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Hi all, this is my long term (ie not enough time to do any work at the moment) project spitfire (in the readers rides section). The aim is to use it as my daily commute car as I do a 55 round trip every day. 40 miles of that is motorway so gentle cruising really helps fuel economy.  Currently the engine is the original 1147cc (67bhp) running twin carbs and dizzy. the aim is to optimise the aerodynamics (nothing daft tho)and lighten the car (standard weight is 800kg). other things like lightweight alternator and lightweight more powerful starter motor have been sourced. a type 9 5 sped box will also be fitted. for the engine I have 2 options:- 1) fabricate a full fuel/spark injection system. I have some homebrew TB's from a Triumph Speed Triple modded like a twin carb setup, I already have a megasquirt setup but would invest in a Canems ECU for ultimate siamesed port fuel management (about £450). I would expect to see around 75-80 bhp plus improvement in torque and fuel economy (hopefully 50+mpg on the daily motorway run, at normal motorway speeds) 2) keep the engined carbed, use an Omex 3d mappable ignition system (already have that) and fit an LPG system. appranrelty this is very easiy to do on carb engines. i'd probably source a herald single carb manifold for this. I need to do further research on the lpg option - as to whether to go dual fuel or just lpg only, and what the pros/cons are. I'd appreciate any pointers to handy websites for running on carb-lpg only have some spitfire pics Matt     |
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bryn
Posted a lot
Posts: 3,913
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Jan 22, 2010 11:27:30 GMT
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Interesting conundrum...  I'd choose option one, so no LPG, optimizing what you have now. The advantages of LPG are obviously the expense, the downside is placing a decent size tank in your boot will pretty much do away with any space... And they're heavy. Plus you do loose performance, so on a vehicle sporting 70-80bhp, roughly 10% is sizeable. Plan A sounds simpler and more efficient in the long run...  Edited because I forgot to say I fricking love that hard top... And wheels. Nice Spitfire |
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Last Edit: Jan 22, 2010 11:28:16 GMT by bryn
Volvo, Buggy, Discovery and an old tractor.
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Hirst
Posted a lot
This avatar is inaccurate, I've never shaved that closely
Posts: 3,930
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Jan 22, 2010 11:51:16 GMT
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My Laurel is LPG from factory.  Only runs on LPG and has a carb with manual choke - amazingly, it just operates like a petrol car in the same situation. Has an absolutely enormous tank in the boot, but the boot is massive anyway as it's a great big 4-door saloon. I understand the main problem with converting cars to LPG-only is that the valves wear down more quickly. I'm told that cars designed to only run LPG have stronger valves to counteract it, but I'm no expert. Overall I've come to quite like LPG, it's quite good. |
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Jan 22, 2010 12:07:14 GMT
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I agree with both of the above.
LPG definitely has its place. Hirst's taxi, my Volvo, old jags.. big old cars that need the conversion to be affordable to run on a daily basis.
A spitfire on LPG just wouldn't be right.. I'd go with plan one for the reasons Bryn has said, except I'd make it even simpler by suggesting looking for an overdrive 'box from later spitfire/toledo/dolomite rather than going for the 5 speed conversion.
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...proper medallion man chest wig motoring. |
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Jan 22, 2010 12:39:58 GMT
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oooohh this is getting interesting and quite a dilemma. On the one hand converting to fuel injection would be the easier option as i've done it before. with weight savings I reckon I could easily crack 50+mpg.......my smart roadster used to regularly get 51mpg with more weight than the spit (80bhp 80lbft torque). Going LPG, i'd possibly go the whole hog and run just LPG, so ditch the petrol tank and replace with a LPG cylinder. the tank goes between the wheel arches just behind where the black axle plate is and the gas filler would use the existing filler hole on the rear deck.  Upping the compression ratio compensates for power loss apparently but I'm not sure at this stage you much have to increase it by. I need to do more homework but i've been told that LPG is 110 octane so modding the engine to run just on LPG overcomes the power loss and worse economy issues of dual-fuel engines...... i think i could keep costs down by sourcing the lpg kit second hand and doing the fit myself, just leaving the final set up and insopection for a professional. Gearbox - yeah, this is a problem. i could just fit an overdrive but they are pain when they go wrong and expension to fix and are heavy. the type 9 is more involved but lighter and cheaper to fix/replace. the jury's still out on this i think |
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Prud
Part of things
Posts: 308
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Jan 22, 2010 13:05:48 GMT
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K.I.S.S. - Translates to "Keep it simple stupid". To chase economy...
1) Lose every ounce of weight you can spare. If you're the only one riding in it, bin everything inside the car bar the drivers seat, seat belts and the dash board.
2) Change the diff ratio (either the gears or just bigger tyres) to lower the engines RPM at freeway speeds.
3) Give the car more gears. The type 9 will be probably be perfect for this.
4) Make the engine breathe well (free flowing exhaust and intake, don't need to worry about cams or other fancy gear) so it doesnt have to work as hard.
I don't know how much an LPG conversion is worth in the UK. But in Oz, it works out at around 150,000-200,000 km of driving before the initial outlay is paid off. Basically its a lot of hassel for not too much gain. LPG doesn't have the lubricating qualities of leaded petrol, so yes you will have to fit hardened valve seats - same as an unleaded head conversion. If the LPG isn't watered down (and in most places it is), it will support astronomical compression ratios. But building an engine to take advantage of that is yet another expense for the conversion to overcome. On standard compression ratios the engine will be thirstier, but the lower cost of LPG means that $ per km is slightly in LPG's favour.
In reality, I wouldn't bother. For economy, I'd focus on making the car breathe well and lowering the revs at highway speeds as much as possible.
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