87, 89 or 93 octane????

[05LegacyGT330Ci]

Well the cars that I use 93 in all are worthy of using it. The Tacoma did get 87 cuz thats all it needed. I would never go below grades though was the point.

[Trent Bates]

Got it! Sorry for the misconception! In a similar vein, I wish there was an ultra-low octane gas available for some of the cars I've had to drive! Some 80 RON would be nice!

 

For most of us, an extra $2-$3 per week really isn't a big deal. I'd imagine that the people that really get upset by it are driving 40-50 MPG cars.

[SubaruFlamingo]

here we have 87, 89, and 93, we've been using 89 for 6 months... and reading this makes me think. I know in the STi you should use atleast 91, but talking with the (knowledgable) dealer, he said 89 would be fine. But aside from engine hurt, is the performance with 93 noticably different aside from numbers? Can you feel it?

[Th3Franz]

Only 93 for my baby.

[Mono]

The only way to know if it's pulling timing and thus reducing power, is to measure your fuel economy over a period time with each fuel. I emphasize the words "over a period of time".

 

If it's truly pulling timing and not operating at optimum power, then you will lose fuel economy.

 

I still doubt you will notice any measurable difference if you maintain you same driving habits. And besides, if there is a knock sensor, then you can't do damage to your engine during this test.

[Mono]

I was referencing the static compression ratios. I took this from Hot Rodding Magazine:

 

"Dynamic Vs. Static Compression Ratio

 

Static compression ratio is what you build into the engine by way of dome volume, combustion chamber volume and compressed head gasket volume. Dynamic compression ratio is the maximum pressure that develops inside the cylinder during the compression stroke. Dynamic compression ratio is influenced by the overall engine breathing efficiency (engines with good flowing ports along with an equally matched induction system) and also by the camshaft profile (how much air the camshaft allows to enter the engine and when it enters the engine). Ring seal is also a factor (as rings seat, they leak less, and increase dynamic compression ratio; as rings wear, they leak more, which decreases dynamic compression ratio). Air density is the final factor in the dynamic compression ratio equation. The "better" the air, the superior the dynamic compression ratio. All of these elements must be taken into account when determining the engine compression ratio. The ultimate goal, according to Reher-Morrison is to create as much dynamic compression ratio as possible, without creating detonation."

[Xaqtly]

Actually, if your car requires premium and you're putting in regular, there's a very good chance your ECU is in fact pulling timing out. That's what it does. Your knock sensor is what tells your ECU to pull timing out so you don't detonate. That doesn't mean you're guaranteed to not damage your engine though, nothing is going to guarantee that.

 

The real question is, why would you run less than premium when you know that it will force your ECU to constantly compensate for the lack of octane by pulling timing? I reiterate that the Legacy is designed to run on premium. Running on less than premium forces the ECU to compensate in order to maintain a safety margin, and even then you're pushing it. If it's a hot day and you're pushing the car, even reducing the timing may not be enough to prevent detonation, depending on how wide that safety margin is.

[Euclid]

I'll make is easy, KNOCK SENSORS CAN NOT ALWAYS PREVENT ENGINE DAMAGE also if the knock sensor DOES prevent engine damage YOU MAKE LESS POWER

 

This is like saying "well my car has a rev limiter so I'm safe from over revving my car." That may be true, but that rev limiter isn't going to prevent your engine from blowing up when you're in 3rd at redline and put it in 2nd instead of 4th.

[Red98GT]

ONLY 93 when I am filling.

 

The way I look at it is if others want to fill using 87, then they are more then welcome to do so on their GT, hopefully others will not be convinced by them to follow suit.

 

-- Bill

[KTM 525]

From what I have read the Subaru motor design is non-interference motor. The LS1 on the other hand had more than it's share of the 3rd to 2nd downshift and bent pushrods were the result much of the time. Nothing catastrophic but an ache no less. If you had to run a low grade gas in a pinch I think you would be ok if you went real easy on the throttle keeping the R's down. My car will get the good stuff whats an extra 4 bucks on a tankfull?

[Trent Bates]

Again, I don't know how my Subaru will handle knock yet so I have to speak from experiences with "other" cars.

