I view it differently as lugging an engine below 2,000 RPM’s continually is not the best for an engine unless cruising on an interstate. I often run sport mode in suburban / city driving to keep the engine at or above 2,000 RPM’s as well as to lesson continual downshifting . Driving in sport mode may also lesson engine deposit build up versus lugging the engine all of the time (just my opinion).
Sport mode and fuel dilution
- Thread starter WarEagle
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This would be an interesting science project take 4 quarts of oil add 6.5 ounces of gasoline (5 per cent). Put this mixture on a heating plate on a scale and measure the weight at various time and temperature combinations.
Is fuel dilution when oil smells like fuel and your dipstick has either gotten shorter or you're at the point of overfilled? or can it just be oil smelling like fuel? the few direct injected vehicles I had and the current the oil will smell a bit like fuel but the dipstick level doesn't move up or down in the 5,000 mile oil change intervals.
Smelling like fuel is a clue. Testing for it with gas chromatograph (not Blackstone flash method) is the confirmation.
Level- you may be burning oil BUT the fuel is taking up the difference so appears to stay the same.
My daughter-in-laws Tucson on VERY short trips you could see the oil level rise by a decent amount over the full mark on dipstick and that was on less than 5k changes.
Level- you may be burning oil BUT the fuel is taking up the difference so appears to stay the same.
My daughter-in-laws Tucson on VERY short trips you could see the oil level rise by a decent amount over the full mark on dipstick and that was on less than 5k changes.
Because it’s not like mixing new pure base oil and pure new gasoline. The oil has it's additives - new and partially spent and the gasoline is a mix of partially burned hydrocarbons, tiny amount of water at start, ethanol, and has it's additives as well. Your sump can and is quite a soup really. So not so easily to just evaporate or distill off the gasoline when the purpose of the oil is to, well hold together as best it can to lubricate and cool as designed.
Think of the cogeners when distilling alcohol, as a much more simplistic concept.
exactly.
@Chief2025 the flashpoint of gasoline is irrelevant. What matters is the vapor pressure of a gasoline/oil mix at varying temperatures.
What you'll find is that gasoline is a mixture of varying hydrocarbons just like diesel fuel and motor oil are. And so you cannot "cook off" gasoline-- you actually only cook off certain portions off it. Exactly the way a distillation column works. The light fractions of fuel evaporate very quickly. The heavier fractions never evaporate.
By way of general education--
Mixtures of hydrocarbons have a "distillation curve" that shows the range evaporate and boiling points for different components of the blend. It's common for a distillation curve to be defined by "initial boiling point" or IBP, T10 (the temperature where 10% of it is evaporated, T50, T90 and the FBP or final boiling point. These five points and the interpolation between them are generally what is meant by "distillation curve" in the context of fuels and oils.
Here's some homework.
What you will find is that the T50 point of gasoline is right around typical oil operating temperatures of 90-110C. That means your standard oil temp is only hot enough to cook off about half of the fuel's blend of hydrocarbons; the heavier half will stay in the oil almost indefinitely.
Thumbs
In ours temperature seems to affect this more than anything. It’s the main reason I’ve opted to use -20 and 5,000 miles oil changes year around. Heavily diluted -20 stayed in -20 range while -16 dropped below -16 range. I prefer the added protection, and don’t want to bother swapping around, but to be fair the -16 did hold up well.
Advancement in technology has rendered some old rules (high compression motors require high octane fuel)....well....old. Lots of high compression motors safely run on regular fuel nowadays. You could say it's more or less the norm to use it to get more power and fuel efficiency from small displacement motors (without forced injection). Sophisticated electronics instantly adjust timing, direct injection prevents preignition, there's variable valve timing, engine temps are better controlled etc. Combined, all allow the use of regular fuels (which contain more energy) to be used without preignition and dreaded engine knock. On the other hand all these improvements, as we know too well, have raised problems of their own (fuel dilution, carbon buildup, VVT and cylinder deactivation mechanical issues, etc.). Mazda Skyactiv motors have had high (13 to 1 or so) compression ratios since 2011/2012 and have proven very reliable.
Some people put a lot of trust in knock sensors and computer controlled timing. I don't.
Even if manual calls for 87? I’ve always read that there is no benefit of higher octane if vehicle doesn’t call for it.
What would be the advantages of premium fuel? I’m pretty ignorant on octane (and a lot of other things)
Generally higher concentrations of the detergents in premium fuel. Shell and Exxon explicitly mention this.
Patman
Staff member
If you’re using Costco 87 you’ll end up getting a higher level of detergents than a lot of other brands use in their premium. It really doesn’t make sense to spend 50 cents a gallon more for premium on an engine that is tuned to run on 87. That higher octane is wasted, it won’t result in more power.
I understand that, but thought @MParr was referring to something other than detergents. Something in relation to the compression and/or fuel dilution.
I’m not too concerned about cleaning as we generally run top tier 87 in the Kia and Hyundai and tank full of something like Techron before each oil change
Thanks. That’s what I had always thought. No Costco near me (closest one is ~40 miles) so I use other top tier
ETA: used to be shell as they were everywhere as Circle Ks but they all are Circle K branded gas now. Mostly Marathon/Arco now as they’re much more common around here. Have a pretty decent rewards program as well
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There is no need for it unless it's specifically called for. Direct injection doesn't care about compression ratios since the fuel is injected at the moment just before ignition. Tech's been improved on and proven over the years. Mazda's have the highest compression ratios for gas engines and they purr like kittens on 87.
Static compression is only loosely tied to cylinder pressure.
Thinking of octane only in term of static compression ignores the critical roles of valve timing and intake temperature.
It’s obsolete thinking in an age where every engine has variable valve timing.
I don't think it's a secret, or it shouldn't be, when lower grade gas is run and the knock sensor tells the computer to adjust engine parameters, the mixture is richened. It's not just one thing such as dialing back the spark timing. Running the engine at a hot temperature would burn off some fuel, however if it keeps throwing a rich mixture back in there, at what point do you get ahead?
SammyChevelleTypeS3
$50 site donor 2023
Lots of folks do not realize even the carbureted engines always had some amounts of fuel in the sump. Of course not nearly the rates that SOME of the GDI engines are seeing.
What is interesting is that some automakers with GDI engines have manufactured and programed them where the owners do not see any or near the amounts of what some of them experience. Our Honda 1.5 GDI-T has the fuel smell all the time so I do short oci.
Yet we have the new 2025 Hyundai GDI 2.4 engine seeing none or at least undetectable without samples. So far I have only put around 4000 miles on it and I check it (OCD!) at least once a week and have yet to catch a hint of any fuel smells. Running only SHELL gasoline as I have done in all vehicles for the past 25-30 years.
What is interesting is that some automakers with GDI engines have manufactured and programed them where the owners do not see any or near the amounts of what some of them experience. Our Honda 1.5 GDI-T has the fuel smell all the time so I do short oci.
Yet we have the new 2025 Hyundai GDI 2.4 engine seeing none or at least undetectable without samples. So far I have only put around 4000 miles on it and I check it (OCD!) at least once a week and have yet to catch a hint of any fuel smells. Running only SHELL gasoline as I have done in all vehicles for the past 25-30 years.
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