Mixing gasoline with different octane ratings

Life Cars & Motorcycles
1

I feel like an idiot starting this thread, because I’m sure I know the answer to my own question. But then again, I’ve been wrong before, so here goes…

I just bought a new Suzuki Jimney to replace my aging Mitsubishi Freeca. The Freeca wasn’t a bad vehicle, but it was past its prime, and an accident I had two weeks ago finally sealed its fate.

OK, so I’m ready to drive out of the dealer showroom with my new Jimney, but I notice that the gas tank is almost empty. So I’m going to fill it up, and I ask the nice sales lady what octane rated fuel I should use (ie 92, 95 or 98). I was a little taken aback when she said 98, but OK, I don’t drive that much, so I’ll bite the bullet and go for the expensive stuff.

Where it got weird was when the sales lady said that I should decide if I’m going to use either 95 or 98, and then stick with one or the other for the life of the vehicle. “Never mix 95 with 98, it’s bad for the engine,” she stated with confidence.

Well, I’m not an expert on gasoline formulas, but quite frankly that sounded like a load of crap. So I decided to ask the Suzuki mechanic. To my surprise, he backed her up.

I’m still having a hard time buying it. I think I understand pretty well what octane ratings mean. It’s a measure of the volatility of gasoline - a higher octane rating means less volatile, thus less liable to cause pre-ignition, which can damage an engine. Octane ratings are determined by blending in certain additives into the gasoline. In the past it was lead, but that causes air pollution and possible brain damage, so these days the chemical of choice (at least in Taiwan) is MTBE:

en.wikipedia.org/wiki/Methyl_tert-butyl_ether

In the USA and other western countries, it’s becoming more common to use ethanol for this purpose. But as far as I know, Taiwan only uses MTBE, no matter you purchase 92, 95 or 98.

My understanding is that a higher octane rating simply means that more MTBE is added to the mix. Thus, if I blend half a tank of 95 octane fuel with 98 octane, I should get a mixture that is about 96.5 octane. I explained this to the mechanic at Suzuki, but he completely disagreed. He claims that 95 and 98 octane fuel are “totally different” and “shouldn’t be mixed.”

OK, I know they color code the fuel - 92 is bluish, 95 is orange, and actually I don’t know what color is 98 because today was the first time I bought it and I haven’t yet poured any into a gas can to see what it looks like. But I assume that the color coding is simply a dye added for identification purposes, and is not a chemical that affects the octane rating.

So am I totally misinformed? Or is the Suzuki mechanic the one who’s got it wrong?

2

Higher octane gas is used in engines that require more compression, to avoid premature detonation. Putting a higher octane gas doesn’t usually affect the engine, at most, you’ll lose a little bit of power. On the other hand, putting a lower octane gas can trash your engine if it detonates before its time.

In fact, my bike used 95 octane gas, but the mechanic recommended filling the tank with 98 from time to time (every couple of months). It seems that the composition of the gas helps clean the engine’s interior. did it for years, and never had a problem with the engine.

3

Some of your terms/definitions are off, as is usual with any discussion of octane, but I’d agree that the mixes octane could be calculated as a simple average, assuming equal volumes.

I’ve seen it stated that this is how mid-octane grades are produced commercially at the pump, (at least in the US) since they only have tankage for low and high octane fuel.

There are allegedly mixing/density difference and non-linearities with leaded fuel, which isn’t going to be available in Taiwan. but I’d be surprised if that applies to normally available stuff.

4

FAIK, using lower then specified octane gas can damage an engine. But using higher then specified won’t damage the engine. It just may be wasting a small bit of money. There is supposedly no benefit to using higher then specified octane gas. Usually an engine will specify an octane required. Any over (in whatever percentage over) shouldn’t be a problem.

I have an Acura RSX (Honda Integra) which is tuned for regular gas. I often use mid-grade and sometimes premium. Personally I believe I get slightly better mileage when using mid-grade or premium then regular gas, and slightly better performance. But my bud who worked at an oil refinery debunks this. He feels that using higher then specified just wastes money with no benefit. Now if I had the RSX Type S, I would have to put in Premium fuel every time as that engine is tuned ONLY for Premium.

Did you get an owners manual with your car? There may be some info there.

When I got my Telstar in Taiwan, it only specified octane must be over 90. So i regularly used 95 for the first few years and then later switched to 92. And i didn’t notice anything. I did feel that 92 seemed to run smoother for that engine. But i may have imagined that. Mileage was the same and all. So later i just used mostly 92. But i mixed and matched with no issues.

