87, 89 or 93 octane????

[hal9e3]

Mono, just because an evil oil company tries to hoodwink an unsuspecting public into buying their pricier gasolines doesn't mean it's not the right thing to do:) Also, there can be a 'quality' difference between gasoline grades. I don't know if this is true anymore but at one point Chevron only put their Techron cleaning additive in the higher grades of gas, not regular. At least, that's what the pumps said. I just looked at a Chevron press release for Techron and they said it was added to all grades. However, it also said that premium grade contained more Techron. In any case this indicates that there can be a difference in gas quality between grades.

[mvigneau]

Interesting.

 

Too Much Boost

With air being pumped into the cylinders under pressure by the turbocharger, and then being further compressed by the piston (see How Car Engines Work for a demonstration), there is more danger of knock. Knocking happens because as you compress air, the temperature of the air increases. The temperature may increase enough to ignite the fuel before the spark plug fires. Cars with turbochargers often need to run on higher octane fuel to avoid knock. If the boost pressure is really high, the compression ratio of the engine may have to be reduced to avoid knocking.

[IwannaSportSedan]

Just FYI,

 

I have had local stations and some convenience chains that don't use major brand gas that tend to add water or something to the lowest grade gas, to top up their storage tanks, and sell slightly less gasoline for the price. It is kinda like pumping food full of water before freezing it to up the net weight. It adds to the profit margin of the low grade gas. High octane gas has a slightly higher profit margin built in, that is true, but without profits, you'd run out of gas before finding an open gas station.

 

I frequent no-name stations less and less, (i tend to prefer Amoco/BP, or Shell first, Conoco Philips otherwise.) but I used to get a "bad tank of gas" every once in a while from those no-name places, and yes, it is noticeable.

 

It is true that a car can't directly tell what octane gas you are using, but it is more than just the knock sensor that comes into play. Higher octane gas burning slower in a lower compression engine can affect the level of unburned fuel in the exhaust, and the 02 sensor can tell that, big time. That is what emissions control systems are designed to do.

 

Vice versa, low octane in a turbocharged, or other higher performance engine that is tuned for high octane gas will run differently, possibly hotter and/or leaner on low octane, and the 02 sensors can tell that, as well, and will tell the ECU to dump more fuel in. The 02 sensors affect fuel flow via the ECU fuel maps and injector control more often than the knock sensor reacts, anyway.

 

Emissions control systems are very sensitive to unburned hydrocarbons in the exhaust, (extrapolated from changes in oxygen percentage levels) and will affect the ECU's management of the engine even before "pinging" is detected by the knock sensor.

 

About gasoline grades...

Mid grade gas is usually fairly clean, often has alcohol (whether or not it helps your engine, it helps support the price of corn for the farmers, which is still very low) and at brand-name distributors, like Shell, Conoco/Philips, Exxon/Mobile. BP/Amoco tend to have at least some detergent adative packages added, to help the fuel system stay reasonably clean. (Fuel injector cleaner, etc, is usually a variant of the same stuff, MTBE, or something similar) High octane usually has more or better adatives.

 

As I said, in Iowa, the State gasoline taxes are reduced for ethanol containing mid-grade gas, to bring the cost of ethanol mid-grade down to the same price (sometimes a penny or two less) as low octane gas.

Helping support Corn demand, and giving less to an already overtaxing state government is fine by me. I am a state university employee, and got less than 2% cost of living increase in my salary in the last 16 months. I actually am losing economic buying power to the state, anyways... but that is another matter.

 

BTW, in some states, taxes on gasoline can approach 30%. That is before local option taxes, corporate taxes of the gasoline retailer, federal taxes, taxes that the distributor passes on to the retailer in the price, and all sorts of other charges that have nothing really to do with petrochemical production or distribution. I am not saying that all taxes are wrong inherently, but that seems like a lot of tax revenue, and it really affects people, even if they don't notice.

 

The government(s) gets far more of your money from your gas purchase than the businesses that you patronize do in profit. Honest profits on a highly competetive commodity don't bother me nearly as much as overtaxation on a product that is not really means-adjusted, as most taxes on retail products aren't.

 

 

Also, another tip for folks...

 

if you see a tank truck filling the underground storage tanks at a gas station.... Go to the next one.

 

When filling those tanks, if they are empty and drawing from the bottom, and just by dumping fluids into that tank, any sediments or "gunk" in those storage tanks gets stirred up into suspension in the gasoline, and pumped into your fuel tank.

