Originally Posted by badgers_nadgers
Won't the knock sensor take care of this anyway?
The knock sensor works the other way around, if you put in fuel with an octane less than what the ECU expects, the fuel will self-ignite before the spark plugs have fired. The knock sensor notices this, and makes the engine spark earlier, when the air/fuel mix is less compressed. You get quite a bit less power/torque and fuel economy.
But if you put in a higher octane fuel than it is mapped for, the spark ignites the fuel and the engine runs as the map is written. It never knocks, but the engine doesn't realise it could
delay the spark to achieve higher compression, therefore more power, torque and possibly economy.
My 3.0 is mapped for 97 octane, if I run on 95 I think it triggers the knock sensor, because I get atrocious fuel economy and noticeably less power, as if the engine is sparking early before the air/fuel mixture is properly compressed.. I did one tank on ASDA 95 and got 15mpg not even booting it, just driving normally, mix of urban/A roads.
On Tesco 99 I get about 20mpg urban and upto 30mpg on a long run, which is more like reasonable..
They are going to size the system up to the engine though so enough fuel is given to meet the current mapping. They said I can expect at least 90% of the power with the set up they will do and about 10% less mpg than I get with petrol.
Down my way it is just over 50p per litre and petrol is about 102p.
I probably do about 15,000 miles per year which in petrol would cost £2784 but with LPG would cost £1517. That is over £1200 saved, which over 3 years works out to be more than £3600! It is not as if petrol power is not the press of a button away either so I can't see a down side to it. I can run a high performance, stunning looking car for the cost of a small engined mondeo.
I wouldn't trust the figures given to you by the people trying to sell you/fit the LPG kit.. They will set the LPG system up to inject the correct amount of fuel for the reading made by your MAF, they can't inject more fuel to get more power as the air to combust it is not there. The reason you get less power with LPG is purely down to chemistry, there is less stored energy in a set mass of LPG than there is in the same mass of petrol. But you can get a higher % of fuel completely combusted with LPG if
you retard the spark, I would assume the LPG kit you are looking at's ECU does this, the old cheaper kits didn't..
I've looked into this, on a few other forums as well as what I've read on here, a few people have had 2.5 V6s LPG'd and they average about 18mpg, which is equivalent in £s terms to getting between 36 and 38mpg on petrol. 2.5 V6s aren't particularly more economical than GTAs, partially down to the ratios and partly due to the lack of midrange torque meaning you tend to rev them more, so I'd imagine you could get 18mpg on LPG (cost equivalent of getting 36mpg on petrol) so you'd still be saving quite a bit. Just don't count on saving as much as you've worked out in your post above..
And I forgot about petrol power at the push of a button, thats one plus at least