As a side note. I changed my oil religiously every 3000km. And i liked to mix different brands of oil , even different grades, rather then use one grade. I would put 20/40 in it and put one bottle of 0/50 synthetic, etc , etc.

Never had any issues with that engine. And it had over 400,000km on it when i sold it and it still ran like a gem, the engine.

3000km seemed way too early to change oil. Over here in the USA, recommended oil change is 5000 mile for most engines. Which is 8000km !

So I probably wasted a lot of money I could have saved on oil changes. Should have gone 9000km between changes.

5

Congratulations on the Jimny!

I don’t see why it shouldn’t be OK to mix it, as long as you don’t go below the recommended rating. I can’t believe 98 is recommended, what kind of super duper engine is that? I think it’s probably 95 and then there’s no benefit to using 98 in terms of performance or fuel economy.

This being Taiwan, I’d be more concerned with not getting the fuel mixed with water, or some contaminants harmful to the engine (make sure you refill at reasonably reputable stations that get a lot of traffic, so that the fuel they sell never stays in the underground storage tanks for too long).

6

Would an accidental tank of 92 octane fuel (instead of 95) explain why my motorcycle is really struggling to start every time? It runs well once started but it simply does not want to start even with the kickstart. The problems started when I got a new tank of fuel a couple of weeks ago and I can’t think of anything else that would explain it other than the attendant putting in 92 octane.

7

Probably not octane. But contaminated fuel might explain it.

8

If the problem is the 92 octane, try adding 98. The mix would make the overall octanes to balance. If it’s what Ducked says, though, you’ll need to purge the whole system and refuel it again.

9

[quote=“Doraemonster”]Congratulations on the Jimny!
[/quote]

Thanks for the congrats. I only hope it lives up to its reputation for reliability. Seems to get good reviews for that. But reviewers (as opposed to actual owners) diss the Jimny for being cramped in the rear seat (admittedly true) and being “gutless” on the highway (also true, especially the automatic transmission models which, unfortunately, are the only models sold in Taiwan). By way of compensation, off-road performance is said to be stellar, but I haven’t had a chance to test that yet.

These days they put a lot of crap on the Jimny that I could do without (electric adjustable mirrors, various buzzers and beepers). But I’m glad that I bought the 2014 model (last one left in the showroom) as opposed to the 2015 model. The 2015 ones have replaced the fuel and temperature dial gauges with digital electronic ones which - when they fail - present you with a blank screen.

I also can’t see any reason why mixing 95 with 98 should pose any problem. According to the Suzuki dealer, it would be like mixing 10W40 motor oil with 30W oil. They seem to have a strange idea of what “octane” is.

The engine compression ratio of the Jimny is 9.5:1, which is fairly normal. Nothing “super duper” about it.

[quote]
This being Taiwan, I’d be more concerned with not getting the fuel mixed with water, or some contaminants harmful to the engine (make sure you refill at reasonably reputable stations that get a lot of traffic, so that the fuel they sell never stays in the underground storage tanks for too long).[/quote]

I haven’t yet run into that problem here, but agree that old gasoline shouldn’t be put into an engine. As it ages, gasoline gets “gummy” and can foul fuel injectors. So it’s not a good idea to store gasoline in containers for anything other than a lawn mower or chain saw. In the USA there is something called “Sta-bil” which can be added to stored gasoline to keep it “fresh,” but I haven’t seen it in Taiwan.

cheers,
DB

10

[quote=“Dog’s_Breakfast”]
I’m still having a hard time buying it. I think I understand pretty well what octane ratings mean. It’s a measure of the volatility of gasoline - a higher octane rating means less volatile, thus less liable to cause pre-ignition, which can damage an engine. Octane ratings are determined by blending in certain additives into the gasoline. In the past it was lead, but that causes air pollution and possible brain damage, so these days the chemical of choice (at least in Taiwan) is MTBE:

en.wikipedia.org/wiki/Methyl_tert-butyl_ether

In the USA and other western countries, it’s becoming more common to use ethanol for this purpose. But as far as I know, Taiwan only uses MTBE, no matter you purchase 92, 95 or 98.

My understanding is that a higher octane rating simply means that more MTBE is added to the mix. Thus, if I blend half a tank of 95 octane fuel with 98 octane, I should get a mixture that is about 96.5 octane. I explained this to the mechanic at Suzuki, but he completely disagreed. He claims that 95 and 98 octane fuel are “totally different” and “shouldn’t be mixed.”

OK, I know they color code the fuel - 92 is bluish, 95 is orange, and actually I don’t know what color is 98 because today was the first time I bought it and I haven’t yet poured any into a gas can to see what it looks like. But I assume that the color coding is simply a dye added for identification purposes, and is not a chemical that affects the octane rating.