If there are small enough particulates, it can get through the gas station's filters, and through your car's fuel filter, and start to clog the injectors.

 

Those solids will settle back down to the bottom of the storage tanks in a little while, and be less likely to get transferred to your tank.

 

It may not be that big a deal, and you might go to a station sometimes that has just been filled, or is nearing empty without the tell-tale sign of a tanker truck right there, and there is nothing to be done but hope your fuel filter is a good one, but think of it as a precaution worth paying attention to, even just for a little more peace of mind.

[Xaqtly]

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.

I don't believe anybody claimed that the Legacy has an octane sensor. Look, it's really simple. When you put lower octane gas in the Legacy than what it *requires* (i.e. premium), then you remove the safety margin created by the use of higher octane gas and you promote the onset of detonation. Your knock sensor detects this (note that it's not detecting octane as you suggest), and retards your ignition timing accordingly. How hard is that to understand?

 

Look, whether or not the Legacy GT will knock under anything less than 91 octane remains to been seen.

Not according to you. You're claiming it's fine to run 87 octane with no ill effects, which means you're absolutely positive that you are not getting any knock at all by using the lower octane. So you're using your own unscientific seat of the pants judgement of your car's performance to make the claim that your timing is not actually being retarded by using 87 octane, when it's a generally well understood fact that using gas that's significantly lower octane than what's recommended by the manufacturer will in fact promote knocking and cause the ECU retard the timing. I mean your entire argument is based on the premise that you "think" your timing is not being retarded.

 

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?

Are you suggesting Subaru is in cahoots with the oil companies now? That their requirement for premium fuel in the Legacy GT is based on nothing more than big oil propaganda and greed? Interesting.

[Mono]

Go back and read my first post on page one. I never said I owned a Legacy GT. I said running 87 octane MAY be no different on the Legacy GT than running 91 octane. I also said you would need to test the fuel economy using both fuels for a sustained period to time. I also said I have personally tested this theory on my 2003 Nissan Pathfinder where 91 octane is recommended and saw no reduced fuel economy. I don’t use seat of the pants to “feel” the ECU pull engine timing. That’s just stupid.

 

Once again, if the ECU retards timing, the result will be less engine power. Less engine power will equate to less fuel economy. That’s not an “unscientific seat of the pants” measurement in my book. Is that simply enough for you to understand?

[Mono]

IwannaSportSedan – you made good points and some good tips.

 

I’d also like to add that oil companies don’t add detergents/cleaning agents because of consumer demand. The EPA makes them do this. A car with carbon deposits, dirty intake valves, etc. is a car with excessive pollution. The “dirty engine” cannot burn the fuel as designed and creates more pollution. The EPA does not want this. This is the same reason car manufacturers have to meet stricter emissions requirements year after year. It’s this reason that ALL fuels must meet specific standards to become available for sale. Not just the premium fuels.

 

Do I think there could be an alliance between some of the auto industry leaders and oil companies? You bet I do. All they have to do is tell consumers that premium fuel is recommended and the majority of people will follow like sheep for fear they will damage their engines otherwise. Everyone likes to believe their car is special and one of a kind when in actuality is a mass produced product geared towards a broad market segment. These mass produced products are set up with very conservative tuning by design. If Subaru set up their cars with super aggressive tuning, then they would likely have customers complaining about engine damage if someone used less than 91 octane. Instead, they would rather error on the conservative side and tune the car with some safety margin built in. It’s this reason I believe most cars can run on lower octane without any negative results. However, a limited production vehicle catering to very small market segment (Dodge Viper, Mustang Cobra, Ferrari, Porsche 911, etc.) is going to be tuned more aggressively because it’s much easier to handle the smaller production numbers if complaints arise. I’ll bet there is still some safety margin built into these vehicles as well.

 

You can dyno tune any stock engine and increase horsepower output. A dyno tune a stock car is basically removing the built in safety margin and allowing the engine to perform at peak performance. Some of you may be surprised how much power is given up to meet certain emission standards meet specific MPG targets. The dyno tuning experts are not concerned with fuel economy or controlling emissions like the government regulated auto industry.

[Euclid]

Yeah measuring fuel economy is a real good way to make sure you aren't busting a piston ring. Good luck on your engine ventures when you try your methods on an actual performance engine, not some run of the mill motor.

[Black_Legacy]

92 is all I can get, 92 is what I'll use. And maybe some xylene for autocrosses.

[kenzo]

I use 117, because it's 24 better than 93.

