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fuel economy

 
Old 10-25-2011, 04:22 PM
  #16  
Macster
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Originally Posted by Lance 4c View Post
I average between 13-15 mpg. I tried to be mellow on cruise control on the freeway and can get 17 or so. But.... can't keep off the boost. In a related note, our local fuel stations are starting to boast non- alcohol premium. That said 76 and Conaco only post 91 octane with their non-eythanol gas, compared to the 92 octane Chevron that still has alcohol. The alcohol must have some effect on the octane rating??? It seems like it would be a good thing to have 92 without alcohol.
The octane rating is measured/determined based on the fuel's blend. IOWs, the affect the ethanol has on the gasoline's octane rating has been taken into account.

Ethanol is an octane booster, but what this means is the ethanol is used in place of probably more expensive compounds to raise the gasoline's octane rating.

(There used to be and probably still is a federal subsidy on ethanol. IIRC, it was 60 cents/gallon which gives ethanol an unfair price advantage compared to alternatives. If this subsidy was removed and ethanol had to stand on its own in the market place, it would not stand on its own.)

Generally these engines (make that all modern Porsche engines) do better with the higher octane blends of gasoline. The Porsche engines are really designed to run with 93/94 octane and yet in many areas of the country 91 octane is the highest octane grade available.

Unless the engine reacts unfavorably to the 92 octane gas with some ethanol in it you will probably do better feeding your car's engine a steady diet of this gasoline. The higher octane will allow the engine controller to keep the ignition timing advanced more than it would with a lower octane gasoline under the same operating conditions and this results in improved driveabilty and improved fuel economy. A 'bonus' is the engine runs a bit cooler as retarded timing results in a higher exhaust gas temperature which subjects all components exposed to the exhaust gas to higher than 'normal' operating temperature.

Sincerely,

Macster.
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Old 10-26-2011, 09:40 AM
  #17  
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Unfortunatley ethanol has less BTU then the same amount of gasoline, so you suffer a bit. MTBE's was the ingredient that ethanol has replaced. MTBE was the replacement for lead in pump gasoline. In order to make ones engine actually take advantage of the positive properties of ethanol, the engine has to be built and designed from boith a mechanical and electrical perspective. Fuel and advance curves are totally different than what is used in a petro burning engine, Hence the reason for less in mpg and some performance in these partial ethanol fuels. Flex fuel vehicles try and bridge the differences but only partially and really do not conform to a full ethanol only vehicle.
Ethanol does provide octane properties.
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Old 10-26-2011, 01:25 PM
  #18  
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You guys seem to get worse mpg than I'd expect - is it because your fuel is lower octane?

With mixed driving (plenty of short journeys, and some blasts) I average 19mpg. On long motorway journeys I average around 28-29mpg. For it's power, I actually consider my turbo pretty economical with fuel.
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Old 10-26-2011, 02:31 PM
  #19  
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Originally Posted by DaveK View Post
On long motorway journeys I average around 28-29mpg.
At what approximate average speed?
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Old 10-26-2011, 02:35 PM
  #20  
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Also, are you using "Imperial" gallons, which are a bit larger than USA gallons.
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Old 10-26-2011, 02:53 PM
  #21  
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Originally Posted by Kevinmacd View Post
Unfortunatley ethanol has less BTU then the same amount of gasoline, so you suffer a bit. MTBE's was the ingredient that ethanol has replaced. MTBE was the replacement for lead in pump gasoline. In order to make ones engine actually take advantage of the positive properties of ethanol, the engine has to be built and designed from boith a mechanical and electrical perspective. Fuel and advance curves are totally different than what is used in a petro burning engine, Hence the reason for less in mpg and some performance in these partial ethanol fuels. Flex fuel vehicles try and bridge the differences but only partially and really do not conform to a full ethanol only vehicle.
Ethanol does provide octane properties.
Yes, ethanol has less energy and in higher ratios can require different fueling maps and so on, but my experience with 91 octane and 93 octane gasolines both blended with up to 10% ethanol (though my info is most if not all refiners do not blend above the 7% to 8% level) is that the higher octane gasoline makes a noticable difference in the car's gas mileage.

If the choice is 93 octane gasoline sans ethanol vs. anything else then the choice is clear. Ethanol free 93 octane gasoline.

But few of us have that choice anymore.

So, if the choice is between say 91 octane even sans ethanol and 93 octane with ethanol, I believe the better choice would be the 93 octane, simply for the higher octane numbers.

Some of us do not even have the choice of 91 or 93 octane gas.

That the DME can run the more ideal ignition timing map because of the higher octane rating more than makes up for the negatives a slight bit of ethanol brings with it.

Sincerely,

Macster.
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Old 10-26-2011, 03:15 PM
  #22  
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Originally Posted by Dock View Post
At what approximate average speed?
I sit at 90mph as much as I can (allegedly) but average is probably more like 70mph.
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Old 10-26-2011, 03:46 PM
  #23  
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@65-70 I can get 28 depending on the weather and level road. But I never usually drive 70 on the hwy it's 75 or above.
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Old 10-26-2011, 06:47 PM
  #24  
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Originally Posted by DaveK View Post
I sit at 90mph as much as I can (allegedly) but average is probably more like 70mph.
At that average speed I'll average 22-24 mpg with EVOMS Stage 2 programming.
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Old 10-26-2011, 11:27 PM
  #25  
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The small tank is the biggest issue to get used to. I get about 16 in mixed city driving (occasionally goosing it, but not pushing all the time) and upper 20's on the highway. Not bad for a high performance sports car, but if gas mileage is a big concern, get a Prius.
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Old 10-28-2011, 12:39 AM
  #26  
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18mpg over about 5000 miles hwy and city combined. I don't baby the car but I do cruise on the hwy in 6th gear at about 2800 rpm unless I can see far enough ahead to check for the boys in blue.
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Old 10-29-2011, 04:55 PM
  #27  
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Originally Posted by Kevinmacd View Post
@65-70 I can get 28 depending on the weather and level road. But I never usually drive 70 on the hwy it's 75 or above.
This. Routinely get 26-28 on the highway if I dont stomp on it all the time.
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