So am I totally misinformed? Or is the Suzuki mechanic the one who’s got it wrong?[/quote]

I am afraid your dealer and your mechanic are correct (technically). Octane does not provide specific volatility values - it is a measure of “under which condition does the fuel mix explode” so to speak. Pure isooctane is set to octane=100 on a calibrated 1piston engine - or the experimental engine is normed by it :slight_smile:
Now gasoline 92, 95, 98 do not only differ by the amount of additive inside (that’s also not why they are priced differently) but crucially carbohydrates (and what not) inside are different. Those components are separated by e.g. distillation; a higher octane fuel will be more narrow (nowadays) in the range of carbohydrates (and whatnot) inside and are more expensive to get to (generally speaking),
a little similar to various crude oils selling at different price-points. This is also the reason why a liter 95 and a liter 98 don’t give 96.5.; gasoline explosion “points” are not a simple function of volatility (jeez nobody really understood why the old lead additive (Tetraethyllead) worked so well, until recently). In fact gasoline bought in winter will (or used to) have a higher volatility (but same octane) than gasoline bought in summer; e.g. in winter it makes the engine start more smooth (assuming a cold engine that is) vice versa in summer…

Now the sales lady will tell you to pour inside 98, because if you would use 95 you could not possibly get the car’s specification values (fuel consumption, acceleration calibrated on 98 fuel). She is a salesperson after all and wont recommend to not use the car at it’s best.
The reason you can choose 95 or 98 (mayhaps even 92) is because cars build later than in the 90s will have a knock sensor, which will adjust the spark timing in accordance with your fuel mix. Point is that little gadget used to be designed to operate around a certain margin predefined by calibrating it on 95 and 98 (mayhaps even 92). So it ll run into trouble when instead smth. close to 95 or 98 the fuel mix would be around 96.5 :frowning: plus a non standardized ratio might lead to a non uniform explosion or incomplete combustion.

Kinda all that issues have been resolved in modern engines over the past decade+ or so (mostly by engine control units {ECUs}). It is kinda impossible to permanently damaged a petrol engine unless one would fuel up diesel. And it seems ur mechanic is a bit oldschool in this regard. So no worries! :2cents:

11

Most of the ‘super’ fuels are marketing gimmics anyway …

12

[quote=“Dog’s_Breakfast”][quote=“Doraemonster”]I also can’t see any reason why mixing 95 with 98 should pose any problem. According to the Suzuki dealer, it would be like mixing 10W40 motor oil with 30W oil. They seem to have a strange idea of what “octane” is.

[/quote][/quote]

Unlike mixing octanes , viscosity of mixtures is non-linear, skewed heavily toward the lower viscosity component. There’s probably a formula, but its probably complex and I dunno what it is.

(Or maybe there isn’t

profmaster.blogspot.tw/2007/12/h … iquid.html)

Anyway, mixing 10W40 and 30W would raise the low temp viscosity a bit, and lower the high temp viscosity a bit more. Not great but not much of a problem in Taiwan.

It’d also render the mixture more shear stable, which is a good thing.

13

M.Ando is correct (Octane does not provide specific volatility values - it is a measure of “under which condition does the fuel mix explode”).
Most mechanics have outdated/incorrect theories.

I highly doubt mixing 95 and 98 is equivalent to 96.5 (see Ducked post above)

Put the lowest octane required by your manufacturer. On 99.5% of the engines, you won’t see any difference.

No problem to change octane at any time (but i would suggest to empty your tank before changing)
No big problem mixing fuel type.
Putting a higher octane fuel in your engine will not clean it up. For that, just keep your engine at high revs on the highway for several minutes.

14

[quote=“CemXen”]M.Ando is correct (Octane does not provide specific volatility values - it is a measure of “under which condition does the fuel mix explode”).
Most mechanics have outdated/incorrect theories.

I highly doubt mixing 95 and 98 is equivalent to 96.5 (see Ducked post above)

Put the lowest octane required by your manufacturer. On 99.5% of the engines, you won’t see any difference.

No problem to change octane at any time (but I would suggest to empty your tank before changing)
No big problem mixing fuel type.
Putting a higher octane fuel in your engine will not clean it up. For that, just keep your engine at high revs on the highway for several minutes.[/quote]

Can’t work out whether you’re agreeing or disagreeing with me. Anyway, to clarify, I’d normally expect it to be a simple average, so I’d expect equal volumes of 95 and 98 to produce 96.5, so I’m disagreeing with you.