 

If a PCM/ECM is pulling timing enough, it is indeed noticeable without checking your mileage over time or logging the data. You can feel it as you accellerate.

 

As Xaqtly said, using a lower octane fuel will work the knock sensors and thus the PCM/ECM harder. It's silly IMO to force an engine management system to deal with a constant/frequent situation that is potentially damaging to the engine just to save a few bucks per week. A person might as well use the cheapest oil and filters they can find for the engine to save even more money.

 

The point is that the PCM/ECM for an engine is designed to maintain a certain level of timing retardation for maximum performance without getting so close to the "edge" as to allow knock to occur. There is only so far that a PCM/ECM can retard and advance the timing. (I don't know what the extremes are for a boxer engine. Does anyone here know?)

 

For example's sake, Let's say that the system is able to adjust from -40 to +40 degrees. If the average is around 0 (static) and the retardation needs to increase to around -25 at 5K, there's not that much room left to retard. On a hot day, that remaining 15 degree buffer could be used up pretty easily to deal with the hot air's tendancy to ignite the fuel sooner. Lower octane gas is going to drop the average by a certain amount too. If you lose 7 degrees with 89 instead of 93, there's only 8 degrees left over in this fictitious scenario.

 

If a person drives like a little old lady all the time they could probably get away with lower octane stuff. But then I have to ask why such a person would spend the extra money for an intercooled turbo engine instead of the base engine. My Wife's 2003 Legacy only requires regular fuel.

It can be argued that the intercooler is there primarily to offset the hot intake air predetonation problem BTW so we can add it's temperature variations to the ECM adjustment range as well.

 

If the knock sensor mechanism was able to only remove the bad without removing some good as well AND it was a truly dependable system that never failed and never reacted too slowly, I would say it's okay to put whatever grade you wanted in there. I don't trust it that far!

[billyjoejimbob]

I buy the mid-grade at high priced stations and 93 octane at Costco. No problems. Its a car, not my sole source of self worth for god's sake!

[Beanboy]

"For optimum engine performance and driveability, it is required (for 2.5 liter turbo models) and recommended (for 3.0 liter models) that you use premium grade unleaded gasoline."

 

The above is from the manual.

 

Many cars say use 91 is recommended/for optimum performance. Subaru kicks it up a notch for the 2.5L turbo saying that it is required.

 

Whether or not you lose power, Subaru thought it was worthly enough to make the distinction. Seems to me that it means the ECU will pull back performance if you use less then 91 on the turbo models, but on H6 ones, performance might be reduced.

 

That's enough for me to go for 93, except for where 91 is available.

 

-B

[IwannaSportSedan]

how much do you save on the midgrade vs premium at the other stations? Mid-grade here is ethanol, and tax relieved to be as cheap as low-octane. Still it is at most about $.10 a gallon difference.

 

Different octane gasolines exist for a technical reason, not because people like to sell three grades of gasoline at all times (it increases the number of tanks, and the capacity that a station has to keep on hand at any given time. Inventory cost money.) It is because some cars need it, and others don't.

 

A car that needs high octane, like a turbocharged car, or a car with high specific output (hp/liter) needs gasoline that won't explode too early. Detonation is a serious problem, it can bend valves, and break pistons, or damage the head or intake manifold.

That is essentially what octane ratings mean, the higher number means it is somewhat more difficult to detonate. Once the gasoline is mixed in the cylinder, the high compression of such an engine is more than enough to ignite the mixture at the right time.

 

Now, overestimating octane can actually hurt mileage on an engine that is calibrated for less octane, like with less static compression, and no forced induction. The higher octane gas will burn slower, and the ecu will dump more fuel in to compensate (burning more gasoline more slowly per stroke will consume fuel faster, hence, less MPG.) On a carbureted car, you have to press the gas pedal harder to get the same speed or accelleration, vs running lower octane gas. Fuel injection does the same thing electronically by widening the injector pulse.

