I had read a few posts where some folks had substituted the use of factory indorsed Premium and now run with regular gasoline in their Jags with success. Most stated that there was no perceptible “seat-of-the-pants” difference when using regular. I thought I too would give regular a try; after all if I can save a few bucks, why not? I suppose it is the type of pants that you wear; I certainly could not tell a difference with the pants that I wear.

I drive somewhat conservatively; perhaps if I flogged the throttle all the time I could tell a difference in performance.

The following is a reprint from Advanced Engine Performance Diagnosis, by James D. Halderman, ISBN: 0-13-503688-7 on page 203, convinced me that I should keep a Jagness attitude and go back to using only premium.


"Can Regular-grade gasoline be used if Premium is the recommended grade?

Yes. It is usually possible to use regular or midgrade (plus) grade gasoline in most newer vehicles without danger of damage to the engine. Most vehicles built since the 1990s are equipped with at least one knock sensor. If a lower octane gasoline than specified is used, the engine ignition timing setting will usually cause the engine to spark knock, also called detonation or ping. This spark knock is detected by the knock sensor(s), which sends a signal to the computer. The computer then retards the ignition timing until the spark knock stops.

NOTE: Some scan tools will show the "estimated octane rating" of the fuel being used, which is based on knock sensor activity.

As a result of this spark timing retardation, the engine torque is reduced. While this reduction in power is seldom noticed, it will reduce fuel economy, often by 4 to 5 miles per gallon. If premium gasoline is then used, the PCM will gradually permit the engine to operate at the more advanced ignition timing setting. Therefore, it may take several tanks of premium gasoline to restore normal fuel economy. For best overall performance, use the grade of gasoline recommended by the vehicle manufacturer."


This reference by Halderman converted me back to strictly running Premium as the most healthy octane to maintain your Jagness.

 

The loss in performance would go hand in hand with reduced mileage. Both are a function of the knock sensor retarding the ignition.

I do not agree with Halderman's inference that the timing stays retarded for a period of time after swapping back to high octane gas. That's too general a statement that applies to some, not all OEMs.

If you noticed no loss of performance, you should not have experienced loss in fuel mileage.

 

I have been using mid- grade in my XJ6, and have been unable to detect a loss of mileage.

 

Yes, well put Mikey, I have to agree with you. I would think the PCM would respond to knock sensor fluxuation more quickly than three tank fulls of Premium.

 

I have some experience programming other PCM's...primarily GM products. The knock sensing systems are essentially a protection device. The programming I've seen pulls timing rapidly on the first sign of detonation...often as much as 15 degrees of overall timing. The systems then adds back the advance slowly, all the while sampling for signs of detonation. The important thing to note is that the systems I've seen revert to standard, or programmed, timing on restart and don't pull advance unless there is a sign of detonation. The idea that it would take 3 tankfuls to return to normal is ludicrous. All i can think here is that the author thinks it takes 3 tanks to flush out the offending fuel...again ludicrous.

Just my opinion, of course...

 

All well and fine, except:

1) according to Jaguar technical documentation, the response to knock is additional fueling to cool the cylinder charge

2) constantly depending on a safety system seems rather silly when it is possible to avoid invoking said safety system

3) if one constantly claims that the manufacturer's oil viscosity and grade recommendations are not to be ignored it seems rather hypocritical and contradictory to then say the manufacturer's octane recommendations can be ignored.

 

Yup, no idea where the theory came from.

 

Additional fueling is just one of several scenarios available....the major problem with that is that a overly enrichened mixture tends to overheat the catalytic converters. I'm sure some combination of overfueling and pulling the timing is actually occurring.

Even if it is just overfueling, why would it take several tanks to reset? Still seems ridiculous....

I could not agree more with points 2 and 3....

 

In an XJR, don't do it. You will blow it up. The XJR actually runs fairly high boost pressure on top of a fairly stout static compression ratio. The computer simply can't compensate enough to make up the difference as all it can do is retard spark timing and add fuel. But you're then running rich and outside of optimum timing. This will decrease economy and power. And there's only so much you can do with spark timing and fuel, as eventually it will start misfiring, and believe me when I say that it does not have the ability to compensate that much.

The XJ8 has a better chance because of VVT. Since it can actually change the physical phasing of the cam, it can compensate more, and you don't have boost to contend with. You might be able to get away with it in the XJ8, but you will still see reduced mileage and performance as you will not be running optimally.

