Low Octane Fuel Economy Tune?

[bill harvey]

But timing is optimized for the det resistance of higher octane. It's been seen time and time again that the Subaru ECU isn't the fastest when it comes to responding to knock.

 

And the thing about knock, is that the ECU can only adjust for it AFTER it happens. By then, damage has been done.

but you are wrong you can run more timing on same fuel with lower boost. So this would work in theory without retarding the timing. in our cars the timing isn't making us ping its the rise in cylinder pressure due to the boost

[jbrown]

based on assuming 4.25 a gallon for regular and 4.50 for premium It's 59.50 for regular and 63.00 for premium based on 14 gallons... not worth it

[bill harvey]

based on assuming 4.25 a gallon for regular and 4.50 for premium It's 59.50 for regular and 63.00 for premium based on 14 gallons... not worth it

its worth it if thing got bad after you bought the car and can't really afford even the 59.50 anymore. the guy was just asking if there was an option and everyone freaked

[BAC5.2]

but you are wrong you can run more timing on same fuel with lower boost. So this would work in theory without retarding the timing. in our cars the timing isn't making us ping its the rise in cylinder pressure due to the boost

 

Sorry, but you are incorrect.

 

You can blow a motor from det at low boost just as easily as you can at higher boost.

 

If you want to run 87, you HAVE to tune for it. Just turning down the boost is not going to let you safely run 87 octane.

[LosAngelesLGT]

OP spent a reDONKulous amout of money making a Tiburon undriveable and now wants to save money by running the wrong fuel in a LGT without tuning for it. Where is the confusion?

[bill harvey]

Sorry, but you are incorrect.

 

You can blow a motor from det at low boost just as easily as you can at higher boost.

 

If you want to run 87, you HAVE to tune for it. Just turning down the boost is not going to let you safely run 87 octane.

 

well no kidding thats why he is asking about a map for it lol but with less cylinder pressure there will be room for more timing even with lower octane the difference between 87 and 91 is not that big a difference if you cut your boost in half you think you are still going to have detonation? The car doesn't run much timing advance at all

[BAC5.2]

well no kidding thats why he is asking about a map for it lol but with less cylinder pressure there will be room for more timing even with lower octane the difference between 87 and 91 is not that big a difference if you cut your boost in half you think you are still going to have detonation? The car doesn't run much timing advance at all

 

Grammar?

 

The money you save filling up with 87 will be lost with worse economy. Even wastegate boost is going to see more cylinder pressure than a high strung N/A, and timing changes can be pretty drastic.

 

Only takes a degree to pop.

[BradyCat]

rant on - Complaining about $4/gallon? Please... I pay about $1.52/L for 94oct. Zoinks! That's like $5.75/gal. $90/tank. OUCH! Friggin taxes - rant off

 

You American's got it made the way I see it

[ClimberDHexMods]

If you're worried about the $0.30 per gallon difference, or $4.80 per tank difference, then you should have paid $20k more for the prius upgrade so you could save the $4.80 plus $35 per fill-up. $40 per fill-up. This is why America needs to teach budgeting in high school! Actually, this is exactly why America shouldn't. A mass of well-informed, intelligent buyers would be hell on GDP. I know I am.

 

That said, people have and do make maps for 87/85 octane. As with anything, you fill up with the cheaper gas, then tune afr, timing and boost to a point where you have a decent safety margin at WOT and every throttle position inbetween. Want an economy map? Don't push the anti-economy pedal so far down. Good lord where do people come up with this stuff?

