Outer Limits 4x4 Board

I constantly hear from people and mechanical shops saying that LPG requires less air than fuel so you dont have to runt he same CFM throttle body as fuel. Stoich for petrol is 14.7 and 15.8 for LPG and even 17.1 for a 50/50 mix of LPG and Butane, dont know the mix rate but definatly got some Butane in it. So LPG at stoich requires more air to burn all the fuel completly than petrol . The other reason my be that LPG gas vaporised takes up a lot more room in the combustion chamber as volume so cant get as much air in the same space as what fuel would how been to the same engine. Hope this makes sense but would like to know why LPG does not require as more air to engine.

I can't say I've heard this claim before, and can't say if it's right or wrong, but if the stoichiometric ratio is by mass, and the LPG is delivered as a liquid, then (as you suggested) the expanded LPG takes up more space than vapourised petrol, so there's less room for air?

Would also contribute to the crap fuel economy from LPG - less oxygen in each combustion cycle means less fuel is being burnt, so less energy is being released. Engine needs to work harder to achieve the same.

Or not. I'm only guessing.

I get better performance and economy from my lpg in a 4.5 80 series.

EpicRedLux wrote:I get better performance and economy from my lpg in a 4.5 80 series.

You must have the injected system not the mixer. I can vouch that a lpg run engine doesn't like an air leak an will make it run like shit

EpicRedLux wrote:I get better performance and economy from my lpg in a 4.5 80 series.

Really? What sort of difference are we talking?

On a side not I just hooked up a wideband to my GQ. On lpg it runs fine from 10:1 up to 17:1 (obviously power does change but runs fine)

-Scott- wrote:

EpicRedLux wrote:I get better performance and economy from my lpg in a 4.5 80 series.

Really? What sort of difference are we talking?

Tiny gains in fuel economy but oodles better performance. Revs hard right through the range compared to petrol even 98octane which lacks down low. I'm going to go extractors and exhaust soon to try and gain some driveability while running unleaded.

Also off approximately 90-95L of gas I'm getting 400-430kms with 285/75s

When I changed my TB42 GQ from petrol to straight gas I got the same power but still only used 95 lts of gas to 400km's witch was the same as fuel but now half the price to fill. That was using Gas research system, found it faultless and easy to tune.

stilivn wrote:

EpicRedLux wrote:I get better performance and economy from my lpg in a 4.5 80 series.

You must have the injected system not the mixer. I can vouch that a lpg run engine doesn't like an air leak an will make it run like shit

I get 550 km from 90 ltr gas on a mixer system 13 years old according to the installation plate. About half the power than petrol though. And before anyone says it It's running 35's and I drive it normal no grandpa driving.

howsie wrote:

stilivn wrote:

EpicRedLux wrote:I get better performance and economy from my lpg in a 4.5 80 series.

You must have the injected system not the mixer. I can vouch that a lpg run engine doesn't like an air leak an will make it run like shit

I get 550 km from 90 ltr gas on a mixer system 13 years old according to the installation plate. About half the power than petrol though. And before anyone says it It's running 35's and I drive it normal no grandpa driving.

Turn the dizzy forward about 10 degrees and don't use petrol anymore...

If it is all set up right I can't see why you would run a different throttle body compared with petrol. Sure the piston has to draw in LPG as well as air in a mixer type of system but ultimately the amount of gas is matched to the amount of incoming air, so what's the difference?

What is it that I am missing here?

EpicRedLux wrote:

-Scott- wrote:

EpicRedLux wrote:I get better performance and economy from my lpg in a 4.5 80 series.

Really? What sort of difference are we talking?

Also off approximately 90-95L of gas I'm getting 400-430kms with 285/75s

You get worse than that on petrol?

-Scott- wrote:

EpicRedLux wrote:

-Scott- wrote:

EpicRedLux wrote:I get better performance and economy from my lpg in a 4.5 80 series.

Really? What sort of difference are we talking?

Also off approximately 90-95L of gas I'm getting 400-430kms with 285/75s

You get worse than that on petrol?

Yeah i'm pretty sure it's tuned for gas cause it runs like a dog on petrol. Just doesnt want to do anything between idle and 3500rpm

^^^ Isn't the 80 series EFI? No tuning there.

howsie wrote: I get 550 km from 90 ltr gas on a mixer system.

what am I doing wrong, I would only get bout 350km to 90ltrs.

EpicRedLux wrote:

-Scott- wrote:

EpicRedLux wrote:

-Scott- wrote:

EpicRedLux wrote:I get better performance and economy from my lpg in a 4.5 80 series.

Really? What sort of difference are we talking?

Also off approximately 90-95L of gas I'm getting 400-430kms with 285/75s

You get worse than that on petrol?

Yeah i'm pretty sure it's tuned for gas cause it runs like a dog on petrol. Just doesnt want to do anything between idle and 3500rpm

That's usually because the seals have dried up because has been run too much as a general guide you should run one tank petrol to three tanks gas.

stilivn wrote:

howsie wrote: I get 550 km from 90 ltr gas on a mixer system.

what am I doing wrong, I would only get bout 350km to 90ltrs.

Mine is part-time (im not convinced it.makes any real difference and.tuned the mixer for economy), take it to a good gas bloke and get it tuned for economy. I tuned mine myself and have had it checked by gas mechanic It's just in acceptable range. Most gas systems are tuned for a Mixi between power and economy standard.