Second fuel tank in boot

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ralft01
Joined: Mon 02 Jun, 2014 08:00
Posts: 37

  Z3 roadster 2.8

Second fuel tank in boot

Post by ralft01 »

I thought I might be able to have an ECU re-map to improve mpg, but it is not possible with naturally aspirated petrol engines. I had a look at an LPG conversion, but that seems to be a debatable issue. I want to be able to travel further without having to stop to refill; I hate being reminded how much fuel I am using and what it is costing! So I am now looking at fitting an additional fuel tank into the boot, connected into and supplying fuel to the existing tank. In my absolute ignorance, I am picturing an alloy tank sitting in the boot with a pipe from it to the fuel tank through a hole in the boot floor.
Please save me from my ignorance and tell me what is wrong with my picture!
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pingu
Joined: Fri 30 Apr, 2004 16:01
Posts: 3412

  M roadster S50

Re: Second fuel tank in boot

Post by pingu »

I've found that the problem with auxiliary tanks is designing a way to fill them that is user-friendly. I used to have one in the back of my van when I ran a dual-fuel system.

If you don't have an M and you are prepared to lose your spare wheel, I'd investigate using a toroidal tank, like the LPG tanks. An old LPG tank may even be able to be converted to hold petrol rather than gas. Then you will need a small pump that will transfer the fuel to the main tank. It will have the added advantage of being stored outside the car, so you won't get any fumes.
Pingu
ralft01
Joined: Mon 02 Jun, 2014 08:00
Posts: 37

  Z3 roadster 2.8

Re: Second fuel tank in boot

Post by ralft01 »

I like that idea! Keeping it all on the outside seems sensible. I wonder if I could use the existing fuel sender to signal the auxiliary pump to supply fuel, and then use it again to signal stop!
Maybe I could add a second filler point, as is done with an LPG conversion.

Any thoughts as to how to/if to proceed gratefully received.
Hairyscreech
Joined: Thu 16 Apr, 2015 12:08
Posts: 69

  Z3 roadster 2.8

Re: Second fuel tank in boot

Post by Hairyscreech »

If you are going to the trouble of this then surely an LPG kit would be the far better option.
Reduce your fuel bills and extend your range by simply dual fuelling?
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Brian H
Joined: Tue 16 Dec, 2008 19:55
Posts: 2505

  Z3 roadster 3.0i

Re: Second fuel tank in boot

Post by Brian H »

Hairyscreech wrote:If you are going to the trouble of this then surely an LPG kit would be the far better option.
Reduce your fuel bills and extend your range by simply dual fuelling?
I maybe think far safer too!
Mike Fishwick
Joined: Fri 19 Jun, 2009 10:27
Posts: 2093

  Z3 roadster 2.8
Location: Daglan, France

Re: Second fuel tank in boot

Post by Mike Fishwick »

A good remap will give a small improvement in fuel consumption, and generally mprove the engine all round, but is more noticable on the larger engines. A well-trained right foot is the best value in terms of fuel economy.

Anyway, the best way to plumb an auxiliary tank is for it to have its own fuel pump, and feed both pumps via a changeover switch, so that when the red light comes on you can select the other tank. The fuel return line from the engine would have to be split, feeding back to each tank via a solenoid valve energised from the fuel pump of each tank.

What fuel consumption are you getting - and what engine do you have? Unless you want to drive more than 350 miles in one go, there seems little point in a bigger fuel capacity - Even in France there are enough petrol stations around, and the UK is full of them. Do you live in the Australian outback?
A Z3 is not just for Christmas - it's for life!
peter2b
Joined: Sat 01 Nov, 2014 18:47
Posts: 963

  Z3 roadster 2.2i
Location: cheshire

Re: Second fuel tank in boot

Post by peter2b »

Bump, I've just got back from a nice trip down south over the last weekend about 400 miles averaged 35mpg is that good or bad? Most roads were A roads
Peter2b
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BladeRunner919
Joined: Fri 17 Feb, 2012 20:18
Posts: 2225

  Z3 roadster 1.9

Re: Second fuel tank in boot

Post by BladeRunner919 »

peter2b wrote:Bump, I've just got back from a nice trip down south over the last weekend about 400 miles averaged 35mpg is that good or bad? Most roads were A roads
Peter2b
Sounds good to me. But how many smiles per gallon did you get?? :D
Hairyscreech
Joined: Thu 16 Apr, 2015 12:08
Posts: 69

  Z3 roadster 2.8

Re: Second fuel tank in boot

Post by Hairyscreech »

That's not bad. The 2.8 does 31-35mpg on a run.

Did a swift trip up to south wales the other week and got 33mpg, for the power output that's not bad.
peter2b
Joined: Sat 01 Nov, 2014 18:47
Posts: 963

  Z3 roadster 2.2i
Location: cheshire

Re: Second fuel tank in boot

Post by peter2b »

I was grinning so much that If I didn't have ears the top of my head would have droped off
Peter2b
ralft01
Joined: Mon 02 Jun, 2014 08:00
Posts: 37

  Z3 roadster 2.8

Re: Second fuel tank in boot

Post by ralft01 »

