91 or 93 Octane
06-19-2019, 05:03 PM
#32
Rennlist Member
The DME (ECU) on any newer Porsche continuously adjusts the timing for the octane level and air density. The car will safely operate without any issues on 91 octane even at sea level. It may produce a little more power with 93, but 91 will certainly not cause it any harm. If you're tracking the car and are after every 10th of a second then you may want to put in 93 octane, but for normal operation (i.e. daily driving) 91 is all it needs.
As for the choice between using 93 with Ethanol and using 91 without Ethanol and adding aftermarket octane boosters, always go for the 93 with Ethanol. As I said earlier, nothing good can ever come from any commercially sold octane booster. Those things are based on either MMT or aromatics like Toulene or Xylene. They only have a minimal impact on raising the actual octane level (10 points increase is 0.1 octane -- so if a bottle says that it will raise the octane by 30 points, you are raising the fuel octane rating by a whopping 0.3), but MMT combustion creates solid deposits on spark plugs, valves and piston crowns and can score cylinder walls, while aromatics are basically corrosive paint strippers that will eat through aluminum, magnesium, rubber and plastics. The damage they cause far outweighs any potential benefit. So - based on the fuel that is available here, first choice would be Ethanol-free 91 (or higher if you can get it and you want the extra 3 hp), and then 93/94 with Ethanol.
Gas stations that cater to the boating crowds often have Ethanol-free gas, so if you have a cottage on / near a lake you might want to check your local marine gas station. The gas station operators may not know if they have Ethanol-free gas or not, but if you ask them how often they check for water in their storage tank and the answer is "twice a day", their gas contains Ethanol. If the answer is "once a week", the gas is Ethanol-free.
As for the choice between using 93 with Ethanol and using 91 without Ethanol and adding aftermarket octane boosters, always go for the 93 with Ethanol. As I said earlier, nothing good can ever come from any commercially sold octane booster. Those things are based on either MMT or aromatics like Toulene or Xylene. They only have a minimal impact on raising the actual octane level (10 points increase is 0.1 octane -- so if a bottle says that it will raise the octane by 30 points, you are raising the fuel octane rating by a whopping 0.3), but MMT combustion creates solid deposits on spark plugs, valves and piston crowns and can score cylinder walls, while aromatics are basically corrosive paint strippers that will eat through aluminum, magnesium, rubber and plastics. The damage they cause far outweighs any potential benefit. So - based on the fuel that is available here, first choice would be Ethanol-free 91 (or higher if you can get it and you want the extra 3 hp), and then 93/94 with Ethanol.
Gas stations that cater to the boating crowds often have Ethanol-free gas, so if you have a cottage on / near a lake you might want to check your local marine gas station. The gas station operators may not know if they have Ethanol-free gas or not, but if you ask them how often they check for water in their storage tank and the answer is "twice a day", their gas contains Ethanol. If the answer is "once a week", the gas is Ethanol-free.
JMO
06-19-2019, 07:23 PM
#33
Burning Brakes
Agree mostly to this but a bit of a blanket statement regarding octane. The more HP per litre your car is putting out, the bigger the impact on performance will be if you don't use what's recommended. As an example, my 997 Turbo says 93 octane in the manual, and if you have to, for short journey/light throttle 91 is OK. Before I put on my exhaust and did a tune, bone stock with Ultra 94 in the tank, it was pulling 2-3 degrees of timing from cylinders 3 and 6 in the mid range at WOT on a relatively cool day. Imagine what it would be doing with 3 points less octane in 30 degree heat. The rated 480 hp would plummet. On long term use of 91, the ECU would adapt the timing and the car would end up running less HP than intended, all the time. We all buy Porsche's for performance, if it's a beater daily driver who cares. For my weekend toy, I want it to be the best it can be.
JMO
JMO
There are numerous dyno charts that compare power losses / gains as a result of fuel octane differences. Here's just one example: https://themustangsource.com/forums/...octane-487398/. Not sure that I'd call a possible 3 hp drop "plummet", but you may be correct that all other things being equal, you could be leaving a few hp on the table - in particular at lower temperatures / denser air. The possible power loss is because your ECU may be able to advance the timing a bit more with higher octane fuel IF IT'S TUNED FOR IT, not because the fuel has any more energy. Higher octane fuel is only less susceptible to knock - nothing else.
P.S. And, the ECU adaptation is instantaneous, not over time.
06-19-2019, 08:43 PM
#34
Rennlist Member
The link you provided above was for a 2010 Mustang GT, which is tuned from the factory to run 87. Of course running 93 won't make more power! The ecu won't be pulling any timing from the base map at all, and 93 has less energy. Remember, this is a Porsche forum, they don't run engines that are put in an F150. 
Yes knock sensors pull timing instantaneously when the engine pings, but modern computers "learn" driving habits and sensor inputs and adjust the maps to compensate. If the knock sensors are continuously pulling timing the ignition map will adjust the timing in those area of the map so the engine isn't "living on the sensor" all the time in its adaptation. Ask any tuner how retarding timing affects HP. It's significant.
Agreed on the point that if car isn't designed for higher octane it isn't necessary. I just don't believe you can throw whatever octane in and the knock sensors will sort it out on a P car, BMW or any European car.
Yes knock sensors pull timing instantaneously when the engine pings, but modern computers "learn" driving habits and sensor inputs and adjust the maps to compensate. If the knock sensors are continuously pulling timing the ignition map will adjust the timing in those area of the map so the engine isn't "living on the sensor" all the time in its adaptation. Ask any tuner how retarding timing affects HP. It's significant.
Agreed on the point that if car isn't designed for higher octane it isn't necessary. I just don't believe you can throw whatever octane in and the knock sensors will sort it out on a P car, BMW or any European car.
