Higher octane gas in a car built for regular

Factual Questions
21

I agree. Octane rating alone says absolutely nothing about burn rate.

I’m not saying it does. What I am saying is that I have read that higher octane fuels, for whatever reason (whatever they added to make it higher in octane or whatever), also tend to be slower burning.

The octane rating itself says absolutely nothing about burn rate though. It’s entirely possible to have a higher octane fuel that burns either faster or slower, or even at the same rate, as a lower octane fuel.

ETA: This is in response to jz78817’s post. The post after that wasn’t there when I started typing.

22

I believe higher octane fuel would allow you to run your car on a slightly more advanced timing setting and improve performance but violate smog specifications.

23

given the emissions implications I wouldn’t expect road gasoline to vary that much between octane levels.

24

An engine designed for low octane gas will have 2 things going on
Lower compression ratio
A retarded spark advance curve

High Octane gasoline burns SLOWER, low octane burns really fast.
When gas burns slower the spark needs to happen sooner so combustion reaches peak pressure at the correct time, or else you waste gas, lose power, and maybe even raise emissions due to improper burning.

The engine also will not have the compression ratio to take advantage of the higher octanes different burn properties, so even if you can correct the timing curve, you will at best make the same power, and still may have worse emissions due to improper burn.
Which in turn may give you a dirtier engine.

High octane fuel has to be compressed higher and ignited earlier to work as it should.

Run an engine with the fuel it was designed for, for best results.

25

More importantly, i want to know where this man lives, the price difference where i live would be more than a dollar to fill my motorcycle even.

Price this morning was like 2.02 regular 2.65 plus 3.05 Premium 4.25 Diesel

26

This is not true. The laminar flame speed of gasoline does not vary significantly with octane.

Also not true. An engine optimized for high-octane fuel will have higher compression a greater spark advance in order to take advantage of the auto-ignition resistance conferred by high-octane fuel, but an engine designed to tolerate low-octane fuel will not perform appreciably differently when operated with high-octane fuel (unless the high octane is conferred by the addition of ethanol, which significantly alters the fuel’s energy density).

Ignition delay (the ability of a fuel to resist ignition after being suddenly subjected to high pressures and temperatures) is not the same thing as flame speed (the rate at which the flame front propagates through the mixture after ignition has taken place). Octane-related ignition delay is not an issue when it comes to igniting the mixture with something as hot as an electrical arc.

27

Southern California. Regular is $2.73/gal. Plus is $2.83. Premium $2.93

That’s Arco top tier gas from a branded station. The fill-up took 10.4 gallons.

eta: The car is a 2016 Nissan Rogue, so not old and not high-performance. I figured the plus gas wouldn’t make any difference, but it’s nice to have that confirmed.

28

I am poor and drive old cars & trucks with few brain boxes and only carburetors.
I have a 334 mile loop that I drive regularly and always refuel at the same places with the same grade of fuel.

Why you may ask?

Because using fuel with ethanol preforms less than straight gas without ethanol, I have some long steep grades and the amount of throttle to maintain speed, coolant temps, etc are all better with straight gas, Milage is enough different to balance the cost difference, Same bucks per trip because of the more ethanol fuel needed to make the trip at asll different weights I have made.

Small engines are ethanol free at all times (like all my stuff, they are old) and my motorcycles (3) are always ethanol free if at all possible. I will use additives if I have to use fuel with ethanol.

As to octane ratings, compression and air temp on air cooled engines determine which I will spend more for when dealing with different needs on engines with much RPM variations input from me.

If you have modern engines and never use them but gently, nothing really changes so go as cheap as you can and with as much sludge in the fuel (ethanol) as you can get so it is the lowest price.

‘They’ have done everything possible to date to take any thinking out of caring for or making decisions about what to do how with your transportation/work vehicles engines.

How many race engines us fuel with ethanol at any octane rating?

Might be a reason even if the cost is not ignored.

