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Old 04-01-2005, 07:23 PM   #1
ShawnRS
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Question STI: What if tuned for lower Octane

Hi there,

I know this wll deemed a stupid question but...

Say, I really want an STI (only) but I like to take long road trips to out of the way places. Many of the gas stations best gas is 91 octane. But few up north (Say NWT) only carry 89 at their single pump.
What if I had the STI tuned for, say, 89 octane. How much performance would be lost?

This is only a question.

Thanks
Shawnrs
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Old 04-01-2005, 07:50 PM   #2
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You'd probably want to not run much boost, so you could try some sort of map that ran low boost, and slightly retarded timing and get away with it.
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Old 04-01-2005, 09:44 PM   #3
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The difference if you could find a tuner and gas to do so would be about -10WHP or so. That's just a guess on my part though as the difference between 91 and 93 octane tunes is usually only 5WHP.
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Old 04-02-2005, 01:57 AM   #4
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water injection should solve the lower octane problem
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Old 04-02-2005, 04:15 PM   #5
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Quote:
Originally Posted by Unabomber
The difference if you could find a tuner and gas to do so would be about -10WHP or so. That's just a guess on my part though as the difference between 91 and 93 octane tunes is usually only 5WHP.
That's true, but it's because peak HP occurs very high in the RPM range. Lower octane will mean that the power curve must be much flatter thru the mid-range, where peak cylinder pressures are highest (and must be reduced to avoid detonation). Torque numbers will become much lower, HP numbers will not be as strongly affected.

You might consider buying a bunch of Torco (octane booster). It is very effective in quantities of around 32 oz. per tankful. It's not the stuff they sell in little bottles at the auto parts store. Torco would probably have to be ordered and delivered to you.
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Old 04-02-2005, 08:20 PM   #6
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Wow no harsh comments. Thanks

I read on some forum that guys have added an alchol injection system, some how to their fuel system (someplace). Would this allow for the use of lower Octane? Because as said above the car only needs it when one is in the higher revs.

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Old 04-03-2005, 01:05 AM   #7
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The car doesnt only need octane at specific rpms. Alcohol would work, and so would water injection, but torco would probably be your best best for trips.
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Old 04-03-2005, 04:03 AM   #8
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Quote:
Originally Posted by santofontana
The car doesnt only need octane at specific rpms.
That's a fact. Octane requirements are highest when cylinder pressures are highest. Highest cylinder pressures are generally around the RPM range corresponding to peak torque. Figure about 3500-4000 RPM on a WRX or STi.
Quote:
Originally Posted by santofontana
Alcohol would work, and so would water injection, but torco would probably be your best best for trips.
Agree, for several reasons. Water or alcohol injection will void your warranty, aftermarket engine management will be required, and this is not a plug and play solution. It's not a good choice if your car is not also your hobby, IMHO.

Toluene and xylene are very effective octane boosters, and perhaps the most commonly used, but they require at least 4 times the volume of liquid as Torco for a given amount of octane increase. That makes them impractical and somewhat dangerous for long trips. I've never used Torco, but there have been several good discussions about it on this board.

Finally, an answer to a question you didn't ask: There are many cars better suited to long trips thru 89octaneland than the STi. A normally aspirated car that makes it's power with large displacement is generally more tolerant of low octane than a turbocharged car.




Edit: I mistakenly wrote this as if it was still in the n00b&faq forum, where it was originally posted. Had I remembered where this was going, it would not have sounded so much like IrvingTheExplainer.

Last edited by 2Stroke; 04-03-2005 at 06:02 AM.
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Old 04-06-2005, 08:53 PM   #9
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I just wanted to thanks to those who answered my questions and through some options out there.
See I only do about five boony land trips a year. The rest of the time I am in the city. I only have the funds and parking for one car. Thats why I specified the STI. It fills all my wants (Fast, three diffs, practical and looks) and is OH so close to being affordable. But alas the STI will not work for me until I can afford two cars, one being an STI.
I'll stick with my RS.

Shawnrs

[quote=2Stroke]That's a fact. Octane requirements are highest when cylinder pressures are highest. Highest cylinder pressures are generally around the RPM range corresponding to peak torque. Figure about 3500-4000 RPM on a WRX or STi.
Agree, for several reasons. Water or alcohol injection will void your warranty, aftermarket engine management will be required, and this is not a plug and play solution. It's not a good choice if your car is not also your hobby, IMHO.

Toluene and xylene are very effective octane boosters, and perhaps the most commonly used, but they require at least 4 times the volume of liquid as Torco for a given amount of octane increase. That makes them impractical and somewhat dangerous for long trips. I've never used Torco, but there have been several good discussions about it on this board.

Finally, an answer to a question you didn't ask: There are many cars better suited to long trips thru 89octaneland than the STi. A normally aspirated car that makes it's power with large displacement is generally more tolerant of low octane than a turbocharged car.
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Old 04-07-2005, 01:19 PM   #10
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Quote:
Originally Posted by ShawnRS
See I only do about five boony land trips a year. The rest of the time I am in the city. I only have the funds and parking for one car. Thats why I specified the STI. It fills all my wants (Fast, three diffs, practical and looks) and is OH so close to being affordable. But alas the STI will not work for me until I can afford two cars, one being an STI.
I'll stick with my RS.
If you know that you're running lower octane than specified, it's pretty easy to avoid detonation prone situations. Note that the ECU will eventually tune itself around the lower octane as well, but you will get some detonation while it does so.

The easiest way to avoid detonation is to use light throttle and let the engine rev higher to make power. Avoid anything more than half-throttle and more than 5-7 PSI of boost.

For example, when driving normally you may use 5-10 PSI of boost and shift around 3500 RPM. Instead, try to keep out of boost (5PSI max) and shift higher, say 4000-4500 RPM.

Adding a KnockLink or similar will also help you monitor conditions which cause the engine to detonate so you can ease up before possibly causing engine damage.

Get the STi if you can afford it!

-Dave
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Old 01-28-2009, 12:32 PM   #11
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not to resurect a dead thread or anything but this came up in a search and i just got an idea. it seems like it might be a complicated tune but what if we were to come up with a way to completly vent boost pressure, like an over-rideable BPV, and then have the engine tuned to run NA, i have the accessport so it could just be one of the maps. like base wastegate presssure to keep the turbo from overspooling (if that's even possible/a problem) sure, we would be sacrificing fuel economy, and severly sacrificing performance, but the lower compression ratio of the turbo engine should allow us to use the cheapest gas around while it's on the non turbo map. could we make this effective? i figure a 300 HP car minus the turbo would still make around 120-160HP (on a very low estimate), so it would baisicly be an impreza or civic?

any ideas. now if i only had a cool tuner or the cash to throw around for multiple protunes...
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Old 01-28-2009, 12:45 PM   #12
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you'd have to have the WG held open so boost couldnt build and then the timing maps would need to be re-done to compensate

it can be done but its kind of silly. on a 2.0 it would be ridicously sluggish, on a 2.5 it sounds alittle more plausible
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Old 02-13-2009, 09:01 PM   #13
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yes it would be sluggish but if it lets you use 85 octane doing long commutes it might be worth it. think mpg and economy. then when you fill it up with REAL gas , just switch maps to your stage 2 or whatever. just an idea, i seem to understand that turbo engines are generally lower compression than na motors(i know in the end it's higher because of the turbo's pressure forcing half an atmosphere of extra air in(15psi), but if we vent the turbo pressure then it's back to na.

just wondering if any tuners would be willing to give it a shot, like put a filter on instead of the BPV/BOV and tune it to work as best as they can with cheap gas and no knock.
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