XKR 4.0 + shell Vpower

[NST]

MrsNST phoned me last night to tell me the jag was driving really well after i filled it up for her. she said it feels more responsive (not sports mode), engine feels smoother and it feels a touch faster.
I've been filling the jag up with shell V Power the last couple of times to give it go and see if it makes a difference. i've not really driven the car for while but the wife been racking up the miles recently.

her comments made me wonder if the higher octane is making a difference?

any comments? does the engine have a knock sensor?

NST

[Triple7]

I think it does make a difference. It did with my S2000 from a few years ago. Wringing it's neck at 9000rpm you could notice the difference. Alas on a V8 with 420BHP I'll save the 8p a litre extra.

G

[a8hex]

Triple7 said:

I think it does make a difference. It did with my S2000 from a few years ago. Wringing it's neck at 9000rpm you could notice the difference. Alas on a V8 with 420BHP I'll save the 8p a litre extra.

G

No idea if it'll make any difference with the Jag engine, but you can find that the more expensive fuel is actually cheaper.

Years ago the misses (B8) and I ran a series of Vauxhall Cavalier SRis (Shape introduced on F plate) and it made a huge difference whether you ran them on 95 octane or 98 octane fuel. Not only did the performance change a lot but so did the mileage. On 98 octane it would happily do over 500 miles on a tank, with 95 I'd struggle to get 350.

When I bought my XJ the salesman said it wouldn't make any difference and I've never tried it. NST have you noticed a difference in the mileage?

[Triple7]

I might run a few tanks through and see if I can get above 26MPG!

G

[Jaguar steve]

a8hex said:

Triple7 said:

I think it does make a difference. It did with my S2000 from a few years ago. Wringing it's neck at 9000rpm you could notice the difference. Alas on a V8 with 420BHP I'll save the 8p a litre extra.

G

No idea if it'll make any difference with the Jag engine, but you can find that the more expensive fuel is actually cheaper.

Years ago the misses (B8) and I ran a series of Vauxhall Cavalier SRis (Shape introduced on F plate) and it made a huge difference whether you ran them on 95 octane or 98 octane fuel. Not only did the performance change a lot but so did the mileage. On 98 octane it would happily do over 500 miles on a tank, with 95 I'd struggle to get 350.

When I bought my XJ the salesman said it wouldn't make any difference and I've never tried it. NST have you noticed a difference in the mileage?

None of the Early GM OHC engines ran very well on 95 octane fuel. I noticed the difference right through the range when I was working on them - don't forget you could still buy full-fat leaded 4 star (98) octane fuel in 'them days, and it was what most engines of the era were designed for. Vauxhall introduced a adaptor swich on the engine managment system following complaints which allowed you to retard the ignition timing by around 2 deg to accomodate lower octane fuel but it was a very crude compromise. It was mainly Japanese engines of that era would happily run on a lower (and cheaper) octane fuel. I'm not surprised you got better consumption with 98 octane, it's what the engine was designed for in the first place and I know that a sensibly driven Cavalier SRi was capable of a easy 40MPG +

I've tried around 1000 miles of BP super unleaded 99 octane in my XJ compared to its usual diet of Sainsbury's unleaded and couldn't tell any difference in either performance of fuel consumption.

[a8hex]

Jaguar steve said:

I know that a sensibly driven Cavalier SRi was capable of a easy 40MPG +

Sensibly driven... Nah.
The way I found to get the best economy out of the things was to nail it! Max bore acceleration when ever you need to accelerate and then never slow down unless you absolutely have to.
Sadly the improvement in fuel economy found using this technique was off set by the need to replace the front tyres every 9000miles, which was about every 2 months for me back then.

[Jaguar steve]

a8hex said:

Jaguar steve said:

I know that a sensibly driven Cavalier SRi was capable of a easy 40MPG +

Sensibly driven... Nah.
The way I found to get the best economy out of the things was to nail it! Max bore acceleration when ever you need to accelerate and then never slow down unless you absolutely have to.
Sadly the improvement in fuel economy found using this technique was off set by the need to replace the front tyres every 9000miles, which was about every 2 months for me back then.

That driving style worked for me too....especially in customers cars on road tests

[NST]

a8hex said:

When I bought my XJ the salesman said it wouldn't make any difference and I've never tried it. NST have you noticed a difference in the mileage?

MrNST says no improvement, i will check the trip tonight anything above 18.7mpg i will be impressed! her main comment is the better response and smoothness, the engine doesn't exactly lack smoothness!

update, the trip is showing an improvement of 1.1mpg, which imho means no improvement..

Edited by NST on Wednesday 13th February 07:46

Edited by NST on Wednesday 13th February 08:35

[GavinPearson]

The engines are mapped on 95 & 98 and use the knock sensors to try to work towards the 98 settings. Good fuel results in improved power over the rated amount quoted on 95 octane. So does running in the cold. But it may not improve fuel efficiency.

[NST]

GavinPearson said:

The engines are mapped on 95 & 98 and use the knock sensors to try to work towards the 98 settings. Good fuel results in improved power over the rated amount quoted on 95 octane. So does running in the cold. But it may not improve fuel efficiency.

thanks for the info Gavin

[sunday-driver]

The is actually a very interesting subject - because it's hard to prove economy or performance in day to day driving and because the price differential is a good 5 or 7% .

I run my 4.2 XJR and cannot tell the difference between regular and super (Shell V or BP Optimax). Thinking about it, the previous owner said the same when I bought it. However, it was a teeny bit rougher on cold start up after a service and running Optimax for a tank resolved it.

On my previous car, 330i, i ran Optimax all the time mainly because the handbook says too and partly because they are a wee bit gutless so needed a bit of help. When I ran it on regular I have to admit I couldn't really prove the difference but kinda felt it was subtley better performance-wise.

I read about a [What Car?] study on this and in lab conditions the academics actually recorded worse performance and economy on their standalone VW 1.8 engine. The response from fuel manufacturers was "you need to run it over a long period".

Basically, my view is that if used regularly it is a better grade/product but buy it because you want a small performance boost or you want to treat / clean the engine with the additives they put in - as long as you don't care about the poor value for money.

In the real world I don't think it's worth the extra, so I no longer use it regularly. I *definately* don't get a 5% improvement in performance or economy (and the XJR is fast enough anyway), so the value just ain't there.

[Phoenix Red]

The cheapest and quickest way to improve your MPG is to make sure that you have the correct pressure in your tyres.
Everytime my MPG drops to around 18 I know it is time to put some air back in to my tyres. If you are running on 20" tyres having the correct pressure in really makes a difference. Also the outside airtemp has a massive effect on the air pressure.

Dave