If you can get 43mpg out of an XJS....

If you can get 43mpg out of an XJS....

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adam quantrill

Original Poster:

11,535 posts

241 months

Monday 20th October 2014
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I came across this article on The Register where it's claimed you can get 43mpg out of an XJS!

http://www.theregister.co.uk/2014/10/19/crawling_f...

This seemed pretty incredible and there are a number of efficiency savings across many different areas, mainly to do with weight savings and reducing drag.

While I'm not advocating increasing the MPG in the Wedge - the other savings are well worthwhile in order to increase acceleration and top end.

The ones that seem worth investigating are:

- Wiring harness - replace with an aluminium bus bar (well, a pair in our case, supply and ground) and a signal bus plus local controllers. I would shorten the injection harness too - there's a half metre of it flapping about in the passenger footwell.

- Electric coolant pump - works better at idle speeds when you need it most. Engine warms up quicker and doesn't get overcooled.

- Radiator shutters - to reduce drag

- Front end undertray - covers under-bonnet area - further reduces drag.

- Increase bottom-end torque with a better EFI computer.

- Smaller and lighter starter motor, and more efficient alternator.






RCK974X

2,521 posts

148 months

Monday 20th October 2014
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Yeah, saw that too - fascinating stuff.

I been wondering what improvement on average a modern digital engine management system would get by itself - from 2.8 Cologne in Granada, it was about 10% from carb to K series, and another 3-5% going EEC-IV, but that's with same engine with almost no other changes, so I bet there's quite a bit more possible.

I wonder how much weight could be trimmed off the wedge ? Probably quite a bit.



mrzigazaga

18,534 posts

164 months

Monday 20th October 2014
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Well i have lost 2.5 stone in weight and my Wedge is definitely faster.. smile

I think a modern ECU set up would be beneficial..On a Wedge..Yes..On an SX Wedge definitely..One of my biggest fears with the supercharging of Delilah was detonation..With a modern ignition system and ECU it wouldn't of been a worry..Mechanical systems are way too slow to react in time..If my ECU ever gives out then i will go MS or similar..

stevoj

798 posts

160 months

Monday 20th October 2014
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It'll never work! if you want 43mpg just buy the XJS biggrin

mrzigazaga

18,534 posts

164 months

Monday 20th October 2014
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To be fair i don't care about too much about MPG...Its all about SPG...Smiles per gallon..smile...Fuel is what it is and if i was that bothered i would go for an LPG conversion..I gas..(Pun).. then it would be "Smiles = Miles"...Mind you 28mpg is good going..I get around 20 in my 350i...I think...

Sireatalot

627 posts

218 months

Tuesday 21st October 2014
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On the way up to the Ace Cafe couple of months back, I managed to get approx 37 mpg from the 350i. that was steady driving @70 the whole way, with the roof up for the M4 section of drive.

( thats better than my Celica !! )

Convert

3,747 posts

217 months

Tuesday 21st October 2014
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MPG!

I've just bought another V8. A 4ltr, that weighs twice as much as the Wedge, has an auto box, off road tyres and the aerodynamics of a house brick.. I reckon I'll get 10-12mpg if i'm lucky.


Don't worry I've kept the Wedge.

Just bought a Rangie for winter use.

Well I figured as I now drive a hybrid company car I need to keep doing my bit for global wombling. biggrin

Sireatalot

627 posts

218 months

Tuesday 21st October 2014
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adam quantrill said:
I came across this article on The Register where it's claimed you can get 43mpg out of an XJS!

http://www.theregister.co.uk/2014/10/19/crawling_f...
Just thought I'd give this a read over lunch.... very interesting stuff there and they all seem feasible modifications to make on Wedges.

however.. after tying to understand the figures, my brain hurts..

Brummmie

5,284 posts

220 months

Tuesday 21st October 2014
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I bet he drove it like the prop was made of sugar glass to get those figures though...?

Wedg1e

26,760 posts

264 months

Tuesday 21st October 2014
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I've just picked up my new Connect and some versions (not this one) have the radiator shutters etc..
The old Connect and presumably a number of other Fords have for a good while had the ECU-controlled alternator which only charges when told to do so - it's also driven by a one-way clutch!

I made the new injection harness for my 390 a good yard shorter than the one I removed.
There's no doubt that a closed-loop injection system can offer better economy but (as you find with a lot of new model cars now) there's often a trade-off in the way the power is delivered; no more shove in the back but a linear progression.