X amount of fuel has X amount of energy by volume. Going to use in in your drive to work car, your 5000 HP drag racer, in your 260 HP aircraft that max RPM is less than 3000 RPM and 99% of the time is at 23-2400 RPM and which will you be OK with ethanol as 10% or more of each volume???

We have a place local that sells as high as 106 octane fuel. No one I know puts it in their new ECO friendly autos that their wife drives.

But… They put it in things that they want to preform.

YMMV ← Bawahahaha ( I could not resist. )

29

NASCAR uses E15.

30

That is not always the case, as I posted above. In the above C&D article, the director of Director of Powertrain Engineering at Bosch said some cars calibrated for regular fuel will produce more horsepower on premium.

This was also confirmed in the below test by Edmunds, with accompanying dyno graphs from many repeated runs with each octane. While not always (nor even usually) the case, some cars designed to tolerate low-octane fuel will produce more power on high-octane fuel:

https://www.edmunds.com/volkswagen/gti/2010/long-term-road-test/2010-volkswagen-gti-the-effect-of-octane-on-its-power.html

31

Indy cars run E98.

32

Indy cars switched from methanol to ethanol in 2007. Some races actually use 100% ethanol.

Of course if you’re driving an older car, the engine may not be set up for any kind of alcohol, in which case E-anything would be a bad idea.

33

Outside the US and Brazil, ethanol is not in wide use as a fuel additive (the US produces more fuel ethanol than the rest of the world combined) so there’s no reason for racing vehicles to run on it.

34

I expressed myself poorly, sorry for that. What I should have written was this:

An engine that is not designed to take advantage of the properties of high-octane fuel (by advancing spark timing when able) will not perform appreciably differently when operated with high-octane fuel. In other words, if you’ve got a low compression ratio and one spark timing map (or just one spark timing, e.g. small/handheld engines) which together permit safe operation on low-octane fuel, then high-octane fuel won’t make a difference.

The C&D article you linked to notes that the small differences they observed in the low-performance vehicles (some positive, and some negative) could be explained as normal test-to-test variability.

The Edmunds article documented a peak difference of 5% for running the GTI on 87 vs. 91 octane, but they noted a temperature difference of 9 degrees between the two test days; that temperature difference alone could account for a difference of 1.6% in ambient air density (plus some change in rolling resistance and tire slip rate on the dyno rolls due to tire pressure, tread grip and carcass pliability). No mention was made of humidity variation, which also affects air density. In short, they’ve left a lot of room for day-to-day variability in test results. If they tightened up their testing to provide more consistent conditions and still observed a difference, then it’s likely the ECM is retarding spark timing to cope with 87 octane fuel. Instrumentation to record spark timing during testing would confirm/refute this.

35

That’s not true - here in Oz ethanol is a widely used additive, most pumps have a “E10” (10% ethanol) option, and some have E85 available.

Probably not as high a percentage as the US and Brazil, but certainly in everyday use.

cheers,

36

Hemmings

yes, true “to work as it should”
putting 110 octane gasoline into an engine with 7.5:1 comp 14deg btdc max advance
does not work as it should.
Sure, the engine runs, and poops all your $$ right out the tail pipe for 0 returns for your effort.

Change that to 12:1 or 14:1 and say 40ish deg of advance (fudging the math on the advance) so we got full force in the right place.
Now it’s working like it should aside from probably needing to correct my generic timing value.

Unless we accidently put the 87 in it, then we got lunch

37

They did for a number of years (2006-2011), but during the switch to the current chassis/engine package, they started running E85, partly because Honda and Chevy like the street-car relevance of E85, which a lot of people can buy at the pump.

https://www.icis.com/resources/news/2013/05/06/9665346/indycar-series-to-keep-using-e85-as-its-standard-fuel-execs/

38

The author’s background is unclear, but he’s demonstrably wrong on some important points:

The addition of ethanol increases flame speed. There are numerous scholarly research publications confirming this, such as this one; see Figure 2C, which clearly shows that the addition of ethanol results in a higher flame speed.

