CAHIBOstep, TTAC’s biggest fan on our Facebook page, writes:
My owner’s manual recommends 91 octane gasoline for my turbocharged five-pot (Volvo T5). There’s no 91 octane gas in Illinois, only 87, 89 and 93. When I was at the gas station the other day, I decided that instead of buying 93, I would mix equal parts 89 and 93 to get 91 (89 + 93 = 182/2 = 91).
Does this actually work? Does this make me the cheapest person in the world?
Sajeev’s answer:
You can make 91 octane with high and low octane gas, since mid-grade gasoline is mixed inside the pump like that. The quality/calibration of the pump is always a concern. Since Chi-town has 93 octane, the Pistonhead answer is to get your ECU re-flashed to take advantage of the extra two octane points. Which is a nice upgrade for many cars, but huge step up for a turbocharged Swedish meatball that’ll crank up the boost accordingly.
I’m sure a Volvo tuner like “IPD” can hook you up, and I wouldn’t be surprised if there’s a more generic Euro tuner worthy of your hard earned cash too. Automakers dumb down their spark timing to compensate for regions that use 91 octane. The number of regional blends and octane ratings in this country are staggering.
Bonus! A Piston Slap nugget of wisdom from TTAC’s resident Volvo-guru, Alex Dykes:
All of Volvo’s turbo engines require a minimum of 87 octane gasoline but recommend 91 octane to achieve rated power figures. As a rule of thumb, the more modern the engine, the better the systems are able to adjust to varying fuel grades. The thing to remember when feeding your turbo engine a lower octane fuel is that because of the turbo the effective compression is fairly high and pre-ignition is a concern, the engines will retard timing to reduce this which in turn reduces power and fuel economy slightly. If you feel really cheap, the difference in fuel economy may not make up for the cheaper cost per gallon.
On octane, its important to remember that each state sets the definition of regular/midgrade/premium in their market. Some states follow the SAE guidelines of 87/89/91 and some do not. States like California have very strict standards on the actual vs advertised octane numbers and other states do not, so a 91 octane in CA is going to be very close to 91 but 93 in some other state could easily test at a lower octane number. Also, for our friends in the EU, the USA uses Pump Octane Number PON aka Anti-Knock Index or AKI while other countries use RON or MON, these numbers cannot be directly compared.
[Send your technical queries to sajeev.mehta@thetruthaboutcars.com]
“USA uses Pump Octane Number PON aka Anti-Knock Index or AKI while other countries use RON or MON, these numbers cannot be directly compared.”
Very true. The US PON is the average of the Research and Motor octane number tests (RON & MON).
Octane requirement is also highly dependent upon driving style.
I have owned several cars that either ‘required’ or ‘recommended’ premium fuel (91 AKI).
I frequently ran them on regular (87 AKI) and noticed no difference.
However, I live in very temperate coastal California and drive like a grandmother.
If you live in warmer climates and/or drive more aggressively, your need for higher-octane fuel is greater.
Buy the 93 and deal with the little bit of extra cost. If you’re mixing regular and premium to get the 91, what are you saving, maybe 2-5 bucks a tank? 87 may not affect performance, 93 will not. I’d just go with the known fact and play it safe. Also, make sure you’re using quality gasoline. I don’t know what’s available in your area, but the “big guys” like Exxon/Mobil, Shell, or Texaco/Chevron are the ones to stick by. Buying some no-name gas or the unknown crap that Wal-Mart’s got in their tanks isn’t always advised.
As a former resident of Colorado, I’ll also note that the octane ratings available at high altitude are typically lower than the SAE recommendation (often 85/87/89). In Denver, Salt Lake, and Albuquerque the atmospheric pressure is lower, so the engine control system leans out the mixture (reduces fuel). This reduces combustion pressure and requires less knock prevention.
Once again, turbocharged engines could theoretically force more air into the engine at altitude, and overcome this shortcoming. But since turbo motors are such a small portion of the vehicle fleet, higher octanes are rarely available at the pump.
I haven’t bothered lately, but I found that I got reduced MPG in my BMW when I used low octane gas… I didn’t really notice that much difference in power, but then I’m not that agressive with the accelerator.
The computer adjusts the timing automatically, of course, so it ran fine….
I went back to premium, as the reduced cost didn’t really offset the reduced mileage (as I recall).
YMMV
Most gas stations have tanks of regular and premium. The midgrade fuel that comes out of the pump is just a blend of those two fuels.
You can use more octane than you need, obviously (although I’ve been told that you should not use excessive octane in a rotary; it supposedly causes problems, but that frankly makes no sense to me.)
