What is the general opinion of my American friends on the fuel to use in my 2004 4.2 XK8. the manual states 95 octane, so that what I've been using. I run my BMW 740i on 98 octane and it runs better than on 95 octane but the BMW manual does recommend the 98 octane. I guess the knock sensors adjust the mixture in the BM it feels more responsive and returns better MPG. Will the Jag benefit from 98 octane.

Tony

 

Give it a try. It won't hurt anything.

 

...cept his pocketbook, what's the difference in price per gallon (or liter) for the different octanes 740i?

Consistently, I do think the higher octane fuel will pay itself off with better fuel mileage, more efficient injectors, less frequent plug fouling, etc.

 

I would be interested in the premium in premium in GB.

Forever the US premium was $0.20 per gallon which has crept up to $0.30, or occasionally, $0.40 per gallon (US). That is a little less than about a 10% premium over the US 87 octane (RON+MON/2) at the moment.

Since fuel is significantly more expensive 'over there' what is the percentage premium for premium?

And, NO, a car will not benefit from an octane rating above that specified by the manufacturer.

and . . . . .

After a fuel shortage a year and a half ago, when only 87 octane was available for more than a month, I have been using the low priced spread with no decrease in mpg or performance. But I do not ask much of the car.

 

Where do you find 95 or 98 octane gas in the US? 93 is the highest octane I see at most stations.

 

US calculates octane differently

 

These are unfortunately the most common myths and misunderstandings all rolled into one.

Any fuel from a quality supplier has sufficient additives to keep injectors and plugs clean. Additional cleaners (if present) are just wasted. There is no inherent connection between high octane and high levels of cleaning additives.

High octane gas simply contains additives or is formulated to be more resistant to detonation and/or pre-ignition. There is nothing in it's make up that will give more power, economy, help starting, smoother idle etc.

All engines will possibly suffer from detonation or pre-ignition with the right combination of load, throttle opening, ignition advance, ambient temperature and a thousand other factors. If an engine is able to apply full ignition advance without suffering detonation/pre-ignition using a given octane rating gas, raising the octane levels further will do absolutely nothing.

Most engines operate nowhere near their detonation/pre-ignition threshold 99% of the time. Under those circumstances, regular unleaded (85/87 octane) would be more than sufficient. The remaining 1% of the duty cycle is climbing hills, rapid acceleration, any situation where heavy throttle and load is involved. In this small fraction of operation high test gas is actually of benefit as noted above.

Perhaps some day cars will incorporate a system to inject a octane booster directly into the combustion chamber using an on-demand system. We could do away with multiple octane levels gas at the pump and just load up on the lowest possible level and away we go. Not likely to happen though.

 

Very well and authoritatively said.

Enough with the octane discussions. Now if we could get rid of the oil discussions.

 

I wholeheartedly agree. Seems to be very popular lately.