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 Post subject: Re: petrol hungry
Posted: Wed Aug 10, 2011 12:32 am 
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Faulty ignition can cause very bad fuel consumption too...
Check and or replace leads if they look original...
02 sensors should be replace every 100.000 klrs...

 

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 Post subject: Re: petrol hungry
Posted: Wed Aug 10, 2011 6:58 pm 
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xafalcon wrote:
xr8185 wrote:
ps. its always running on high octane 98, it that helps


This actually hurts economy rather than helping. It's designed to run 91, that is what you should feed it. Higher octane fuel burns slower, so each litre of gas does less work as the piston travels further down the bore before peak cylinder pressure is reached.


hmm, this is intresting.... so your saying any vehcile thats designed to run on 91 will be worse off running on 98..??? well I better tell my mate that worked for Shell (retired) who's a chemical Engineer stop using 98 in his Hyundi as he'll get better fuel economy going back to 91....right!!
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 Post subject: Re: petrol hungry
Posted: Wed Aug 10, 2011 9:09 pm 
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AUXR8 2 wrote:
xafalcon wrote:
xr8185 wrote:
ps. its always running on high octane 98, it that helps


This actually hurts economy rather than helping. It's designed to run 91, that is what you should feed it. Higher octane fuel burns slower, so each litre of gas does less work as the piston travels further down the bore before peak cylinder pressure is reached.


hmm, this is intresting.... so your saying any vehcile thats designed to run on 91 will be worse off running on 98..??? well I better tell my mate that worked for Shell (retired) who's a chemical Engineer stop using 98 in his Hyundi as he'll get better fuel economy going back to 91....right!!


so if this is true gthen how can ford claim on the fgg xr6 they are 195 kw but if ou use 95 fuel it goes to 198 kw and if you use 98 fuel it will go to 202 kw.? surely the motor cant identify what fuel is in your tank

 

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 Post subject: Re: petrol hungry
Posted: Wed Aug 10, 2011 9:16 pm 
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AUXR8 2 wrote:
xafalcon wrote:
xr8185 wrote:
ps. its always running on high octane 98, it that helps


This actually hurts economy rather than helping. It's designed to run 91, that is what you should feed it. Higher octane fuel burns slower, so each litre of gas does less work as the piston travels further down the bore before peak cylinder pressure is reached.


hmm, this is intresting.... so your saying any vehcile thats designed to run on 91 will be worse off running on 98..??? well I better tell my mate that worked for Shell (retired) who's a chemical Engineer stop using 98 in his Hyundi as he'll get better fuel economy going back to 91....right!!


Correct. As noted above, less work is extracted due to the slower burn rate. And additionally, adding extra timing advance will not recoup this as it increases -ve work before the piston reaches TDC. The only way to make more power out of higher octane fuel is by increasing thermal efficiency through increased dynamic compression.

If it was as easy as feeding an engine higher octane fuel to extract more work out of a gallon of gas, Detroit would be all over it like a rash as they have to meet average fleet milage figures.

In summary. Any given engine will make maximum power from the lowest octane fuel that still allows an optimum spark advance curve

 

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 Post subject: Re: petrol hungry
Posted: Wed Aug 10, 2011 10:22 pm 
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It's not quite as easy as that either..Most modern engines will advance to ping then drop back...
A slower burning fuel can push piston down the bore longer..
I say can as there are so many variables it's not funny...
One part is detonation threshold while the other part is the energy in the fuel..

Any air leaks between engine and 02 sensor cracked extractors can suck in air which gives senser's a lean reading
which will make them give a richer mixture to burn the oxygen...

 

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 Post subject: Re: petrol hungry
Posted: Thu Aug 11, 2011 7:45 pm 
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No, a slower burning fuel will always extert less net push (work) than a faster burning fuel as it builds less peak cylinder pressure (and importantly, less area under the cylinder pressure curve = conversion of petrol into mechanical energy) assuming the spark timing curve is optimised for the engine and fuel. Advancing the timing mechanically or electronically still doesn't change the burn speed, just the point ATDC when peak pressure happens. Advancing spark increases the -ve work as the production of combustion gases starts earlier, pushing down on the still rising piston.

The only way to make more power out of a slower burning fuel is to increase thermal efficiency by increasing dynamic compression through forced induction, or increasing static compression (without then loosing it all with a bigger cam, which is why it's dynamic compression being referenced above). FWIW One other way is running the engine hotter so more energy goes into expansion of gasses, less into the radiator. But this isn't relevant here.

Octane in petrol is generally increased by addition of oxygenated compounds or branched chain hydrocarbons. The former is way cheaper than the latter, so that's whats typically used. Unfortunately the addition of oxygenates (primarily ethanol) reduces the energy of the fuel as it is already partially oxidised. And even if "top shelf" branched chain hydrocarbons are used to boost octane, there are no more carbon-carbon bonds to oxidise than in lower octane petrol. So the combustion nets the same heat output (more or less).