 

Now my 4.0 liter ranger is a truck motor, and tuned to run on old turpentine all day long, and is tuned for torque at lower RPMs. so I put the cheap 89 octane ethanol in it. If ethanol midgrade was more expensive, I would probably put the cheapo 87 octane in it. it has a 19.5 gallon tank, and typically gets 14-18mpg, and even less this time of the year, with 4WD in the snow.

Filling a larger tank more often, yeah, I want to save some dough, and technically, the higher octane would be more wasteful, and even run worse than the lower grade gas. All signs point to cheap.

 

The Miata is normally aspirated, and doesn't call for premium, so the engine is not really tuned up enough to really nead ultra high octane. It, however is a smaller, higher revving motor, with a little more static compression, and is also driven differently, so I put 89 minimum in it, and 91 if that isn't available. IF I ever get my wish to turbocharge it, it will be on a strict diet of premium at that point, due to the added dynamic compression. Without that added compression, it doesn't need the extra octane, and again is doing no performance favors.

I also like the fact that the mid-grade stuff usually has the detergent chemical package added to it, and is more likely to be clean, and non-watered down.

 

Now, with a Legacy 2.5i, that would be similar to the Miata's situation. The GT add the all important turbocharger, though. That changes things significantly.

 

The reason the GT makes something like 67% more horsepower than the 2.5i is because of the added compression that the turbo's compressor shoves in there. You don't get ~80 extra horsepower for free. It has to be fed properly, an have some of the excess heat removed. Significant increases mean significant considerations.

 

With air (the oxidizer) compressed and heated, when fuel is added, it is much more likely to combine, and explode. It is kinda like a small bomb like that.

 

Ideally, inside the cylinder, when the piston is at the right position, and the valves are closed, that is what we want to happen. If that explosion happens to early, in the intake tract, or while the intake valve is not seated, bad things tend to happen. bend valves, broken piston crowns, intake manifolds and head damaged. engine computers will try to save you by tuning itself down, but they aren't majicians, and can't change the laws of physics or chemistry.

 

It is better to be safe and spend the extra ~$1.60 per tankfull ($.10 for estimated 16 gallon fill, just a rough guess) for the correct gasoline that the Legacy GT asks for, than to risk damage.

 

BTW, the lower performance of the engine running detuned will probably would rob you of enough fuel mileage over time, by having your foot further into the gas pedal to cruise and go up hills, to nearly consume that $1.60/tank by making you fill up slightly more often anyway. There is no such thing as a free lunch.

[jerseyglock]

Listen to Euclid, its all about KNOCK Sensor!!! When detect knock(s) from engine, timing will retard and less HP

[some6uy008]

Only 91 here in No. California

[STG]

The manuel recommends at least 91 octane but in a pinch, you can use 89 but it may affect performance

Who is this Manuel fellow and does he work for S.O.A.? Did he get that info from the Legacy owner's manual?

[STG]

EASY Euclid.... I'm with you on the premium gas and performance. Wouldn't risk putting anything less in for it to be cheap or to hinder performance. Being this is the first performance car I've owned, my question is more from curiousity.

The Legacy is a Performance Car??

 

Let's get real here. While it performs exceptionally well for what it is, few people would categorize it as a performance car. The Legacy is a very nice and competent $30K sedan.

 

Unless the car can scare you (like a Shelby 427 Cobra with 600 hp at 2,500 lbs.) it ain't a performance car!

 

By the way 91 octane won't hurt the car. That's all we have in California

[agctr]

Purely running on Shell Optimax 98Ron here in Australia and getting great mileage and power.

Adam.

[MSB]

I guess the engine must be able to make the adjustments comfortably, as for komeng's JDM specB they recommend 100RON, and all they have in Brunei is 98RON, as per Oz , and I doubt whether they would import the cars if the fuel would affect the engine and its warranty.

[c-lo]

Ummm, thanks everyone..... I've learned alot about using different grades of gas.

[Mono]

Your car doesn’t have an octane sensor, it has a knock sensor. The ECU doesn’t sense the lower octane fuel and pull timing. It’s not like it goes, hey, this is 87 octane and my manufacture says I need at least 91 octane, so now I’m going to pull timing”. It will only pull timing if it picks up engine knock. So, you don’t automatically lose power just because you are running lower octane.