And most systems like these will run through a loop. It's expecting premium as that's what it was tuned for, so when you put regular in it it figures you are in an emergency situation and can't get premium, or it's just bad fuel. It will detect knock, pull timing and add fuel to compensate until knock goes below the danger threshold. Then it will begin leaning it back out and adding timing to try to get back to normal, as it expects the lower knock resistance fuel situation to be temporary. So it will try to get back to normal until it knocks again, at which point the cycle starts over. So while running on regular you are getting several knock events every day, and each one takes its toll. You are damaging the engine with every one, day after day.

 

If this was true, the owner would hear/feel/see the difference. Doesn't seem to be the case.

 

As stated, it's probably okay in the naturally aspirated cars. There's a lot less to cause knock, and they're better able to deal with it thanks to the VVT. Probably okay, and the difference in performance will be small. You won't be running optimally, but if you don't care about the small decrease in performance, regular is way the hell cheaper.

But I can guarantee you it's not okay in the supercharged cars. You might not even notice the knock, but it's there, and you will damage your engine in short order, if not catastrophically on the first cold day when you're making an extra couple pounds of boost.

 

You've got proof you can share with us?

 

It's running 10-12 pounds of boost on top of 8.9:1 compression. That's a lot of knock potential. 87 octane is going to knock, no way around it. Anyone who's ever done anything with forced induction will agree. You can try it if you like, but you will damage your engine. And I doubt there's much empirical evidence out there, as I would hope no one would be foolish enough to risk blowing up their engine to save a hundred bucks a year in fuel cost.

 

What you describe would be accurate if there was no knock control at all (as in 'old school' engines) or the control systems installed on modern engines were not adequate. I've seen no evidence of the latter being true.

For reference- here's Jaguar's current thinking on the subject of octane, emphasis is mine.

"OCTANE RATING
Premium unleaded gasoline with a Cost of Living Council (CLC) or Anti Knock Index (AKI) octane rating of 91 or higher should be used. Even a very small amount of leaded fuel will damage your vehicle’s emission control system and could invalidate the emissions warranty. In addition, leaded fuel will damage the oxygen sensors in the fuel injection system and also seriously damage the catalytic converter.

Note:
Federal law requires that gasoline octane ratings be posted on the pumps.
Using unleaded fuel with a lower than recommended octane rating, can cause a
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 knock is detected, even when using fuel with the recommended octane rating,
consult your Dealer/Authorized Repairer to have the problem corrected. Failure to do so is misuse of the vehicle, for which Jaguar Land Rover Limited is not responsible.

Note:
An occasional light engine knock when accelerating or climbing hills is acceptable."

I wonder what technical changes they made to the engines to accommodate this- or do they possibly spy on this forum and are getting tired of the debate?

 

I always run Premium in my xj8. used a lower octane one in a emergency, and it had a great loss in power, and knocked like hell. Also had a decrease in average miles per gallon... I am very happy with 22 miles per gallon around town, dropping down to 16 was not acceptable... I went right back to premium, and never went back... love my 2001 xj8 British racing green. (AKA Green Shadow)

 

Thereby nullifying any purported cost savings gained from buying Costco gas. The tank costs less to fill, you don't get as far, and have less fun on the way.


Changes are immaterial once the car has left the factory with the original or updated recommendations for that particular model. They probably don't give a second thought to any fool who continues to beat a dead horse.

 

I too have a 2000 XJ8 4.0 and I have been running regular unleaded. No loss of power, mileage appears to be about the same 22-23.5. Of course I'm not gunning it and racing to the next light. This is just a cruising car.
That's what the 70 Cougar XR7 is for.

 

exactly


++

 

Jaguar sells cars all over the world .. I can tell you they sell cars (and did the x308) in places where premium is not available .. or the delivery is spotty .. so they must be able to get them to run. This is true for most I would think. MB and Porsche publish the use of regular in the owners manuals and discuss it on european delivery --- they tell you it will not hurt the engine but various "performance" issues can occur. This is also in the USA manuals.

That being said ... my XJR's will not run properly on regular. I have had to run regular in both MB's and Porsches and the MB's don't seem to care and the Porsches don't want to idle properly ... only did it a couple of times. I have a house out in the country and they run out of premium occasionally. We have an old MB V6 that runs on regular up there all the time and I can't tell much difference.

 

No idea why this brand was dragged out for another beating, but Costco is one of the all hailed 'Top Tier' suppliers...................

http://www.costco.com/kirkland-signature-gasoline.html

How about that. Exactly what's been said all along yet gets regularly dismissed by those that can only focus on what 'might' or could 'happen'.