 

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 Post subject: Re: petrol hungry
Posted: Thu Aug 11, 2011 8:26 pm 
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Anyhoo! I've always thought that the fuel figures claimed here at times were a bit much, having said that I did get 400kms from a tank of fuel (octane rating withheld) in my 5.0 NF on hols once also she had 3.23:1 gears at the time.

 

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 Post subject: Re: petrol hungry
Posted: Thu Aug 11, 2011 10:22 pm 
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XAFalcon..
Yea...I'm thinking forced induction.. Which as you said is right...
When these fuels start working better...At the expense of consumption..

 

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Project 1UZ-EF has started.. S475 Turbo 4.0 V8 Mustang Celica.....

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 Post subject: Re: petrol hungry
Posted: Fri Aug 12, 2011 1:37 am 
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On long drives in my EL XR8 when stock, I could get over 600km out of a tank. Around town, I think I'd get closer to 500

 

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 Post subject: Re: petrol hungry
Posted: Fri Aug 12, 2011 8:35 am 
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i used to get bout 550 out of the gt round town and nearly 700 on the highway
brothers xr8 struggles to get 500 in any driving
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 Post subject: Re: petrol hungry
Posted: Fri Aug 12, 2011 2:58 pm 
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You can talk about the theory of higher octane fuel all day but every vehicle I have and every other car I know of using higher octane fuel than standard gets better fuel economy and power than it did with standard fuel.
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 Post subject: Re: petrol hungry
Posted: Fri Aug 12, 2011 6:55 pm 
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XR9UTE wrote:
You can talk about the theory of higher octane fuel all day but every vehicle I have and every other car I know of using higher octane fuel than standard gets better fuel economy and power than it did with standard fuel.


Well I thought that was half the point of it really! :wink:

 

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 Post subject: Re: petrol hungry
Posted: Fri Aug 12, 2011 10:42 pm 
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Benny D wrote:
AUXR8 2 wrote:
xafalcon wrote:
xr8185 wrote:
ps. its always running on high octane 98, it that helps


This actually hurts economy rather than helping. It's designed to run 91, that is what you should feed it. Higher octane fuel burns slower, so each litre of gas does less work as the piston travels further down the bore before peak cylinder pressure is reached.


hmm, this is intresting.... so your saying any vehcile thats designed to run on 91 will be worse off running on 98..??? well I better tell my mate that worked for Shell (retired) who's a chemical Engineer stop using 98 in his Hyundi as he'll get better fuel economy going back to 91....right!!


so if this is true gthen how can ford claim on the fgg xr6 they are 195 kw but if ou use 95 fuel it goes to 198 kw and if you use 98 fuel it will go to 202 kw.? surely the motor cant identify what fuel is in your tank


Yes the motor can identify what fuel you are using, its called a knock sensor. Modern cars keep the advance just under where it will ping. The better the fuel, the more advance the engine will introduce. Hence the 202kw figure when running on 98.

I run 98 on my XR8 and have also manually bumped the timing to 34degrees.
Car gets better economy and has more poke.

 

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 Post subject: Re: petrol hungry
Posted: Fri Aug 12, 2011 11:55 pm 
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Sturmovik wrote:
Benny D wrote:
AUXR8 2 wrote:
xafalcon wrote:
xr8185 wrote:
ps. its always running on high octane 98, it that helps


This actually hurts economy rather than helping. It's designed to run 91, that is what you should feed it. Higher octane fuel burns slower, so each litre of gas does less work as the piston travels further down the bore before peak cylinder pressure is reached.


hmm, this is intresting.... so your saying any vehcile thats designed to run on 91 will be worse off running on 98..??? well I better tell my mate that worked for Shell (retired) who's a chemical Engineer stop using 98 in his Hyundi as he'll get better fuel economy going back to 91....right!!


so if this is true gthen how can ford claim on the fgg xr6 they are 195 kw but if ou use 95 fuel it goes to 198 kw and if you use 98 fuel it will go to 202 kw.? surely the motor cant identify what fuel is in your tank


Yes the motor can identify what fuel you are using, its called a knock sensor. Modern cars keep the advance just under where it will ping. The better the fuel, the more advance the engine will introduce. Hence the 202kw figure when running on 98.

I run 98 on my XR8 and have also manually bumped the timing to 34degrees.
Car gets better economy and has more poke.


This shouldn't happen for you, though, going by what xafalcon is saying. Really, you should be running 91 in your XR8, as that's what it was designed to run on and you won't achieve anything by running 98. If anything, you should have less poke. It seems your real world experience says otherwise... As does mine.

 

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 Post subject: Re: petrol hungry
Posted: Sat Aug 13, 2011 12:55 am 
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xafalcon is full of theory, but not much else IMO.

Dating back well into the 90s, the first mod Mustangers did was to bump the timing and run premium on the Windsors. It gained 5-10hp on the dyno, and shaved tenths off their 1/4mile.
Thousands of real World experiences cannot be wrong

 

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