Gas situation.. need your thought.

[Conga1991]

So just moved down to Arizona. My Dad decided to fill up my tank, thinking that he knew I use Premium, but puts Unleaded in. I have a 7mgte by the way, pretty much stock besides fmic and exhaust. Would running that kind of gas until its used up be bad for my engine?? I haven't noticed a difference while driving it other then I'm getting much better gas mileage, hopefully its not running lean ): I dont have an A/F sensor/gauge so I wont be able to find out. Just want your thoughts if running this gas is bad or not?

[cre]

Different gas grades aren't going to affect your A/R at all. Lower octane fuel has more kinetic energy and is less stable... You'll most likely see better mileage (as with most older cars) and slightly less power. As for options: 1) baby it until it's empty 2) add two to three gallons of Toluene (available at Home Depot, Lowe's, Ace) to significantly raise the octane.... You'll still need to baby it until it has time to thoroughly mix in.

[Conga1991]

Cre! You've done it again, like always!! The Guru for answering almost every question presented on here! When putting the additive to the tank should I put some premium gas in it to help dilute it? or should it be fine by just adding it in without adding gas?

Thank you in advance!!

[cre]

You're not going to be able to get enough premium in there with a tank already full of regular and a couple gallons of toluene. Don't worry about topping it off if it's already quite full. Actually, it'll mix better if there's a little room for it to splash around. Worst case scenario is that you end up running 105 octane for a a couple hundred miles.

How much fuel is in the tank? I had assumed 3/4 or more. If it's less you'll want to adjust how much toluene you add accordingly.

[Conga1991]

Bringing this post back up from the dead. You said that with lower octane gas raises mpg. if I use Unleaded gas and mix with toluene will that still improve my mpg or will it be the same as if using premium gas??

I am currently using premium and im not even getting around 20mpg. I mainly drive in town and only use the freeway for less then 10 mins a day to get to school.

What are some ways to improve my mpg?? I am using used stock injectors that have about 180k on them, upgraded fuel pump to walbro 255, upgraded turbo to 57 trim, driftmotion intercooler set-up, and 3.5 inch exhaust with ddp. Not sure if this can be the cause but whenever I hit WOT in 3rd or higher I hit fuel cut, doesnt happen in 1st or 2nd though. cel comes on and its the knock sensor or the wiring, pretty sure. Hope this helps and whoever can give their .2 it is greatly appreciated!

[cre]

I'll post more later. But for now, general rule of thumb: Higher octane = less energy... It has less kinetic energy, not to be confused with the engine's being able to make power. Ideally, as I've said before, you want to run the lowest octane possible for your setup with the least amount of ping. Running 105 octane fuel on a stock 7M-GE will result in terrible mileage and even worse performance.

It's a balancing act and there's no simple cheat beyond keeping combustion chamber temps down (Ceramic coated air ducts, good IC, air filter not in the engine bay, EGR system intact, coasting with the clutch out, water/meth injection) and then run only the octane you need. I live a mile higher than sea level... I've run 87 octane on a few higher rated vehicles and found more response with no discernible difference in the data logs as far as the ECU adding or pulling timing.... Now going from a recommended 93 to 89 or 87 is something that's safe here due to the altitude and you can find publications by various manufacturers and oil companies stating what octane to use for a given altitude. Factory recommendations assume sea level. Many newer vehicles are better able to ride that performance threshold though and are able to make good power while maintaining equal mileage in which case I'd suggest sticking with premium if you can afford it.... but the MKIII isn't that smart. Normally I still run 91 octane (or 89 when I can find it. 91, 87 and 85 are the common ones here) as it's cheap insurance and I'm not trying to squeeze every bit of performance or mileage out of my car.

A code 52 will SLAUGHTER your mileage! Fix it ASAP.

Seriously though, your mileage doesn't sound off to me at all. I don't know how your driving style is but city is always the worst.

Do you have any means of fuel management installed? If not, add that and tuning to the list.

[pandrade]

so adding toluene to 87 octane gives you both benifits of the better mileadge and octane boost?

what would you recomend for street 87 and the toluene mix, what should i use for autocrossing weekend?

do you know any articles that i could read up on ( got me intristed in this. old guys at my shop say they mix acetone it was like 1 oz for every 2 gal. Im not trying that unless i actualy read about it first)

[cre]

I'm tired as hell and have to get up early so I'm posting this as is... If there are any inconsistencies I'll correct 'em tomorrow at some point.

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No, adding pretty much any fuel stabilizer will reduce your mileage.

The only way that some vehicles see improved mileage from higher octane fuels is with modern computer controlled valve timing, being able to lean out the fuel mixture by selectively increasing the amount of gas fed back via the EGR system and a much broader control over the ignition advance. EGR control also allows a virtual reduction in engine displacement. Exhaust gas has nothing in it to burn, it's inert. So, pump 1 liter into a 2 liter engine each cycle along with the 1 liter of viable mixture and PRESTO! You're now driving a 1 liter engine! And it's running much cooler so you can inject a little less fuel too... BAM! 1 liter, low consumption. BUT when you stomp on the throttle it's miraculously a rich running 2 liter turbo charged monster leaping from 40hp to 300hp... 60MPG to 10MPG. Not a real world example but not an exaggeration. This is what many auto manufacturers have done to achieve some of the wicked mileage you see on reasonably powerful cars today. You run a lower octane in these newer cars and they take a hit on both ends... can't run so lean and can't push as hard.

For most common pre2000 cars you can forget the thought of higher octane improving your mileage unless you're running too low of an octane already. Run too low and you get detonation/ping, the ECU pulls timing and ADDS more fuel to get the engine running safely again and thus you get worse mileage. Again: It's a balancing act.

Higher octane is SAFE... And pretty cheap insurance, but there are reasons why you may not want to run it. On my MKIII I ran an AFR of 16.1:1 at lean cruise, I think about 15.3:1 regular closed loop cruise and I think the idle was 15:1 or so. On a road trip my mileage on 87 octane was crazy. Of course it was also tuned for some performance and the minute I started driving aggressively 30% throttle or higher it would go to 13:1 and richer and richer as it started boosting and my mileage would plummet. :P Now on this car with a standalone or advanced fuel piggyback you don't want to run so lean if you're also running low octane fuel... balance. You also do NOT want to run a very lean cruise if your EGR is not functional and unobstructed... you NEED that extra cylinder cooling. I was running the MAFT Pro, for those who are going to ask.