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Old 01-28-2014, 11:50 PM   #1
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FUEL + TUNE

Okay so I know you can retune your car at any time. and you can use different tunes depending what fuel your using... now some of the gas stations around here only sell regular and don't have mid grade or premium... My question is... low low does your fuel have to be before adding fuel and changing your tune... if im running a tune for premium and I hit a gas station that has only regular... if I am not on complete empty will it affect the cars performance. hope that made sense lol
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Old 01-29-2014, 12:12 AM   #2
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Just keep a tune for regular gas on your handheld along with your tune for better gas. That way if your in that situation it's just a min to switch

But to answer your Question I don't think anything will happen if you go easy on it till you get the right octane.

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Old 01-29-2014, 12:21 AM   #3
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I guess My question is
if and when changing tunes. Obviously draining your tank and lines is not practical. How much fuel mixed is acceptable... if im running a 93 tune and go to the gas station with a quarter tank and fill up with 83. cant I switch my tune at that time? and vise versa.. if I have a quarter tank of 83 and fill up with 93 can I retune to 93 while having a mix of both fuels
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Old 01-29-2014, 12:33 AM   #4
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In short, no. You never want to fuel up with LESSER octane than what your car/tune needs. However, you can fuel up with MORE octane than what your car/tune needs, it's fine.

So in this case, you'd tune to 87 from 93 (with a quarter left of 93 in the tank), and then go to the gas station and fill up with 87. To go back to 93, you'd keep the 87 tune, drain the tank as much as possible, fill up with 93, finish that tank off too, fill up with 93 again, THEN tune back to 93 (to be safe).

Essentially mixing octane ratings will equal out to a rating in-between. For example, half a tank of 89 + half a tank of 91 may equal out to 90 etc... Experts correct me if I'm wrong.
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Old 01-29-2014, 12:43 PM   #5
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If you are running a 93 octane tune DO NOT put 87 in it PERIOD. Not sure why you'd think it would be ok.
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Old 01-30-2014, 05:09 AM   #6
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Man its like you don't read. Obv if I have a 93 tune im not going to be filling up wit 87 ...
What Im asking is when you do want to switch tunes weather it be from 93 to 87 or 87 to 93 how much of a fuel mixture is acceptable... because obviously I am not going to drain my tank and lines before switching tunes..
example: running a 93 tune driving down a highway and running low on gas.. quarter tank or less... and the gas station only offers 87 ... witch happens depending on where you are... can I fill with 87 and change my tune while at the gas pump?.... and vise versa... If say I am using a 87 tune and decide I want to use my performance tune or race tune that requires 93... do I have to wait until my tank is basically out of fuel before I switch tunes... because not very often do I have that low of fuel in my car... hope you can understand that I tried to simplify it.
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Old 01-30-2014, 05:57 AM   #7
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I can't really help, but I had kind of the same question and thought I'd add to yours. Would it be acceptable to throw in a bottle or two of octane booster? I've never been sure how much those little things actually "boost" the octane.

Say I had 1/4 tank of 89 left on the street tune, but wanted to head to the strip and change to the 91 race tune. Could I fill up with 3/4 tank of 91 and add the bottle of booster and be good? I think we have 1 station in town that sells 93 or else I'd run that.
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Old 01-30-2014, 08:22 AM   #8
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Originally Posted by KraftDinner View Post
Man its like you don't read. Obv if I have a 93 tune im not going to be filling up wit 87 ...
What Im asking is when you do want to switch tunes weather it be from 93 to 87 or 87 to 93 how much of a fuel mixture is acceptable... because obviously I am not going to drain my tank and lines before switching tunes..
example: running a 93 tune driving down a highway and running low on gas.. quarter tank or less... and the gas station only offers 87 ... witch happens depending on where you are... can I fill with 87 and change my tune while at the gas pump?.... and vise versa... If say I am using a 87 tune and decide I want to use my performance tune or race tune that requires 93... do I have to wait until my tank is basically out of fuel before I switch tunes... because not very often do I have that low of fuel in my car... hope you can understand that I tried to simplify it.
You can switch DOWN to the 87 tune with no issues, but when I want to run 91 octane and I've had 87 I run the 87 tune for one or two tanks while I keep refilling with 91
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Old 01-30-2014, 08:29 AM   #9
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that's kind of annoying haha but I guess at least then you know what is in the tank
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Old 01-30-2014, 08:33 AM   #10
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Also when running a 93 octane tune you should save your fuel receipts for at least a week or so when driving the thing. This way if a gas station is putting 87 in all their tanks like a bunch of jackasses (it happens) you have a piece of paper saying you got fuel from them on X date and the grade and the motor blew on this date.