 

Many of you seem pretty well educated on octane and understand it’s purpose. However, I am sure there were some people reading this thread that learned some valuable information along the way. Unfortunately, there are lots of car owners who seriously believe that if they purchase the most expensive fuel sold, their cars will be faster and run better overall. I see guys run 100 octane at the track in their normal aspirated stock engines and think they are magically run faster in the ¼ mile. What a waste of money, but can you blame them? Advertising plays a huge role in this misconception. Yes, companies carry different fuels for different applications, but take a closer look at how they market their more expensive fuels. Have you ever seen an commercial for their 87 octane fuels? Of course not, the premium means more profit for them. I also never see any TV ad for premium fuel discuss the octane characteristics of their fuel and why you as a consumer may or may not need to use higher octane. Instead they promote the cleaner engine concept and why that’s “so good for your engine”. Guess what, all fuels have to meet specific EPA standards to carry detergents and cleaning agents, not just the premium fuels. They forget to mention that in their ads because they want you to purchase the premium fuels.

 

Look, I do believe some cars may perform better with higher octane fuel. But just don’t get sucked into hype of premium fuel because of the misleading advertising out there. And lastly, the manufacturer recommendations are conservative.

[Euclid]

^^^

 

I don't think anyone is saying cars can "sense" which octane you put in it. I also don't think anyone is trying to say that you have to put the premium in because it's *the* thing to do and will give your car that extra oomph. I do agree with you however that there are many people who do believe that putting in higher octane fuel will increase your cars performance. Which unless you have a highly modified car that requires race fuel you're just wasting your money. But you seem to be stuck on this "it's just for profit" marketing mentality.

This isn't a discussion of whether or not putting premium in the LGT is because of gas company hype (it’s not), it's a discussion of the turbo engine in the LGT being designed for premium fuel (it’s a turbo!). And putting in a lower grade of fuel greatly increases the risk of pinging and detonation. THIS is where the knock sensor comes into play. You don't seem to quite understand how this affects the mechanics of the engine. It's not a MPG thing, or a "it seems to drive ok" thing. It's about the tolerances of your engine, and longevity.

 

 

*side note* c-lo, I wasn’t picking on you specifically. I was just using your words as an example of what many many people out there think.

 

There seem to be the 4 major “debates” on every car message board out there.

1- Proper break-in

2- How to warm up the car

3- First oil change

4- Which octane gas to put in the car

[hal9e3]

A couple of points. If you use gasoline in your vehicle that has a DIFFERENT octane rating than the manufacturer's recommendation you will PROBABLY suffer reduced power and gas mileage. Car and Driver published a test proving this within the last couple of years. They used a Honda Accord and I believe a Mustang GT as the regular/premium test subjects. Also, the octane rating for gasoline in the US is the average of two different tests. You've probably seen the little (R + M)/2 sticker on pumps. One test is run at a higher RPM than the other and is more strenuous. This means that two different gasolines with the same octane rating can in fact be different. If you've experienced inconsistent performance with gasoline purchased at different gas stations, or even at different times, this could be the culprit.

[Mono]

Running on less than premium forces the ECU to compensate in order to maintain a safety margin, and even then you're pushing it.

 

Right here. This is not true unless it knocks. Just running less than premium does force the ECU to compensate. The ECU cannnot detect the different octane. This is nonsense.

 

Look, whether or not the Legacy GT will knock under anything less than 91 octane remains to been seen. I thought that in order to respond to this post properly, it was better to provide some information about the different fuels. That's what I've done. I do know that I have personally owned vehicles where higher octane is recommended by the manufacturer, and I have ignored those conservative recommendations seeing no ill effects on performance or longevity.

 

I also know that the advertising for premium fuel is purposely misleading to the public to attract more sales. Again, why don't we ever see any adds for the 87 octane fuels?

 

Lastly, 87 octane is not a lower "grade" as you stated in your last post, it's simply a lower octane. The fuel quality is no different from 87 to 93. Using terms like "lower grade" is exactly what the fuel companies want you to believe so you purchase their more expensive "higher grade" fuels.