Single or multipoint

[robbie77]

Evening all.

What are the main differences between a single and multipoint lpg system?

My reason for asking is that i am looking at getting a RRC 3.9 with a singlepoint system.

My only experience of lpg was a few years ago when i had a 3.5 v8i disco.
I had it converted professionally and from what i can remember it was a multipoint system that had a lambda sensor.
The system cost me about £1600 fitted and i was told that the engine my car had was a good one to convert, with no real noticable difference.
However, my car didn't like motorway speeds from that point on and would overheat at speeds over 50mph.
I got rid of the car sharpish as i was doing 600 motorway miles each weekend, and i lost £2500 on the car in 3 months!

Should i be wary of the RRC with a singlepoint system? Will it make it to and maintain 70mph happily? The only thing that i know for sure is that it's singlepoint with a 95l boot mounted tank.

Your opinions on this are welcome

Robbie

[TEMPL4R]

Simon Hobson is the lad to ask R.

Apart from the obvious, one injector in the intake or one for each cylinder, that is my input and only knowledge with LPG.

Chris

[SimonHobson]

What are the main differences between a single and multipoint lpg system?

My reason for asking is that i am looking at getting a RRC 3.9 with a singlepoint system.

My only experience of lpg was a few years ago when i had a 3.5 v8i disco.
I had it converted professionally and from what i can remember it was a multipoint system that had a lambda sensor.
The system cost me about £1600 fitted and i was told that the engine my car had was a good one to convert, with no real noticable difference.
However, my car didn't like motorway speeds from that point on and would overheat at speeds over 50mph.
I got rid of the car sharpish as i was doing 600 motorway miles each weekend, and i lost £2500 on the car in 3 months!

Should i be wary of the RRC with a singlepoint system? Will it make it to and maintain 70mph happily? The only thing that i know for sure is that it's singlepoint with a 95l boot mounted tank.
Starting at the back, no you should NOT be worried about it. If it is set up right then it will feel just the same to drive as a non-converted one. MY mate runs a 3.9 with single point system, it goes "quite nicely" :D


The differences are :
A single point mixer system uses a mixer ring in the intake which adds gas by venturi action sucking the gas through holes - just like a carb uses venturi action to suck petrol through the main jet. It's cheap, simple, and fairly easy to tune even when open loop.

A multipoint injection system has a set of injection nozzles, one in each intake port, which are controlled by a box of electronics.

In both cases the gas is converted to vapour by a water heated reducer. There is a third option which injects liquid gas into the intake, but these are uncommon and tend to be troublesome.

There are pro's and cons both ways. Mixers are cheap and simple, but do introduce some restriction into the intake - and the mixer must be correctly sized. Injection systems cost a lot more, but then they have better control of fuel delivery which is essential for modern vehicles with sophisticated performance monitoring & emissions control. Also, on some engines a mixer system results in a large (sometimes variable) volume of explosive gas/air mixture in the intake tract, and a backfire can be 'interesting' - especially if you have a weak plastic intake manifold :eek:


For a 3.5i Disco, there is no real reason to require an injection system - it's overkill and expensive. Don't forget that the 14CUX injection system on these is really nothing more than an "electronic carburettor", and apart from idle speed control, it can be switched off altogether when running on gas. I'm running that very same engine in my 110 using a closed loop mixer system - and it will hold 70 on the motorway without any trouble at all.

The fact that you had overheating problems suggests that the system wasn't set up properly. One possibility is that the system just wasn't correctly sized - either the reducer wasn't big enough to supply the vapour, or the injectors weren't big enough to meter it.

Sadly, such problems as this have got gas a bad name - it's just too easy to get a system "not right". The same would be true if people were fitting aftermarket carbs or petrol injection systems, and the only reason we don't get the same problems is that this doesn't happen since all petrol cars come from the factory with a working fuel system.


When you are looking at the RR, ask for the conversion certificate. If the owner doesn't have one, factor in the cost and inconvenience of getting the system inspected and a certificate written. However, be prepared for a "sucking through teeth" response as one of my readers found out (http://www.diy-lpg.co.uk/articles/files/how_not_to_biy_it.html) :rolleyes:

I have a couple of articles on my site (here (http://www.diy-lpg.co.uk/articles/files/installation_rules_pt1.html) and here (http://www.diy-lpg.co.uk/articles/files/installation_rules_pt2.html)) that should show you what to look for on the gas system. Forearmed with this knowledge, you should be able to spot if the vehicle is a "good job" or a rolling nightmare.

[robbie77]

Thanks Simon, what a cracking response.

I remember when i had my disco converted lpg was fairly new to the masses (2002) and i trusted the dealer as they were the most renowned in the area.

I will look carefully at the RRC to make sure that i aren't just buying problems.

Robbie

[SimonHobson]

Thanks Simon, what a cracking response.
Thanks :o

I remember when i had my disco converted lpg was fairly new to the masses (2002) and i trusted the dealer as they were the most renowned in the area.
Nah, it was old hat by then :D It's just that lots of people still hadn't heard about it.

Thinking back, I think I must have bought my Disco around 1998 - it was a 1990 3.5 carb model. Looking in the price guides, there was about a £1200 premium on the diesel (or discount on the V8 depending on your point of view). Since the LPG kit only cost me £700 I'd saved £500 up front :) After that it was V8 joy at diesel costs :D

[robbie77]

If memory serves me right, i was getting around 200 miles for £20 but with an engine that would make small children cry at the noise of it starting up :eek:

I used to love driving through tunnels with the windows down just to hear the engine :D

[wylie_eyed]

If memory serves me right, i was getting around 200 miles for £20 but with an engine that would make small children cry at the noise of it starting up :eek:

I used to love driving through tunnels with the windows down just to hear the engine :D
Oh yeah baby nothing like a V8 on song :D