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Octane ratings for our cars

 
Old 10-05-2017, 09:48 AM
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Andre The Giant
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Default Octane ratings for our cars

I wanted to get the information from a reliable source, Porsche, so I called them up and asked what fuel octane should I be using in my MY 1985 32V V8 and they specified an Octane rating of 91. This information was given by Porsche Classics in Las Vegas.

He also mentioned that if I was able to use a fuel with 94 Octane, i would be getting about 7 HP more !
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Old 10-05-2017, 10:15 AM
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FredR
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Originally Posted by Andre The Giant View Post

He also mentioned that if I was able to use a fuel with 94 Octane, i would be getting about 7 HP more !
I'd like to know how that is remotely possible. given in your part of the world they tend to add ethanol to get the octane rating up when the stuff has about 2/3rds of the heat of combustion compared to a pure gasoline blend. Even so, you would still need a gasoline with better energy generation- maybe that is possible in your area.

Octane helps the engine run more power adders such as compression ratio and ignition advance.

I am not sure about your model but the S4's/GTS's seem to be optimally tuned for 98 RON [93 PON] and will comfortably run 94 RON [91 PON] but does so by pulling some timing on top end. Your model cannot pull timing so Porsche made sure the tune was safe and thus it is possible to add more timing but then you are ever more closer to knock initiation.
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Old 10-05-2017, 10:28 AM
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hwyengr
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The most reliable source there is, the Owner's Manual, says minimum 92. I would imagine that since you can only readily get 91 on the west coast, that's just what the Vegas-based dealer says since they couldn't require a fuel you can't easily get.

Here in the Midwest, though, it's 93 all day long!
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Old 10-05-2017, 10:44 AM
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Originally Posted by FredR View Post
I'd like to know how that is remotely possible. given in your part of the world they tend to add ethanol to get the octane rating up when the stuff has about 2/3rds of the heat of combustion compared to a pure gasoline blend. Even so, you would still need a gasoline with better energy generation- maybe that is possible in your area.

Octane helps the engine run more power adders such as compression ratio and ignition advance.

I am not sure about your model but the S4's/GTS's seem to be optimally tuned for 98 RON [93 PON] and will comfortably run 94 RON [91 PON] but does so by pulling some timing on top end. Your model cannot pull timing so Porsche made sure the tune was safe and thus it is possible to add more timing but then you are ever more closer to knock initiation.
I am assuming they are considering that either 91 or 94 octane will contain 10% ethanol. The car is optimally tuned for 92 and although Ethanol has more oxygen fueling the combustion but as you said 30% less energy per unit so if you are running 91 most likely they are saying power will be down 7 hp vs adding 7 hp if you use 94. Although 7 hp seems a bit much.

My B-in-law works for the Energy department and specializes in alternative fuels. He wants me to convert my track car to E85 which is 105 Octane but then I would have to truck around 55 gallon drums of the stuff since I would be burning more of it.
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