2.5 vs S2 fuel consumption
- Thread starter dann944
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Copperman05
New member
Dont worry I couldnt find it either, then for ages I thought I had a sticky throttle until I realised the carpet was getting in the way of the pedal..[&:]
Edd
Edd
I couldnt find the reset on my turbo! Thanks for asking the question. I never found much difference with 95-97 ron fuel, afaik there should be no difference in the lux as it doesnt have a knock sensor. Without a knock sensor the car cannot adjust its ignition timing therefore cannot take advantage of the higher octane fuel. Therefore there should be no noticable difference, unless there are other differences between the fuels with different octane ratings. I was never sure but I think the varience in the lux's performance comes from the external air temp/moisture etc as the DME isnt great at compensating for it.
Hi Tom
Thanks for those comments. All I can say is the car was definatley cosiderably smoother and quicker to pick up with half and half 97 / 99. could this explain it showing as a little rich on the dyno?
I did change the air filter before getting the 99 but didnt notice any improvement on th 6 mile trip to the petrol station.
Any other thoughts would be appreciated
Mike
Thanks for those comments. All I can say is the car was definatley cosiderably smoother and quicker to pick up with half and half 97 / 99. could this explain it showing as a little rich on the dyno?
I did change the air filter before getting the 99 but didnt notice any improvement on th 6 mile trip to the petrol station.
Any other thoughts would be appreciated
Mike
Starting at the basics, the difference between octane rating of fuel AFAIK is simply the addition of additives to the fuel to prevent knock. Knock being the detonation/explosion of the fuel in the cylinder.
http://en.wikipedia.org/wiki/Octane_rating
There are possibly other differences in fuel brands, such as additives, that could also have an effect on performance. I'm not sure about this, perhaps others could comment. The reason that certain cars benefit from a high octane fuel is that their clever engine management systems can optimise their ignition timing. Optimising the ignition timing of the engine gives you the best combination of cylinder pressures over the power stroke optimising the use of the energy produced by the combustion process. This makes the engine more efficient and typicaly more powerful. The higher cylinder pressure makes the fuel mixture more likely to detonate and create knock. When such clever engine management systsems detect knock they retard timing to prevent engine damage. This optimisation process requires the ability to detect knock so you can protect the engine. The 944 lux's DME does not include knock detection and thus does not make the changes to ignition that would be needed to get the benefit of the high octane fuel.
Unless their are other differences in the fuels I cannot see any basis for an increase in performance. I would love to know of their are any other differences in fuels or if my understanding of whats going on is wrong. Especially as at times I could have sworn my lux did run better on high octane fuel. Although I always put it down to placebo until I later thought it was down to atmospheric conditions.
Edit:
Heres one that might go some way to explaining it. The 944's DME compensates for air temp based on the air temp at engine startup. If the intake air temp changes during driving the DME will not compensate for it. Maybe the act of stopping at the petrol station and restarting the engine and thus the air temp setting is what we are noticing on the subsequent drive?
http://en.wikipedia.org/wiki/Octane_rating
There are possibly other differences in fuel brands, such as additives, that could also have an effect on performance. I'm not sure about this, perhaps others could comment. The reason that certain cars benefit from a high octane fuel is that their clever engine management systems can optimise their ignition timing. Optimising the ignition timing of the engine gives you the best combination of cylinder pressures over the power stroke optimising the use of the energy produced by the combustion process. This makes the engine more efficient and typicaly more powerful. The higher cylinder pressure makes the fuel mixture more likely to detonate and create knock. When such clever engine management systsems detect knock they retard timing to prevent engine damage. This optimisation process requires the ability to detect knock so you can protect the engine. The 944 lux's DME does not include knock detection and thus does not make the changes to ignition that would be needed to get the benefit of the high octane fuel.
Unless their are other differences in the fuels I cannot see any basis for an increase in performance. I would love to know of their are any other differences in fuels or if my understanding of whats going on is wrong. Especially as at times I could have sworn my lux did run better on high octane fuel. Although I always put it down to placebo until I later thought it was down to atmospheric conditions.
Edit:
Heres one that might go some way to explaining it. The 944's DME compensates for air temp based on the air temp at engine startup. If the intake air temp changes during driving the DME will not compensate for it. Maybe the act of stopping at the petrol station and restarting the engine and thus the air temp setting is what we are noticing on the subsequent drive?
Thanks for that Tom and congrats on the new one!
On the drive up to the Dyno Day, it felt smoother, more powerful and revved easier than at any time since I've had it and sunday morning I went out for drive with my wife and without prompting she said it seemed smoother, so I'm really intirigued!
I suppose I'll have to use this tank and go back to 97 for a bit and repeat the exercise.
At least I know its running a little rich so its safe even on the 99 stuff.
Just as an aside I checked your pics page and wondered what the power was on your Lux on the Dyno. Mine did 148 crank /121 wheels on Saturday.
Thanks for your time and comments
Mike
On the drive up to the Dyno Day, it felt smoother, more powerful and revved easier than at any time since I've had it and sunday morning I went out for drive with my wife and without prompting she said it seemed smoother, so I'm really intirigued!
I suppose I'll have to use this tank and go back to 97 for a bit and repeat the exercise.
At least I know its running a little rich so its safe even on the 99 stuff.
Just as an aside I checked your pics page and wondered what the power was on your Lux on the Dyno. Mine did 148 crank /121 wheels on Saturday.
Thanks for your time and comments
Mike
Np mike, I'm interested as I always thought my car felt smoother on high octane too. Mine dyno'd 150 at the crank and about 120 at the wheels.
My car also ran rich, that's normal at WOT, but at low speed in my opinion its a design feature of the early injection system. Running rich allows the engine to cope better with the laggy air flow meter, which is only used at low throttle. As the air flow meter takes a while to respond to change in air flow running rich allows the injection system to catch up. I swapped to the later injection system and the car leaned out quite a bit, as a result the low end was much less torquey than with the early injection system.
I'm not sure what you mean when you say the car runs rich so is safe on the 99 octane?
My car also ran rich, that's normal at WOT, but at low speed in my opinion its a design feature of the early injection system. Running rich allows the engine to cope better with the laggy air flow meter, which is only used at low throttle. As the air flow meter takes a while to respond to change in air flow running rich allows the injection system to catch up. I swapped to the later injection system and the car leaned out quite a bit, as a result the low end was much less torquey than with the early injection system.
I'm not sure what you mean when you say the car runs rich so is safe on the 99 octane?
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