I envy the 2.8 driver who gets over 30mpg! I have a 2.8 Automatic, permanently set on Economy mode. On a long motorway run, driving sensibly and within speed limits, I might achieve 24 mpg! Around town, still driving with a soft (educated?) right foot I might get 21 - 23 mpg. My local BMW specialist assures me that the transmission changeover is behaving as it should. I have tried driving on the pseudo-manual setting to prolong the time in high gear, but doesn't seem to make a difference. On the upside, the tachometer never gets even close to red!
With regard to to LPG conversion! Lots of discussion topics on line across a whole range of vehicles. Since the government is removing all incentives for non-electric vehicles, and specifically are not encouraging LPG (apparently favouring LNG), the view is that LPG filling stations will disappear. The one local to me has discontinued LPG filling. On top of that, it is not cheap to get a decent conversion.
Advice from a raceway tank maker is that it is a lot more complex than just feeding from an auxiliary tank. All sorts of emission factors become involved. The suggestion of a separate system with a change-over switch would also have to cater for this.
On reflection, it was a mistake to buy an automatic. Although comfortable to drive, it takes too much control away from the driver.
Bonzo
Joined: Wed 21 Mar, 2012 21:52
Posts: 866

  Z3 roadster 2.8
Location: Norfolk

Re: Second fuel tank in boot

Post by Bonzo »

ralft01 wrote:.
On reflection, it was a mistake to buy an automatic. Although comfortable to drive, it takes too much control away from the driver.
My thoughts entirely. The only time I would consider an auto box is when I am unable to change gear! The 2.8 has such good low down torque anyway, you can potter along at 1000 revs with no probs. Give it some right foot and it just goes, a few seconds later pulling like a train!

Bonzo
Z3lda (Zelda), 1998 2.8, Montreal Blue, Beige Oregon leather interior and M steering wheel, Sports Seats, Wood trim, Chrome Line Interior, Centre armrest with Cupholder, Alpina Softline wheels, Chromed Angel Eyes, De- tango'd, Rear stone guards, Sport aerial, ASC front strut brace, Strong Strut Butt Strut and Body Brace, Uprated brake fluid, Yellowstuff pads 78k.
Hairyscreech
Joined: Thu 16 Apr, 2015 12:08
Posts: 69

  Z3 roadster 2.8

Re: Second fuel tank in boot

Post by Hairyscreech »

Yikes, that's bad, Glad manual was top of the requirements list. :shock:

It's not a disastrous procedure to convert to manual on the BWMs as long as you have a manual donor.

Around town the manual is not a lot better, 24-26 seems to be average, my run to work is currently 29mpg (up from 22 in the E30!)

Not heard of any issues with LPG around here (wiltshire) but I know there are a lot of automated/account filling stations as lots of the taxis and farmers run on LPG.
You can put a multi point system together for about £600 but its best to check your insurance will cover LPG without a registered installer doing it. The whole registered installer thing is such a scam as well, it's an excuse to charge over £3k for the same job you can do yourself and in some cases I have seen do better and more safely. :head:

Second tank wise, your right, you will struggle with the venting systems if you were to try to be 100% legal. It's not impossible but not worth the hassle.
peter2b
Joined: Sat 01 Nov, 2014 18:47
Posts: 963

  Z3 roadster 2.2i
Location: cheshire

Re: Second fuel tank in boot

Post by peter2b »

You can guess what the government will do as soon as ever one changes to LPG they will put up the price like they did with diesel
Peter2b
John Wilson
Joined: Fri 12 Jun, 2009 12:11
Posts: 252

  Z3 roadster 2.0
Location: Diss

Re: Second fuel tank in boot

Post by John Wilson »

Dont worry about fuel consumption. You can buy a good Z3 for 3 grand instead of something like a focus for 10 grand. That gives you £7000 pounds spare for the difference in fuel comsumption. If youre forever driving a Z to try and get 35 miles per gallon, you would have more fun in a Kia Picanto.
ralft01
Joined: Mon 02 Jun, 2014 08:00
Posts: 37

  Z3 roadster 2.8

Re: Second fuel tank in boot

Post by ralft01 »

John, you are absolutely right! Thank you for putting my head back on straight.
Mike Fishwick
Joined: Fri 19 Jun, 2009 10:27
Posts: 2093

  Z3 roadster 2.8
Location: Daglan, France

Re: Second fuel tank in boot

Post by Mike Fishwick »

I covered 500 miles across southern France over the weekend, mostly on 55 mph country roads with a little on 80 mph autoroutes, driving legally as cameras and les Gendarmes are out in force this summer. On most of the roads we were using you had to work hard to keep above 50 mph anyway. Fuel consumption on the trip computer of my 2.8 (which is calibrated to be within 1 mpg) came out at 39 mpg!

The service indicator had just been reset, I use 2.5 bars in all tyres, which are pretty new, and was using Total Excellium 98 octane, which is the best fuel I have found. All of this helps a little - but it is still pretty good for an old car now classed in the UK as a 'Gas Guzzler!' My usual local running around mpg is in the 32 to 35 mpg.

With fuel consumption which can be as good as that, who needs an extra fuel tank?
A Z3 is not just for Christmas - it's for life!
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pingu
Joined: Fri 30 Apr, 2004 16:01
Posts: 3412

  M roadster S50

Re: Second fuel tank in boot

Post by pingu »

Personally, I don't find the consumption the problem, but the range. I know if you improve consumption, you improve range, but even driving like Miss Daisy, I would still only get about 280 miles from a tank - and I didn't buy the car to drive like that :drive . When we toured Scotland, we had to carry 10 litres of spare just to be safe (and we needed it). I also had to use octane booster as there was no 98 octane available.

There was a smell of fuel for most of the journey as the cans were in the boot. Some numpty owner before me had cut a 12" hole in the bulkhead for a bass speaker and my boot isn't airtight.

I would be interested to see if there is a practical way of externally storing another 10 litres (or so) of fuel.

In BDR, we used to strap jerrycans to the roofs of Land Rovers - perhaps that's the way to go. A 20 litre jerrycan on a luggage rack with a hose running into the main tank. THIS IS NOT A SERIOUS SUGGESTION :D .
Pingu
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