is compressed (which causes a lot of heat to be generated) and self-ignites before the spark plug fires, causing the resulting explosion to start pushing the piston down before it is at top dead center. This results in major forces downward while the piston is still trying to go up.

This can lead to cracked pistons, bent piston connecting rods, and things of the sort.

Fuel that has a higher octane rating takes more compression to self-ignite.

I do not know what compression ratio level 87 octane vs. 91 octane (or higher) is the "magic number" but if you want to enjoy the most of what your high priced European car can deliver, I strongly suggest you use Premium fuel. Any amount of pre-detonation (otherwise known as "knock") will result in timing being pulled out of the engine (this is how the manufacturer attempts to protect engines from bad gas) and you will feel a noticeable lack of power.

 

Oh come on now, here I live in Norcal the difference between 87 and 91 is 20 CENTS PER GALLON. If you're concerned about saving $3.60 at a fillup this may not be the car for you.

 

Umm, no. There is no such term as pre-detonation.

Ther terms are 'pre-ignition' and 'detonation'. Here's two decent links describing each:

Engine Basics: Detonation and Pre-Ignition by Allen W. Cline

https://en.wikipedia.org/wiki/Engine_knocking

Note the difference between 'burning' of the fuel and 'explosion'. Detonation is 'exploding'. Normal combustion, whether at the appropriate time or not, is 'burning' or 'conflagration'.

Higher octane fuel is not less susceptible to pre-ignition, it is less susceptible to detonation.

Understanding these subtleties is essential.

 

Ok Mikey, if you want to run low octane gas in your car and think it will not harm anything, go right ahead.

The simple truth is that ALL of the gas model XFs require Premium gas to run at peak and designed performances.

If the original poster wants to run low grade gas, they can of course, but when they start having drivability issues and take it to the dealer to figure out what is wrong, I hope the solution is simply to use Premium fuel and the engine does need extensive repairs.

Davetibbs said it correctly.

 

If 89 octane was $2 gal and 93 was $5 gal I'd still by the 93..

 

I'm surprised you asked a question with such an obvious answer. That "vast majority of vehicles" are designed and tuned for 87 octane, and consequently most of them run very nicely on that fuel. Engines designed and tuned for 91 octane likewise will mostly do well with their specified fuel. Yes, modern computerized engine management systems can often compensate for lower octane fuel, but not always. If you want to say that there is no reason to burn 91 in an engine designed and tuned for 87 I'll agree with you, but when the manufacturer specs the engine at minimum 91 the spec should be followed. This is especially true with a high performance car such as these.

 

I've never heard of 87 octane causing issues with O2 sensors, what I have heard is leaded fuel causing issues with O2 sensors and catalytic converters. I had a 110 leaded map on my old STi, but it was running catless and I didn't run that map enough for it to be an issue with the O2 sensors.

When a car requires premium fuel it's usually because it is either boosted and / or running higher compression which makes it more prone to knocking. Will an ECU adjust for regular gas if you absolutely have to run a tank or two in an emergency? Yes it will. Is it worth saving a few bucks a fill up to run regular fuel when in reality you will get worse gas mileage when the ECU pulls timing, reduced power and the likelyhood of damaging your engine in the long run (especially now that winter gas is starting to hit the stations)? You can do what you want with your car and justify it however you want but it is silly to risk it.

 

So what is the obvious answer that causes O2 sensors to supposedly burn out, or has WRX provided it for you?

It's leaded gas that kills sensors (and cats) not lower octane.

This subject is a 'deja vu all over again'. Beaten to death starting 10-15 years ago with the S-types XKs and XJs, now here it is again with the XFs. The same old myths and misconceptions are being dragged out.

Yes, old school (pre-computer controlled) high performance engines can easily become damaged through use of low octane fuel. Today's engines do not.

There's not one case of a 4.2l engine being damaged by extensive or continuous use of 87AKI fuel. There's many owners (including myself) who report no apparent change in performance or mileage while using 87. Whether the OP wants/needs to save a few bucks per tank is not the point.

Jag said it quite clearly in the relevant owners manual. That wording is a vast improvement from the S-type equivalent that lead some people to believe the engine would grenade instantly.

 

Honestly, if the relatively minor additional cost of premium fuel is such a big issue for you, then you may want to reconsider the type of car you are looking to buy.

 

Incorrect sir!
Just a quick Wiki But its pretty much the same def as Websters.

In a normal spark-ignition engine, the air-fuel mixture is heated due to being compressed and is then triggered to burn rapidly by the spark plug and ignition system. If it is heated or compressed too much, then it will explode when triggered, or even self-ignite before the ignition system sparks (PRE IGNITION). This causes much higher pressures than engine components are designed for and can cause a "knocking" or "pinging" sound. Knocking can cause major engine damage if severe. The most typically used engine management systems found in automobiles today have a knock sensor that monitors if knock is being produced by the fuel being used. In modern computer controlled engines, the ignition timing will be automatically altered by the engine management system to reduce the knock to an acceptable level.

The higher the Octane, the more resistant the gasoline will be to pre ignition and knocking. And it doesn't require sustained running to cause damage as you state Mikey, I've seen pre ignition break a ring with one quick rev of an engine. Just not worth the risk to save a few cents to me.

 

The vast majority of engines are not high performance. And if you want to know more about how low octane Pre ignition and Detonation can tear an engine apart...

 

Seems a matter of choice (and maybe chance). If the knock sensors and the software - operated out of intended specification - can handle cheap fuel by messing with timing then you've saved a few dollars (if MPG doesn't suffer as much). If this isn't so then getting an engine repaired is quite costly. Do you like the odds?

I'm not putting cheap fuel in my car, but everyone gets to choose for their car......

I would definitely avoid spirited driving on cheap fuel.