There MAY be some exceptions, as when mixing leaded and unleaded petrol, but I’d be surprised if they happen when mixing two octane levels of fuel from the same supplier, which is what is being discussed here.

M.Ando is partly, even mostly, correct, but his terms are off in places (as is usual ); way off in at least one place.

“(Octane does not provide specific volatility values [TRUE] - it is a measure of “under which condition does the fuel mix explode” [UNTRUE]).”

Octane is a measure of a fuels susceptibility to detonation (relative to a reference fuel) , where detonation is a defined, abnormal and detectable behaviour. IC fuel normally “explodes” in an engine.

“Now gasoline 92, 95, 98 do not only differ by the amount of additive inside (that’s also not why they are priced differently) but crucially carbohydrates (and what not) inside are different.”

I don’t believe there is any carbohydrate in petrol. If there is, I’d guess someone has been putting sugar in your fuel tank. I suppose there might be carbohydrate in (bad) biodiesel.

I was recently quite snotty with someone who said they got better performance from upping the octane above manufacturers recommendation, (in a Subaru Forrester), but when I thought about it afterwards, I realised I was giving them the “old school” line, and may well have been talking crap. A modern engine may be capable of advancing its timing to exploit higher octane fuel, under control of a knock sensor, though I don’t know if this Forrester was doing so.

15

Yes, my use of quotation marks is purely accidental… Also “spelling out” things is a great idea, people usually really enjoy reading :slight_smile:

Ah, nevermind… However indeed I dunno how exactly to define a detonation vs. an explosion.

Now that makes me feel a bit lonely and slightly amused. :ponder:

Edit:
Really sorry about the post mess above, thought i was in preview. Plus cant find a “delete post” button

16

Gas is heading down in price these days. Our gas stations (unlike in Taiwan) are all individually priced. 3 gas stations across the street from each other on the same block will have different prices. One mile away another gas station will have a different price.

Generally Chevron, 76 , and Shell will be priced about ten cents per gallon or up to 15 cents a gallon more then “no name” gas. I understand from my bud who used to work at refineries that the base gas is what the no name gas companies sell. Chevron and the like add specific additives to their base gas (which they buy from refineries) basically to help keep engines cleaner. The difference is slight.

IT is hard to specifically deduce that the additives that name brand gasoline adds increases mileage. Because the specific usage from one tank of gas , even on the same car with basically the same routing still varies because of traffic. I usually get between 29 to 30 mpg. IF i had been on longer freeway runs this can go up to 32.8 like I got from my last tank (of no name gas). The previous tank from the same gas station I got only 29.3 thereabouts. This time I made a run to san jose and back (some 120 miles) out of the 350 miles i can get on one tank thereabouts.

I think a lot of what we feel may just be psychological. Like i always feel that I get slightly better mileage when i put in Chevron, versus Shell (and this may actually be true because Shell uses some alcohol in its gas whereas Chevron does not (at least in California, far as I know).

And i always “felt” that Chevron, 76 and Shell yield better mileage per gallon, say up to 1mpg then no name gas. But that has not actually been proven as fact. Because I have been getting stellar MPG from no name gas as well.

Currently i “feel” that mid-grade gets me slightly better pickup and possibly better mileage. But not sure if that is actually true, or is true to so slight a degree that is hard to measure?

Last night I put in midgrade (and there is such a thing here in Calif) and i do feel that my car is slightly more perky then when i pulled into the gas station with regular.

Gas stations here have Premium and REgular and Mid-grade is when the pump draws (supposedly equally) from BOTH tanks.

Premium is usually 20cents more then Regular and midgrade is usually ten cents per gallon more then Regular. Although this is not 100pct the case at each station.

Long and short of it, scientifically it is probably of no benefit to use higher then required octane. But fuel additives the name brand gas companies add to their base gasoline are supposed to keep your valves cleaner and therefore a cleaner engine should yield better mileage.

I still have my gut feeling that using midgrade on my car gets more pick up and go. And possibly better mileage. Even though the facts may not match the perception.

The increase in performance pick up wise and mileage wise may be so slight that our normal driving habits dont really pick it up.

Like my bathroom scale is not accurate enough to detect a difference of a tshirt. LIke taking my tshirt off and weighing myself again didn’t make the readout different to a tenth of a kg. But i am sure that technically there is a difference. Just that my equipment couldn’t measure it.

Same with driving. There could be so slight a difference that is hard to measure in view of all the other irregularities in driving between one tank of gas to the other.

One would have to put the car on a steady speed on a roller and then get a machine sophisticated enough to read the difference between the car using midgrade and using regular over say 100 miles.

And performance wise will probably likewise require the use of some kind of laboratory environment to deduce as well.