 

"these go to 11"

[Black_Legacy]

toulene is only 110 isn't it?

[Colonel Angus]

At the Bloomingdale one they bring it in on an unmarked truck but the driver was wearing BP clothing when filling the station (needless to say I didn’t fill up because its bad to fill the car when they are filling the tanks). I asked the attendant and he say that’s the case. For you executive members this does not count towards your 2% but I do get 2% on the Costco Amex.

 

I have also tried Mobile and BP and I always receive the same shity mileage, im not complaining I tend to shift late…really late…a lot.

 

thanks for the info!

[Rally Performance]

Hello all! Good post, this - some of you guys know your stuff... let me add a few thoughts about how the Subarus cope with timing advance that may help. We've mapped our share of WRXs and are moving into the LGT now (have a pearl white 5mt )

 

First, the Subarus make power very well with a combination of boost and ignition advance (contrasted to, say, an Evo that responds well on just boost). Subaru recognises this and has a very active ignition advance system in the ECU. It works like this, simplified: There is a "base" ignition map and an "advance" ignition map. The ECU starts with the amount of advance allowed on the base map and uses it... but "learns" that it can advance the ignition more in the RPM and load ranges specified on the advance ignition map.

 

The car will pull timing if it senses knock in the band that it is sensed... and does it both short and long term. If over time it is consistently sensing knock in a particular area, it wil learn that out of the equation. Thus, those owners in California who can only run 91 octane will find that their car will adjust over time to that fuel. Still, that car running on 91 will not be as quick as the car in Pennsylvania running 93. It is a quite tangible difference.

 

If a car gets a tank of 91 octane, in our experience, it will be fine - but performance will be degraded for a time after that tank is run until the car re-learns the better gas. In fact, it takes quite a while until the car "trusts" you again... an ECU reset by leaving the battery terminal unhooked for an hour or overnight will "reboot" the ECU so that it will start learning from scratch again. NOT necessary frequently, but if you've gotten some bad fuel it is good to run a full fresh tank through and then do a reset.

 

For mapping purposes, I even ran 89 in my WRX to watch the results. Haven't done this in the LGT yet. I like to test our remaps to ensure that if Grandma puts 89 in, the car reacts safely. I DONT recommend running 89 all the time in the car... especially if you are doing any modifications at all.

 

Finally - to some of the good points that were made in this thread, there were some saying that if you drove conservatively, not getting into boost much, the lower gas is probably ok. I differ here - on the factory map for the WRXs we found that there were points down low in the RPM / load band (in the cruising range) that were very edgy on lower octane fuel. In fact, there was more safety margin at higher RPM / load ranges! As a result, the car's willingness to advance across the entire RPM band, long term, was affected negatively (the advance multiplier dropped) to keep itself happy. Again, it kept itself safe but it ran quite "doggily".

 

SO - to summarize

1.) Lower octane fuel affects performance across the RPM band, in a tangible way;

2.) Higher octane fuel will still offer a bit better safety margin, long term, than lower;

3.) The higher the octane, up to 93 (but not over) that you run, the better performance you will get. Above 93, unless your ECU is custom mapped for it, all you get is more heat;

4.) DON'T use a manual boost controller unless you have proper engine management planning for it. We've serviced enough Subarus blown up by them to know.

5.) (opinion) you can see a little bit of the intelligence in the Subaru ECU... which is why we only sell ECU remaps rather than piggybacks that try to "trick" it into certain adjustments by lying to it through the sensors.

 

Hope this helps!

 

Regards,

Ken

Rally Performance

[John M]

I have a MBC and an EcuTek datalogger. I can prove it's just fine as long as it's done properly.

 

I logged the car in stock form, paying close attention to the knock correction value (where positive is good). The stock setup (93 octane) was between 4 and 5. The MBC at 17 psi was 3 to 4. Still nothing in the negative, or the "you're knocking" area.

 

How is this going to blow? I refuse to pay the $1k ransom for an ECU until I have no other choice (like after a Green & supporting mods). In the meantime I will continue to debunk any tuning myths I run across.

 

Just wait till I undo the variable voltage fuel pump setup. The lag in fuel response caused by that can easily cause intermittent lean conditions. I'll bet that's why most people say "my car is faster if I roll into the throttle than if I just go WOT."

 

Ya gotta watch out for us old ex-DSM owners. We're cheap bastards and insist on trying everything!

[Trent Bates]

Great info Ken! I haven't had my car to the track yet to discover even a portion of that info!