RCK974X

2,521 posts

148 months

Tuesday 21st October 2014
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If you've got a geek mindset (like me...) there is some fascinating stuff on the web about efficiency, energy etc.

I said this somewhere else....but worth repeating....

teacher

In theory, it should not matter at all what engine you have, the fuel economy of your vehicle is determined by three things, its total weight, its total drag, and any 'parasitic' loads (alternator, water pump, power steering, etc). The drag is dependent upon the air speed (aerodynamic) and total rolling resistance (tyres plus all moving parts,bearings etc).

To generate the power to move the vehicle, the engine converts fuel into energy. If you have a modern engine which is more efficient than an older one, it DOESN'T MATTER WHAT SIZE IT IS, you should still get better mpg.

This of course neglects things like aceleration and a heavy right foot, but assumes you are cruising with just enough pedal to continue at the same speed. But it's still valid, and proper science too.

So actually I reckon a 350 will probably be about the same as a 280 overall - the 350 may well be slightly better at cruise as it does have a (basic) electronic control system, although 'K' is not too shabby either.

It's a myth that a big V8 will automatically be a gas guzzler, sure it will be a bit heavier on gas around town, but not as much as you might think. There are lots of true stories of owners changing their carb 4 cyl 'sport' engine for a 5 litre EFi V8, and they are amazed their gas mileage goes UP.

If you reduce any of the above 3 (weight,drag,loads) you're bound to get better mpg, but you still have to drive gently.

Of course tuning your V8 may give more power, but often its overall efficiency actually drops, because you've tuned it for more power at certain revs, and it therefore gets worse elsewhere, unless VERY carefully managed.
Hot cams are typically bad, but gas flowed heads are often quite good (again, think EFFICIENCY, not power)

And that's EXACTLY how the Jag was modded/managed.....

{Rant Over, if you're still reading !!}

Edited by RCK974X on Tuesday 21st October 22:54


Edited by RCK974X on Tuesday 21st October 22:56

RCK974X

2,521 posts

148 months

Tuesday 21st October 2014
quotequote all
Another example is the absolutely STUPID EU rule about low power kettles.

It's a scientific FACT that a low power kettle will use MORE energy than a high power one, to boil the same amount of water.

Go on, someone say why this is.....

Edited by RCK974X on Tuesday 21st October 23:06

Wedg1e

26,760 posts

264 months

Tuesday 21st October 2014
quotequote all
RCK974X said:
Another example is the absolutely STUPID EU rule about low power kettles.

It's a scientific FACT that a low power kettle will use MORE energy than a high power one, to boil the same amount of water.

Go on, someone say why this is.....
The same might be said about the absolutely stupid EU rule on reducing the power of vacuum cleaners: firstly the less powerful one will pick up less and secondly, when the lady of the house realises the carpets aren't as clean as they should be she will insist that the man of the house vacuums more often, thus using more electricity AND wearing out the man concerned... whistle

RCK974X

2,521 posts

148 months

Tuesday 21st October 2014
quotequote all
AND he'll have to wait longer for the bloody kettle to boil......

The EU is one of the reasons why I buggered off to the other side of the planet.
At least I brought the wedge with me.

AceOfHearts

5,818 posts

190 months

Tuesday 21st October 2014
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Great read especially as I use a 3.6 XJS as my daily driver. On a long journey the best I have achieved is 34mpg over 200 miles. Maybe when I have finished my other projects I will have a go at a few of his ideas to my own car for a bit of fun smile

adam quantrill

Original Poster:

11,535 posts

241 months

Wednesday 22nd October 2014
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I have been wondering about electric motors on the front hubs. What donor vehicles can we use, with front wheel drive, where I can change the granny front hub over to a driveshaft front hub, with minimal front suspension/geometry changes?

I'm thinking of hanging the motor off the stub axle at the back, which would then be used for braking+generation (in conjunction with the normal brakes of course) and power delivery. A modest battery pack could be installed in and under the passenger footwell.

This would confer extra advantages = a few more bhp = less oversteer (while the battery has charge!) = maybe delete the starter motor and bump-start instead = charge it in the garage for a few miles range/boost on the batteries.

Another thought is - given my supercharged setup - if I can drive the blower off the recovered energy I don't have to sap as much from the crankshaft... hmmm....