[ClimberDHexMods]

but you are wrong you can run more timing on same fuel with lower boost. So this would work in theory without retarding the timing. in our cars the timing isn't making us ping its the rise in cylinder pressure due to the boost

 

WRONG WRONG WRONG WRONG WRONG

 

It depends on the efficiency of the turbo for your application. Tell me how making more power with more boost if that added boost is in a more efficient area of the compressor map, is bad relative to making that power with advance. Timing, especially 'as aggressive as you can get without knocking' timing in the Subi tuning world is what causes cylinder pressures to be so damn high, much of it wasted energy. The smartest tuners out there have a bias to make power with BOOST over timing. It is a far safer way to make power. The exception of course being if you try to hold boost far outside of efficiency and flood the engine with heat. But the Subaru community appears to have figured out to avoid that in most cases, and in many cases go the opposite way in running excessively shy boost and quietly up the harmless safe timing to create far more cylinder pressure for same engine output than pulling the timing and upping the boost to hit peak efficiency. 15 or 20 or 30 psi is NOTHING compared to what cylinder pressures hit through the combustion cycle, which is largely a function of how much A/F mixture you've ignited and expanded near TDC. I know I'm in the minority in the Subaru community with this train of thought, as all the bros know better and the masses are never wrong. And I've started threads and called sources who lead the industry, all of them standing on the proper side of this issue. Full Race, Element Tuning, Infamous, to name a few.

[ClimberDHexMods]

ask a fellow subaru tuner. Tuning for E85 wouldn't be a problem, that's pretty common amongst subaru performance lovers. You could tune it extremely conservative in terms of timing advance BUT with the nature of E85 itself, you're gunna go through more fuel than if you were to tune it for 87 or 91 specifically.

 

Typically E85 cars get worse mileage, though the fuel is cheaper. So I don't know why you'd go through the trouble of doing so.

 

because it's 104~108 octane?

[bmx045]

"DON'T PUSH THE ANTI-ECONOMY PEDAL DOWN"

 

 

Best quote I've heard on this forum yet. And it's only 6:56am over here.

[DirtyDan]

The need for increased octane in a boosted motor is because of increased cylinder pressure. You could run 85 octane if you took the turbo off one of these motors. It would so slow you'd be watching grass grow as you pull away from a stoplight. But, without the increase in cylinder pressure from increased air supplied by the turbo, you don't need the added octane.

 

If you limit boost to say, 2 psi, your octane requirement to prevent detonation will be less than if you have 4 psi in exactly the same setup. Every increase in psi of boost will have a direct effect on the increased chance for detonation. The increased amount of air into the cylinder and the fact that it is hotter air will increase the potential for detonation.

[Bbc84]

Take off the turbo and top mount off, connect intake to throttle body, fill the car with 87oct. and then go tune

 

Or you can just drive like a grandma. I can get 30mpg out of my Stg 2 LGT by driving conservatively.

[ClimberDHexMods]

The need for increased octane in a boosted motor is because of increased cylinder pressure. You could run 85 octane if you took the turbo off one of these motors. It would so slow you'd be watching grass grow as you pull away from a stoplight. But, without the increase in cylinder pressure from increased air supplied by the turbo, you don't need the added octane.

 

If you limit boost to say, 2 psi, your octane requirement to prevent detonation will be less than if you have 4 psi in exactly the same setup. Every increase in psi of boost will have a direct effect on the increased chance for detonation. The increased amount of air into the cylinder and the fact that it is hotter air will increase the potential for detonation.

 

Cylinder pressure through the combustion cycle is a dynamic and highly variable occurrence. When you simplify it like that, you leave out a lot of the equation, and open the door to a practice of tuning that can over-stress the motor. Sure boost directly increases cylinder pressure, but IGNITION TIMING also directly increases cylinder pressure. To say that boost is the culprit of detonation, and then make up the power with ignition timing (which let's be realistic most people do), is to say wine makes people drunk, so you're only serving beer.

 

With conceptual gaps like this, folks are liable to feel safe running "conservative boost," when in fact their cylinder pressures could be much higher than the guy next door running more boost, less timing, same car same mods same fuel. Everyone is so quick to memorize what boost at what RPM, what does it taper to, etc. Then ask someone what their timing is at target WOT boost, at peak torque, at redline, only people who actually know what they're doing have any idea... now THAT is scary.

[DirtyDan]

My post was based on the same tune with less boost.