He also claims that slowing the combustion process reduces the likelihood of engine knock, which is not true. The goal is to burn the mixture via normal deflagration (a subsonic flame front moving through the mixture) before the last, unburned part of the mixture undergoes detonation (a supersonic flame front moving through the mixture) due to being squeezed and heated by the already-burned part of the mixture and/or hot spots on the surface of the combustion chamber. Higher octane fuels are able to resist ignition by those elevated temperatures and pressures for a slightly longer time, allowing the entire mixture to burn via deflagration. It follows then that you don’t want to “slow the combustion process” to avoid knock, you want to speed it up. So you design your intake ports to assure the generation of plenty of in-cylinder turbulence at TDC, to stretch the flame front - and for large-bore engines, you maybe add a second spark plug so you’ve got two flame fronts instead of one.

Note that while the octane rating matters when resisting ignition by the kinds of temperatures experienced during compression and contact with combustion chamber surfaces, is doesn’t make a difference when considering ignition via a spark plug’s electric arc. The former are measured in hundreds of degrees, the latter in thousands of degrees.

There it is again, he thinks higher octane gas burns slower (wrong), which makes it more resistant to knock (wrong).

It may be that certain compositions of high-octane gasoline do have a lower laminar flame speed, but as shown above, ethanol is one additive that actually increases LFS. jz78817’s quote sums it up best:

39

The part about if the premium has (typically 10%) ethanol and the regular doesn’t is true, 10% ethanol lowers the energy density something like 3%, with that much negative impact on mileage all else equal. However with some brands in some places the premium (or super premium*) doesn’t have ethanol and the regular does. And in other places all grades have it. Here in NJ state law requires it.

Without having tested all kind of cars, Joema’s points and references about the possibility of regular/mid rated cars giving slightly better power or mpg on premium seem plausible. Recent cars that is. If the ECU machinations to adjust the engine can detract slightly going from premium down, it’s possible some cars are set up for normal use on a lower octane because that’s more economical, but their ECU programs and physical set up would still do a little better in power or mpg on higher octane of the same energy content. But as you mentioned, without tight scientific standards on the tests, it’s hard to tell. And the chance that it would offset the price difference in the gas is remote.

Speaking of cars where the manufacturers’ min is 87 that is. Where the recommended is say 91 and min 89 (our BMW 328i for example) 93 might actually make a difference, relatively highly turbocharged engine stock, and in our case with add on tuner which overlays a different control program to close the turbocharger’s waste gate more and give more boost in given conditions. The tuner manufacturer quotes the hp increase on 93.

*for example Shell V-power 93 in some places doesn’t have ethanol when the other Shell grades do, which makes some people believe V-power never has ethanol, but in other places it does.

40

In some cases they will make slightly more horsepower. That was one basis for the SAE horsepower scandal of the mid-2000s, where some Asian manufacturers used premium fuel during HP certification on cars which were specified for regular. This in turn improved HP a bit.

This was a loophole in the SAE J1349 horsepower specification which was later closed. This forced those manufacturers to re-certify their vehicles using regular octane fuel (among other things) which reduced their stated HP – despite no engine changes. This resulted in some 2006 models having reductions from 4 to 20 horsepower compared with 2005 models.

However the HP difference was obscured by other “cheats” during certification, including lower-than-specified oil viscosity. But those manufacturers definitely thought using premium fuel in a car certified for regular would improve their HP ratings, or they wouldn’t have done it.

However I don’t see how this could make any difference in fuel economy. Both premium and regular have about the same BTUs per gallon, so it would seem to require the same fuel flow rate to produce a given amount of work. E10 gasoline (ie 10% ethanol) contains slightly less energy per gallon so might produce a little less mpg, but that’s a separate issue.