If you would like to combine fuels on your own in order to get something between midgrade and premium, then you could certainly do it, assuming that you don’t mind wasting that kind of time just to save a few pennies. I wouldn’t.
The computer adjusts the timing automatically, of course, so it ran fine….
I always understood that the timing is retarded on instruction from the Knock Sensor. So, for an adjustment to occur you gotta be knocking; never good for an engine.
Any thoughts, Piston Slappers?
Johnny Canada : So, for an adjustment to occur you gotta be knocking; never good for an engine. Any thoughts, Piston Slappers?
Not true. Since the knock sensor is screwed into the block, it can “hear” the knock before you will. WELL before. Its like a frickin bloodhound, even hearing valvetrain noise on modified LSX V8s and cutting timing for no good reason.
Canada makes a good point. The ECU cannot detect octane in gasoline, it can only detect pre-detonation, then retard the ignition to just before that point. Not sure I would want my engine always running on the edge of pre-detonation.
Or you could find a Marathon station near you and pay mid-grade price for “premium” fuel.
When I had my Eclipse GSX the station near me (Poplar Grove, IL) had 91 octane mid-grade. It was nice to save 10 cents and get the right fuel.
Yes, you can mix 93 and 89 to get 91.
The ECU will retard and advance timing automatically. Many modern engines tuned for midgrade or premium fuel will advance timing up to the point that knock just starts, then they back off to a “safe” level. This happens very quickly and the system adjusts in such small increments that there is really no danger of damage as long as the fuel is within the ranges of gas sold in the US (approx 85 to 100 AKI).
I had the same experience as Lokki in my BMW and found that using 89 RON degraded the mileage enough to negate any savings, as near as I could calculate.
2000 BMW 528iA (91 RON reommended).
Some 7-Elevens around Dallas were offering 5 different levels of octane at the pumps. An online archive of a news article said they started this in TX and FL back in 2002.
I just noticed it last year when I tried their stations. No need to mix gas in this case IF you can find one in your area. :)
Maybe they have expanded this into other states 7 years on.
“Some 50 7-Eleven gas stations in North Texas and almost 100 in Florida…”
http://www.accessmylibrary.com/coms2/summary_0286-7431946_ITM
(Appears original article is no longer available)
There are also only two tanks of gasoline in the ground, an 87 and a 93 since that is all the refineries make. So 89 or 91 is just a mix anyway.
I vote for reprogramming the car to use the 93:
http://www.ipdusa.com/Volvo-850/Performance/ECU-Upgrades/p-72-143-2183/
Yeah, I am with Redbeard. But use BSR instead of IPD; no need for a laptop and the tune is cheaper.
Don’t waste the money. Keep it at 87 octane and it should be absolutely fine.
The ECU cannot detect octane in gasoline, it can only detect pre-detonation, then retard the ignition to just before that point. Not sure I would want my engine always running on the edge of pre-detonation.
I thought that in most cases the best timing is right at that point, just before detonation occurs, and this is why the shade tree method of timing an engine is to do just that, advance until it pings, and then back off just enough to eliminate the ping in all conditions.
Mixing fuels of different octane does not give you an arithmetic mean — that is, a 50/50 mix of 93-octane and 87-octane fuels does not yield 90 octane, more like 88-89.
Engines always knock at least slightly. Normal knock is very mild, and isn’t audible externally. A knock sensor looks for preignition that is particularly loud or frequent, and signals the engine computer to retard the timing until the knock drops below a safe threshold.
An engine’s octane requirement varies a lot. It varies with altitude (octane requirements are lower at high altitudes), engine condition (older engines started to require higher octane numbers, due primarily to carbon buildup in the combustion chambers, not as much of a problem with modern engines), engine load, and temperature. If you’re climbing a hill or lugging in a low gear, the engine’s propensity to knock increases; if you’re cruising at a modest speed, it’s at a minimum. This is why the “shade-tree” method MadHungarian mentions above is not a great idea…the engine may not ping while it’s running in neutral without a load, but knock like mad going up a hill or a WOT.
Turbocharged or high-compression engines may not be damaged by 87 octane fuel, but it means you’ll be running with retarded timing all the time, which reduces both power and fuel economy. The reason such engines have knock sensors is not to let you cheap out by running regular fuel, but to prevent engine damage if you accidentally run a tank of regular, or if it’s the only thing available in a remote area, and you have no choice.
I would use premium. There are circumstances where using 89 probably wouldn’t make a significant difference (e.g., on a long highway trip at more-or-less legal speeds), but for the most part, if it’s recommended for your engine, not using it is a false economy. (Conversely, in the majority of cases, if the manufacturer recommends 87 or 89 pump octane, there’s nothing to be gained with 91 or 93.)