 

Most people don't have the tools or ability of tune their own cars, so trying to lecture me on the dynamics of tuning an engine with timing etc. is a waste of space. You're not going to impress me with crap like that. You mention nothing about injector pulse, injector impedance, fuel pressure, timing variables, etc. I can get into as technical a discussion as you want, but very few people on here can do anything with that information.

 

I have no intention of getting in a pissing contest with anybody about an economy tune on a car made for decent performance and reasonable economy. If you want maximum economy, then get a different car. Hybrid Honda's are kinda fun and way quicker than a gas version. Personally, I hate FWD.

[ClimberDHexMods]

I agree completely, except on certain points

 

A good small diesel. Would have to be something inexpensive or the OP will spend more on the car than the savings in fuel costs. Even if the trade is only +$10k, it takes a bit for a 20mpg car to surpass a 35mpg car in fuel expense by $10k. Actually 37,500 miles, at $4/gallon. So for every added 37.5k miles of driving under ownership/lease, add $10k of net PITI expense to your car. That is principle, interest (if applicable), taxes (& registration), insurance. I don't mean to nerd out this thread so much, but yeah. You can make it slightly more accurate with NPV

 

And then the fun factor, trying to value a LGT experience vs other cars...

 

Or just learn to hypermile.

[ehsnils]

you guys are complaining about $4.2x...

 

its like $4.40 to $4.5x here at least

 

Try $9.20... :mad:

[ClimberDHexMods]

Try $9.20... :mad:

 

But your country is much more healthy... balances out :lol:

 

I'll blow that extra $6.20 per gallon savings on dialysis later.

[jhawk222]

I got lucky with $4.28 for 93 octane yesterday.

[BAC5.2]

My post was based on the same tune with less boost.

 

Still wrong. Less boost doesn't mean you can run lower octane. The ecu will correct for detonation, but detonation has to occur first. By the time the ECU compensates, damage has occured.

 

Put 87 in the car, and even at low boost you'll be breaking things. Period. Otherwise people with SI-drive cars could run 87 in i-mode. No one does. For a reason. Its a bad idea.

[ClimberDHexMods]

Still wrong. Less boost doesn't mean you can run lower octane. The ecu will correct for detonation, but detonation has to occur first. By the time the ECU compensates, damage has occured.

 

Put 87 in the car, and even at low boost you'll be breaking things. Period. Otherwise people with SI-drive cars could run 87 in i-mode. No one does. For a reason. Its a bad idea.

 

Let's leave it alone. There's no hope of this ending up an intelligent discussion where everyone runs along having learned something. That and this exact discussion has taken place in depth within this forum, so we only need to search a little to see this discussion, instead of reliving it all over.

[goneskiian]

I've got a Shell about a mile from my house that offers Premium for Regular price on Mondays.

 

I also shop at QFC grocery stores and use their "club" card and get another $.10 off per gallon (http://www.shell.us/home/content/usa/products_services/on_the_road/fuels/grocer_rewards/qfc/).

[BAC5.2]

Let's leave it alone. There's no hope of this ending up an intelligent discussion where everyone runs along having learned something. That and this exact discussion has taken place in depth within this forum, so we only need to search a little to see this discussion, instead of reliving it all over.

 

You're right, I keep forgetting that intelligent technical discussion is not appreciated on this board. Silly me.

 

We were in Washington DC last night. The highest price I saw? $4.899/gal for 93 octane.

[ClimberDHexMods]

Chances would be better, for a start, if this were in the tuning forum, where people interested in tuning do check in and make an effort to keep up quality. This main dumping ground section is usually beholden to oil discussion, turbo timers, "Help me spend $500", "I have question and haven't discovered google yet", and HELP my car is broken! Which is all well and good. This forum is nothing more than its members and some fancy buttons.

 

Almost $5/gallon?!?! Too bad it won't change commuting behavior. A traffic jam yesterday is a traffic jam tomorrow.