[Black_Legacy]

there is a lot of long winded posts here, wow!!!11one

[IwannaSportSedan]

the Manual boost controller issue is interesting to read, but without owning a Legacy at this point, it is just interesting reading for me.

 

The more interesting reading is about the variable voltage fuel pump.

 

Does Subaru use a returnless fuel system? Why would one want to lower the voltage to the fuel pump, other than to reduce fuel pressure? I would think a more constant pressure pump, and a good Fuel Pressure regulator would be a better system for a performance engine, especially a modded turbo motor.

 

good stuff, and I am still reading and learning. usually a good thing.

[Rally Performance]

Thanks Trent! Good thoughts from you earlier, was just trying to add some of our experience... everyone is still learning including us, so it's good to share info.

 

John M -

Oh no, not a DSM guy! :O (I had a GVR4 prior to the WRX…) Actually our reflashes are a lot less than $1k... not advocating a standalone ECU, but we do like to program right on the Subaru ECU.

 

I did make a blanket statement that MBCs are evil, but in fairness we are quite conservative with our approach. A smart user who has the monitoring tool that you have can do it safely. For many people, however, it is too often a costly “pay now or pay later” trap on the Subarus. What works for one car won’t often work on another, so be careful. More on that later…

 

On your EcuTek DeltaDash logging – Great tool to use, and you apparently know your stuff. Make sure that your laptop is a fairly quick one, and that you are logging at the fastest speed possible, or in our experience you may well miss knock events with the knock switch(es)… but you will catch most negative knock correction, so that’s good. Look critically at knock correction during part throttle higher boost situations, as you’re doing, and at the transition when the throttle snaps open. Here’s why…

 

 

(…and this is one of our big reasons why we don’t like MBCs without proper mapping on Subarus): If you raise boost particularly in part throttle situations, you can get part-throttle lean conditions. The LGT and other Subarus except the STi cutover to open loop at a fairly high throttle position. This means that as you roll into the throttle, the car is still trying to achieve an AFR of 14.7:1, quite lean if you are suddenly able to make more boost due to the MBC… or an MBC with other mods, etc.

 

Additionally, (this makes it worse) the ECU for emissions reasons doesn’t cut over immediately when that throttle position is reached, but delays for a few seconds. This was a real problem with the 2004 WRX, and was a change from the earlier model years.

 

 

When we do a performance ECU, we can not only tell the car to cut over to open loop (more balanced) fuelling at an EARLIER throttle position, but we can get rid of the evil code that makes that cutover delayed as well. Both of those things are important if you are going to run boost earlier at part throttle conditions.

 

 

Remember – what works for one car may not work for another, given mod level and location. For instance, it sounds like your car is running safely at that boost level with an MBC. If you fill your car with California fuel, you’ll probably see negative knock correction… or perhaps if you take it to significantly higher altitude. Certainly if you add a bellmouth downpipe with a high flow cat like we’re making, you will need to analyze to see if you are still safe. In that situation, with the car’s physical ability to make boost improving, you may have an issue.

 

There’s also that looming desire to crank up the boost on the MBC when a riced-out Civic pulls up next to you.

 

 

SO – here’s why we like proper ECU tuning, rather than an MBC… in a nutshell, it gives better performance with a better safety margin:

 

 

1.)The car responds overall better to both boost AND timing, in contrast to the 4G63 in the Mitsu; engine management can take care of both and do it safely;

 

 

2.)Doing the change with engine management allows you to really enhance earlier onset of boost safely for the reasons above – open loop cutover timing is a particular factor.

3.)Doing the change right on the ECU retains the full adaptability of the ECU depending on changes in fuel, environment and even configuration changes in the car;

 

4.)Then there’s the evil desire to crank the boost (resulting in engine rebuilds…) in the UK in 2002 we serviced one car per week due to MBCs. When the owner wanted an MBC install, the UK techs also began asking when they wanted to schedule the engine work.

 

 

SO – if you use an MBC, do it responsibly and monitor aggressively like John M is doing...

 

 

Regards

 

Ken

 

RP

[Rally Performance]

John M -

 

One more thing - good monitoring of knock correction... in effect when it went down, though, it does show that the car isn't advancing quite as much as it did earlier in that same range. The ECU pulled out a degree of advance that it was comfortable using before.

