Turns out 'recommended' Premium gas doesn't do much

[upstatedoc]

since when did the "regular vs premium octane" debate include coffee?

 

[KIA-StingerGT]

Since when did "regular vs. premium octane" ever become a debate? lol

Owners manual says premium is recommended, not required.

Debate Ended.

 

[pah]

Right, it's not about fuel economy nor fuel efficiency. It's about how the engine was designed and the ECU tuned.

OEMs know some people will put anything that vaguely smells like gas in the tank so they try to protect the engines. Some do it better than others. Too low of octane for a particular engine (note that "too low" is a dynamic thing - depends on a huge variety of factors including the combustion chamber pressure (which is much higher with FI engines) and temp at each moment, spark plug temp, exhaust valve temp, etc etc) will result in the ECU retarding timing. Some handle this more gracefully than others. E.g., my '02 Lincoln simply cannot pull enough timing to run reliably on <91 octane, so it'll have detonation which will destroy rod bearings.

Best case, the ECU pulls enough timing to prevent detonation. It'll constantly be in "Oh Shit" mode and you'll be leaving power on the table. Worst case, the ECU can't keep up with the changing dynamics and you end up with rough running, detonation, and unhappy endings.

The good news is the Stinger ECU is super agressive about adjusting timing. They actually use timing adjustment to modify power output - timing advance drops to about 0degrees during shifts to be nice to the trans. So it's likely to be just fine running on the cheapest of gas, but the ECU will likely mess up from time to time and try to get back to its baseline tune, realize the engine is rattling apart, and back off again. Especially when flooring it.

Note that detonation/ping is different than it was in the '70s. We relied on the "metal can of rattling marbles" sound to tune ignition curves, but by the time you hear the marbles the problem is already extremely bad. Modern systems use a knock sensor and apply rather advanced signal processing to it (it's literally a wound wire that vibrates) to determine when detonation occurs. Note that different ECUs have different signal processors and algorithms. Ford ECUs from the late 90's were awful at this, so most tuners turn off the factory knock sensors.

Anything that vaguely smells like gas? That'd be 87 RON wouldn't it? 91 RON is the entry level petrol in Australia. Our mid spec premium is 95 RON and the top shelf premium is 98 RON. I'd be happy to see 91 RON scrapped and the 95 RON made the entry level. That should stop the Oil Co's gouging motorists by claiming that 95 RON is a premium fuel. As the world moves towards very high power output engines with turbos, and small capacity, I suspect that few if any will run on 91 RON.

Engines have knock sensors to detect engine ping (which is destructive). Running low RON fuel in a turbo engine is likely to trigger the knock sensors that retard spark advance and turbo boost to protect the engine against pinging. Reduced spark means a loss of fuel economy and reduced turbo boost means less engine power. Kia says I can run 91 RON in my V6TT but I use 95 RON. The addition of ethanol increases the RON No. I've inadvertently filled the tank a few times with E10 fuel (10% ethanol) a few times (because the bowser is labelled 95 RON). That causes the fuel economy to drop off.

 

[KIA-StingerGT]

"... Reduced spark means a loss of fuel economy and reduced turbo boost means less engine power. "

The AAA's test result of an average power loss of 1.4 % with Reg fuel means a 5 hp drop with our Stinger GT.

That's more than acceptable to me considering the cost savings.

 

[pah]

Top Shelf premium fuel becomes a MUST with most after market tunes. The fact that Kia sets up Stingers to run on 91 RON leaves plenty of room for the Tune Houses to alter factory settings to boost power.

The big issues with an aftermarket tune are (of course) the possible loss of warranty and the fact that more than a few tuners don't mind knife edge tunes that ping if the fuel is a bit below normal. The factory safety margin is a nice insurance policy against engine damage.