You will not damage the impellers from sucking in cold air, possibly from thermal shock but you would have to be driving a fireball into an ice river.  I can't think of any reasonable situation which would cause that.
 
LPG is not really a clean fuel,  the quality of it changes alot through the year and it does contain a lot of deposits.  Open up a LPG convertor, it will look like sticky tar everywhere, this stuff does rot the diaphrams too.  You can mount a convertor 2 ways, one causes more deposits inside the convertor itself, the other causes the deposits to go more freely into the motor.  Which is worse I don't know.
 
EFI is not an automatic emissions test pass.  The tune itself will dictate the emissions and a "poor" tune can pass whereas a "good" tune could fail.  The more ignition timing the engine runs the more NOX emissions occur.  NOX is the worst of the lot even though it is an indicator of efficency.  This is the reason that most 1970's cars especially Fords ran little timing. 

as far as i understand it is more fuel = more power = more emissions. Increased engine efficiency = more power on less fuel = less emissions.
What about the use of inline devices such as cats? My m5 has 4 of them i guess they gotta do something?
What about ethanol fuels like e10?
I dislike using other fuels than petrol, bit of a purist. But ill keep harping on about this emissions stuff cuz no one likes being down the pits every 3 days.

That statement is only partially true because NOX tends to increase with efficency.  Mechanically all things being equal increasing the timing on any carby motor will generally improve the idle and power.  The combustion temps go up substantially and the NOX goes up too.  This is the reason for EGR.  It purposely introduces exhaust gas back into the combustion to reduce efficency at part throttle and NOX.
 
The only other way to improve emissions on an old donk it to use a more efficent head design. 
 
You should be starting to see what the others are saying, building an powerful engine to meet emission standards, especially today's is a waste of time that is only going to cost you money. 

Ethanol fuels arent pushed because they are particularly clean, just that they are made from renewable sources.
 
E85 might sound great in theory but it has some massive short comings
 
1. It doesnt store at all, so if you buy it you gotta use it. No good in a hobby car that isnt being driven daily
2. The Ethanol and Petrol 'blend' separates over a fairly short period of time and does not 'remix' once separated which significantly alters the octane level through the tank. Ethanol sinks to the bottom because it is heavier.  Vehicles designed to run on this stuff have a sensor in the tank to that the ECU can adjust the furl mixture to cope with the variations in the fuel mix. Good luck achieving this on a carby fed engine.
3. Ethanol is hygroscopic which means it attracts and absorbs moisture. there is a very intersting video on You Tube on this. http://www.youtube.com/watch?v=YeCyFxoWPpo how is this important in a car?  Anything running a carby has an open vented fuel tank which means it is exposed to moisture in the air. Later model EFI stuff has what is referred to as a 'closed loop' system which is not vented to atmosphere.
 
If you doubt any of these statements about Ethanol all you need to do is google 'effects of ethanol' and you will find plenty of evidence.  As long as an alternative is available i wont go near Ethanol blended fuels.
 
For the record, the Youtube video is nothing to do with me or the company i work for. It was actually done by an American competitor to the product we distribute.
 
G

cheers for all the detailed info boys,
ive picked up some reading material regarding F/I and will read through that and see if i cant understand thermal dynamics and the absolute physics related to what im trying to acheive.
 
is there anywhere i can find out more about using lpg as a fuel. as lpg fuel isnt done in the usa and such its hard to get books about using it.  (ive been told good and bad things about this but i really would like to know in detail)
 
my head hurts. i havent studied this much since highschool.

Without going into all the technical info/jargon, If LPG was the answer everyone before you would have been using it. I am all for thinking outside the square and doing things differently however, some things are best left alone in my opinion. Having said all that tho if your that super keen, give it a go and let us all know your outcome. 

well with engines alot of it seems to be tradition, experience or rigerous testing done by large well funded brands to find the best way to manipulate a fuel. most being american brands.
 
lpg as a car fuel is recently being introduced to usa (where fuel is cheap and emissions standards relatively loose. infact every american you mention it with, will think 'sausages') and same with uk. so i can guess there wont be any studies on it or performance enhancement.
RE: http://en.wikipedia.org/wiki/Autogas
in australia the ron is around 105 average from memory.
ive seen a few turbo applications work to a suprising level. as far as i can tell it has good characteristics for a force inducted motor. with great difficulty im trying to find out how much can you get out of it. and if current problems faced with petrol still exist, eg thermal dynamics in relation to emissions and power loss when using a fuel that hits the heat source at 40 below
 
albeit the blends vary from pump to pump and purity is an issue
if im completely outa my tree you can tell me, i just need to know why. im keen as hell, but planning to build a F/I motor with little info about its use is a hard thing, im tempted to just build a turbo pinto with all the usual specs, pull out the decomp plate (or reduce it to a mathematically acheived comp ratio that suits lpg most) and see if it works... or see if i can find 2 pistons and 4 rods imbedded in my shed wall 

Unless you are attempting uber horsepower with a pinto motor you are looking into this too much.  The std pinto motor has a lot of mechanical design limitations, which would cost a lot to fully rectify.
 
If you want a turbo charged street engine look at the one joeyjonsey has done.  You can easily turbocharge vitually any motor, especially for minor power increases of 50-100% with the std motor.
 
The important 3 aspects of any motor for performance are cam, compression, and airflow.  Everything else is just icing, fancy rods and forged pistons increase durability.
 
The pinto cam is pathetically small with very little lift, it is cut on a wide lobe centres and generally installed retarded.   
 
You don't need to really lower your static compression for mild power increases(especially with a LC bottom end),  8.5:1 is generally considered a good compression.  Even then you can fine tune compression with cam lobe centres.  Also if you engine cranks at less than 160psi you can throw 8psi at it with no problems.
 
Finally the turbocharger has the biggest effect on driveablity, ideally this should be sized correctly especially on a small 4 cylinder.  Whats the point of have a turbo that produces max efficiency at 30psi if you are only going to 10psi?  In the same token why have a power peak of 7500rpm when your car spends 90% of its time between 2-4500rpm? 
 
I am in the slow process now of turbocharging a mates mk2.  I am hoping to do it all for less than $3000.  That includes:
 
Fully ported head
Custom cam
Std bottom end
T3 turbo
Fabricated exhaust & manifold
Camira Ecu & wiring loom 
Fuel pump & injectors
Modified carby manifold
Ebay intercooler
 
$1500 of that is going to be the head and cam.  We are doing about 95% of the work ourselves.  Hoping for 220fwhp at about 10psi.
 
 
 
 

sorry to bring up an old thread but it caught my attention that most assume lpg is no good for making hp... heres a video to say it does.
 
http://www.youtube.com/watch?v=siWqyB87LNs