Hi guys,

Can one use 98 Octane Unleaded (Racing Fuel) in the XKR?

 

My question would be, Why would you want too?

There is no need use 98 grade, and thinking that it may improve performance may be a myth and may cause damage to the engine like burning the valves.

 

I believe that South Africa uses the RON system for rating the octane levels of gasoline/petrol as opposed to the AKI system used in North America.

All modern Jags achieve full rated performance on 95 RON fuel so using anything higher will just be a drain on your wallet and will return nothing in terms of increased performance or reduced fuel consumption.

The term 'racing fuel' is a misnomer as it does not turn a vehicle into a 'racing car'.

 

I have used VP MS 109 RON 109, MON 101 R/M 105, ogygen 9% as straight or blended with regular 98E5 ( US 93 ). Race only. Normal boost in the stock XKR do not need high octane fuel but extra oxygen gives 3-4% ( straight VP 109 ) more power. Just ensure that mixture is enough rich. Otherwise...............

 

I use 98 RON here. I've used it in all of my cars for the past 15 years or so, mainly because it is supposed to be slightly cleaner and it is only 4 cents per litre more exey.

I dont know whether there is a difference in performance from 95 RON in the Jags. I have noticed a difference when it is put in the 5 pot turbo volvos that my wife drives (when she hasnt pinched the Jag). I think they run better on it.

The XKR has oodles of grunt with a paucity of traction. Personally I could never justify any expense at all designed to add power, because it would only mean more rubber left on the road and even less forward movement on launch. Though maybe that is because I'm putting in 98 octane...I should try 95.

 

Keep in mind that the octane rating of a fuel is strictly it's resistance to detonation (knocking/pinging) and nothing else. Nothing to do with 'quality' or volume or type of other additives of any type.

The only way an engine can make more power with increased octane rating fuel is if it is experiencing detonation on the lower rated fuel and the knock sensor is commanding a reduction in ignition timing advance. The higher rated fuel can inhibit detonation allowing the ignition to operate at optimal advance.

If the engine is not detonating on a given fuel (such as the 95 RON indicated for Jaguars), there is nothing to be gained by using higher ratings.

 

I have occasionally found that with some 95 RON fuel, I get a 'surge' effect at full throttle, almost like DSC cutting in & out but without the DSC light coming on. I suspect that's actually the knock sensor detecting the start of pre-detonation and backing off the throttle. Not all 95 RON is created equal...

However, unless I'm planning to do some fast driving, I tend to buy 95 anyway, 'cause I'm a tightarse - if I have a tank of that, I keep alert for the surge & back off a bit. But for the autobahn or the track, I'll go for 98 or even 100 if I can get it, because the car runs better under hard acceleration.

 

If you're tracking your car or running it hard, you may see the benefit of the higher octane. Only way you would really notice a benefit from running high octane is by altering timing through your tune. Outside of that, you won't see much difference.

 

The reason I'm asking is that I used 98 on my Z4 M Roadster and still have a few fills left that I've paid for already.

I'm not willing to risk harm to my new toys engine though.

 

Oh, higher octane is no problem - it's lower that causes issues, or too much ethanol. As I said, I've run mine on 100 RON in Germany.