An advantage of higher octane fuels is they generally have more detergents and other additives.
Only Tier 1 fuels spread the additives across all the blends.
In some cases, turbo engines run with low octane fuel will overheat in one or several cylinders. This is due to series cooling systems forcing last cylinder on the loop to run hotter than rest – the manufacturer compensates by putting knock sensor on that hottest running cylinder.
Some Japanese brands, accustomed to running on 98 octane in Japan, run the ragged edge of pre-ignition even when they burn recommended USA fuel. Their reasoning is to keep mixture lean as possible so emission standards are met.
For those cars, the ECU may detect use of higher octane and actually advance engine timing to take advantage of it.
Many, many thanks to Sajeev, Mr. Dykes, and all who commented.
I’ll share a few observations I have made in my various octane experiments.
I ran 87 octane gas in the car for about a year. I started to do it when gas prices shot up. The knock sensor compensated for the reduced octane level, of course. The engine clearly lost some power, but I felt like I had to bite the bullet.
A few months ago I bought a full tank of 93 octane when the station I stopped at was out of 87 and 89. On my way back into the city that evening on an EMPTY Edens Expressway, I found myself in a good-natured game of cat and mouse with some late model variety of MB C-Class.
The first time I really mashed on the accelerator, the car suddenly went like hell! I felt like I was driving KITT in Super Pursuit Mode. It was awesome.
I have found that mixing my own 91 just doesn’t produce the same results on a consistent basis. I had no idea that mid-grade gas is mixed inside the pump as it comes out of the nozzle. I’m guessing that is not a very reliable method, at least among the many thousands of different gas pumps in Chicago. At least that’s my story and I’m sticking to it.
I’m going to use 93 now. I’ll hold off on the ECU re-flash until I can save up some coin (it’s about $1,500 from ipd). It’s true that the extra power I get using 93 isn’t always necessary while plodding around the city, but I figure there’s no use having a turbocharged engine if you can’t take advantage of it when the time is right. Otherwise I should have just gotten the normally aspirated 2.4L.
I’ll probably use 89 on long road trips.
I wouldn’t reflash the ECU, particularly if the car is under warranty.
In much of the US, 91 is the standard octane level for premium; it’s not just in states such as Colorado, where elevation makes a difference, but elsewhere where that formulation is typical. If you plan on driving out of state, you don’t want to have a car that requires 93 octane fuel, as you might not always be able to find it.
If the car runs fine on premium fuel, then leave it alone. If it doesn’t, then there’s something wrong with the car that octane can’t fix.
@Pch101
“If you plan on driving out of state, you don’t want to have a car that requires 93 octane fuel, as you might not always be able to find it.”
I agree with your sentiments about the warranty. That is another reason why the $1,500 isn’t burning a hole in my pocket yet.
However, wouldn’t the knock sensor still compensate for the reduced octane if I had to use 91? I would assume that if you pay decent money for the re-flash, someone would have thought of that. But I also know what they say about making assumptions.
However, wouldn’t the knock sensor still compensate for the reduced octane if I had to use 91?
It should, sure. But if the performance is optimized to run on 93, then you’re going to lose performance when you use a lower octane fuel, which is the case that you have now.
In any case, I would be highly skeptical of chipping anything that doesn’t have a turbo. Most chips are a scam; when they do work, it’s primarily because you’re playing with the boost characteristics of the turbo. If you have a non-turbo Volvo, I wouldn’t even think of wasting your money.
That should make you wonder why the stock characteristics were set as they were in the first place. The engineers are usually shooting for some compromise between performance, durability, economy and visceral appeal, so if the chip ends up putting more stress on the motor and changing the idle characteristics, then it’s matter of choosing a different set of trade offs, not of making it a “better” car.
I tend to have more faith in the guys at the factory than some aftermarket shop, particularly the shops whose products tend not to work in the first place. I know that some guys like to tinker with stuff like that, but I don’t. Suspension mods are one thing, messing with the motor is something else.
@Pch101
“I tend to have more faith in the guys at the factory”
I agree 100%.
My V50 is turbocharged but considering I started with an ’86 240DL in college, I am already living high on the hog, as it were.
I do think it would be fun to have another 30 HP on tap, but I probably should just save the $1,500 to buy a faster car.
I do think it would be fun to have another 30 HP on tap, but I probably should just save the $1,500 to buy a faster car.
Ha, I almost wrote something like that in my last post.
In my opinion, most people who want a faster car should go get a different car that was designed to go faster. The consequences of modifications might be worthwhile to some people, but you have to be able to live with the drawbacks, too.