There's effectively 3 levels of LPG conversion on offer, and the dearer they get, the better they get.

Bottom rung is your good old vapour systems - these are the old convertor & mixer arrangements - there's 2 types - simple converter (usually teamed with a vacuum-operated mixer) and complex convertor - more common on EFI cars, with a "ring" type insert in the inlet stream to feed the gas into it (not vacuum operated). The convertor converts the liquid LPG from the tank into a vapour, which the mixer then feeds into the inlet stream. On a stock motor, these often lose power, as they place a restriction in the inlet stream, to allow the vapour to be drawn into the engine. On V6 commodores, this sort of system is notorious for backfires.

Middle-rung is vapour injection. Saying middle rung is a bit wrong, as it's really a bit higher up the chain though, but not as good as liquid injection.
You still have the convertor which converts the liquid LPG from the tank into a vapour, but instead of a mixer in the inlet stream, you have an injector, and a feed nozzle. The injectors are mounted in banks together, and output the gas vapour via thin tubes that are drilled into the manifold. Better than the basic mixer systems, but the cost has gone up dramatically. These can be easily retro-fitted to many cars - as long as they are EFI, as their computers piggy-back off existing injector signals to know when to trigger.
Economy is vastly improved over the basic type, and you don't lose any power on petrol on a dual fuel car as there's no restriction in the inlet stream. Figures of identical or better than petrol economy are achievable with this system.

Top of the tree is liquid injection - or more correctly termed liquid phase injection. These do away with the convertor completely, and almost mimic a petrol EFI system. They run a high-pressure in-tank pump, which pumps the liquid LPG up to a fuel rail, and injectors then open & close to inject it into the inlet stream. The phase change from liquid to gas - as the liquid evaporates, causes a massive drop in inlet temperatures - almost like an intercooler on a turbo, which increases the efficiancy of this system. Very few people are doing LPI in the aftermarket, but it is out there.
On an OEM level, Holden already have LPI out there in the HSV range, and Ford are almost ready to release their LPI 4.0L (mine is already on order, due in August). Ford's R&D results have been spectacular - with equal or better performance to petrol, and better economy being the highlights of their testing.
I believe Holden are supposed to be working on a similar LPI system for the V6 now as well.


If you plan to keep your VS for quite some time, or do a lot of km, I would consider the vapour injection for it. You won't see change out of $4K, but the savings are there to be had over many years/km if you can justify them.

quote:

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Originally posted by commodorenut
On V6 commodores, this sort of system is notorious for backfires.

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yeah i've had a exploding air box before, lol, scared the f*** out of me!!!

Thanks guys.....Mick that was very educational!!! I didnt know you could get vapour injection. My mate has a normal vapour system...it is worth while to convert it to a vapour injection.

As with my vs.....I have a recon engine and Transmission so I will keep it for a while. It looks like its a very big conversion. With vapour injection can you still have your car as a dual fuel system.

Yes, as commodorenut has said the vapor injection system is your best bet. You can retain your old tank & supply line. The converter is replaced by a smaller but similarly designed 'regulator' & you get a set of injectors, harness, hoses, ECU & dash switch/gauge unit.

The installation, for the most part, is not beyond the average Joe. You could a kit (less tank & filler etc.) fit & wire it yourself & get a gas dude to check it over, secure the lines & certify it.

You won't get any rebate from the Feds if the car already has LPG & a kit (without tank etc.) would cost around $1500 to $2000. At a saving of say $40 a week (conservatively) you will cover its cost in less than a year. If your car has no LPG currently, then you can receive a $1250 rebate covering a substantial portion of the cost.

These kits work very well & are much better than primitive mixer systems. Obviously they only work on EFI cars, because they piggyback the EFI ECU & utilise all the sensors in that system, especially the oxygen sensor. That's why they are so efficient, they don't do away with the $millions of development Holden put into their EFI in the first place. I've had 3 of my vehicles running on injected LPG for the last 3 years & I'm a big fan.

They start up on petrol & switch over to LPG automatically. The switchover is so smooth it can happen at idle & often you can't tell that it has actually changed over if it wasn't for the light on the dash & the associated beep.

Dr Terry.