[John M]

Less advance with more boost is standard operation with most any ECU. Part throttle logs showed virtually no difference before and after the mod. This was using the stock EGT sensor, which will be in place until the cat-less uppipe goes in. While not a perfect data source, it's good for part-throttle comparisons.

 

The knock correction showed marked loss at 18ish psi so I would say 17 is the highest anyone should go without ECU work. My patented assometer says there's a noticable improvement. FWIW, my car at 15 psi ran the same times & mph as have been reported with the AccessPort flashes.

 

The more interesting reading is about the variable voltage fuel pump.

 

Does Subaru use a returnless fuel system? Why would one want to lower the voltage to the fuel pump, other than to reduce fuel pressure? I would think a more constant pressure pump, and a good Fuel Pressure regulator would be a better system for a performance engine, especially a modded turbo motor.

 

We've got a good ol' standard return fuel system. Most manufacturers vary the fuel pump voltage for noise reasons. They don't want somebody to complain about a whining noise in their fancy new car. Lots of cars have it, including the Supra and 3000GT/Stealth. I prefer the pump to receive full voltage all the time. This involves a rewire with at least 10 gauge.

 

Subaru actually turns the pump off & on several times a second for either 33% or 66% duty cycle at part throttle. I'd rather it run all the time and let the FPR return any unused fuel. I will eventually run a Supra pump - I prefer it over a Walbro.

 

Rally Performance - feel free to PM me with info on the ECU mods you offer. Also, if you have any insight on what the ECU does with the manifold pressure reading (besides use it for boost cut), I'm all ears!

 

Oh yeah - and I once owned GVR4 # 746/2000, in addition to a half dozen other 1st gen DSMs and an 89 Mirage turbo.

[Rally Performance]

Less advance with more boost is standard operation with most any ECU. Part throttle logs showed virtually no difference before and after the mod. This was using the stock EGT sensor, which will be in place until the cat-less uppipe goes in. While not a perfect data source, it's good for part-throttle comparisons.

Agreed with all of the above...with the uppipe and other mods you'll likely start to get some changes in part throttle logs.

 

The knock correction showed marked loss at 18ish psi so I would say 17 is the highest anyone should go without ECU work.

Agreed - if their car is in the same configuration as yours, with the same fuel and altitude. Good assessment on your part!

 

FWIW, the nice thing about a well done reflash is the ability to make good power at lower boost levels than needed with boost increases alone. Still, with the WRX for instance, our stage 2 maps target about 16.5 psi. On an open class rally car we'll run 18-20, but then reliability isn't as critical...

 

FWIW, my car at 15 psi ran the same times & mph as have been reported with the AccessPort flashes.

That's good news for us then Our reflashes should be able to walk all over that particular AP flash at 15psi.

 

If people take the same approach that you are, they should have a good margin of safety at setting boost levels with their particular car / configuration etc.

 

KC

Rally Performance

[Xaqtly]

Go back and read my first post on page one. I never said I owned a Legacy GT. I said running 87 octane MAY be no different on the Legacy GT than running 91 octane.

So why are you posting here at all? Let's summarize.

 

* You don't have a Legacy GT.

* You don't have any actual data whatsoever on what running low octane gas in a Legacy GT would do to it.

* Your posts contain information that you got from testing different cars with different motors, different tuning, and no forced induction, yet you're trying to apply that data to the Legacy GT.

* You're saying it may not make a difference despite the fact that your tests were done on a car that "recommends" Premium gas - the Legacy GT *REQUIRES* premium gas. You see the difference there?

 

So if you buy a Legacy GT and you decide to put 87 octane into it, that's your choice. But don't tell people "it may be no different than running 93 octane". On this car, it's very different. If you've been reading all the posts in this thread hopefully you should know that now.

[agctr]

The higher the RON, the better fuel management you will recieve.

Adam.

[SubaruFlamingo]

All "premium fuel required" talk aside, theoretically, is 93 better to use than 89 in the cold conditions? I'd imagine the temperature mixed with the fact that it's redlined many times playing in the snow would lead to it only being better.

[Rally Performance]

It's better in all conditions... I would avoid 89 if possible. See my earlier posts in the thread... up to 93 will definately give better performance, 89 will degrade performance for longer than just the time it's in the tank, unless you do an ECU reset.

 

Ken Cole

Rally Performance

[c-lo]

YOu know, when I asked this question, I didn't know I would be opening up such a big debate! Thanks for everyone's input. I now understand why it's important to use 93 octane. Considering how much fun I'm having with the car I wouldn't take the chance of putting in anything less.

 

carlo