Gas Grade and Mileage

RoshDawg

Something interesting, Porsche (at least on the 991) runs temps of 210-220 for oil when in normal mode to reduce emissions and in turn (I think) increase gas mileage. When selecting sport, oil temp goes down to 180.

Higher oil temp apparently has something to do with emissions.

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SouthArk370Z

From what I have read about some of the MPG contests, higher engine temps = better MPG. Something to do with keeping the combustion gases as hot as possible while in the cylinder for maximum efficiency. IIRC, ∆T and heat loss rate were thrown in there.

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RoshDawg

Nice that makes sense. Judging from your other posts I take it you're an engineer? If so, what kind?

Oh and op, use 91 at least.

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| EVO X MR | '11 WRX STI | 370Z SPRT | 911 (991) C2S |

SouthArk370Z

Not an engineer but spent many years as an Instrument Tech at a hazmat incinerator, so I know a little bit about combustion and heat. I also paid attention in Science classes when attending high school.
I also spent some time in mechanical maintenance, so have been around pumps, valves, flow, &c.
I was always asking the engineers "Why'd ya do it that way?" and most of them LOVE to splain stuff to others. heehee

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RoshDawg

Nice! Haha spot on!

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EndyKwon

So running an oil cooler would make the engine run less efficiently as far as MPG goes?

KERMIT

Heat energy flows from hot to cold and based on the difference in temperature from one side to the other (delta T) and the material properties, it will determine how fast energy is transmitted through the material.

That being said, the heat energy is generated by fuel energy. You want to have more of that energy end up turning your crankshaft as opposed to going into the atmosphere as heat. Running hotter reduces the delta T in the combustion chamber allowing more of the heat to stay in as opposed to going into the coolant. (this is super basic overview).

as a rule of thumb your fuel energy is 30% crankshaft power, 30% cooling losses, 30% exhaust, and 10% friction.

What they are attempting to do with running hotter is take some of the cooling % and put it back into crankshaft power. We are talking very small amounts... but every bit helps to meet the future regulations.

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SouthArk370Z

Not really. The oil has very little to do with cylinder/gas temps and, as KERMIT points out, the effects of engine temperature on efficiency can be subtle.

It's not really something the average weekend mechanic wants to mess with as there are a lot of other variables involved. Changing the engine temp (eg, by installing higher/lower temp thermostat) can affect how the ECM reacts. Plus there are limits to how far you can go before pre-ignition becomes a problem - especially with a high-compression engine.

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