You can report this sort of thing to the authorities and if they inspect the station and find they have 87 in all the tanks you have a great case to go to small claims court for a new engine.
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Old 01-30-2014, 09:49 AM   #11
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Also when running a 93 octane tune you should save your fuel receipts for at least a week or so when driving the thing. This way if a gas station is putting 87 in all their tanks like a bunch of jackasses (it happens) you have a piece of paper saying you got fuel from them on X date and the grade and the motor blew on this date.

You can report this sort of thing to the authorities and if they inspect the station and find they have 87 in all the tanks you have a great case to go to small claims court for a new engine.
Great tip.
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Old 01-30-2014, 09:59 AM   #12
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how easy would it be to actually get the fuel tested loool I donno if anyone would be willing to just go do that.
I usually only buy my gas from like a shell or brand name gas station and not the small ones
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Old 01-30-2014, 12:47 PM   #13
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I personally don't have a tune yet, but was wondering this same question and thought of a different approach. Can someone please confirm if my thinking is right.

What is you go to walmart, orielly's, etc. and buy some octane booster and put like a quarter of that (or whatever the octane point boost per gallon is) in say a quarter tank of 83 and then fill up the rest of the tank with 93?
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Old 01-30-2014, 01:04 PM   #14
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Originally Posted by 04beautyGT View Post
I personally don't have a tune yet, but was wondering this same question and thought of a different approach. Can someone please confirm if my thinking is right.

What is you go to walmart, orielly's, etc. and buy some octane booster and put like a quarter of that (or whatever the octane point boost per gallon is) in say a quarter tank of 83 and then fill up the rest of the tank with 93?

Just buy the correct gas needed. If you can't then tune the car down. Octane booster is worthless.
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Old 01-30-2014, 06:09 PM   #15
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If your tune is optimised for 91, or even 93, octane fuel--and you have to load up with 87 octane swill--then don't worry about it. Just stay away from any hard acceleration (70+% throttle) until you can run through the 87 and get some good stuff in there. If you hear any indication of pinging back off the accelerator NOW.

If you will have to run the swill for some time, then load a proper tune...
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Old 01-30-2014, 06:21 PM   #16
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Considering a 91 or 93 octane tune advances ignition timing, i would suspect if you would go back to 87 octane fuel with a 91 or 93 octane tune you would get some knocking and or pinging, poor performance and bad MPG. could even cause a lean condition but i think the fuel injection would work hard to adjust it depending on the type of fuel injection.

Just my opinion
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Old 01-30-2014, 06:25 PM   #17
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forget what i said listen to Cliffyk he is the man
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Old 01-30-2014, 06:45 PM   #18
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forget what i said listen to Cliffyk he is the man
Thank you for the kudos!

FWIW most 2V tunes, even those optimised for 87 octane, lean out the commanded mix in the Stabilised Open Loop Fuel table by "½ AFR" or a bit more because the stock tune is pig-rich at WOT.

In open loop operation fuel octane has nothing (OK, maybe a tiny bit) to do with changing AFR, in closed loop the O2 feedback will do all it can (up to ±25% more/less fuel) to keep the AFR at 14.7:1.

[edit]
Here is a typical "custom 87 octane" Stabilised Open Loop Fuel table compared to a stock tune. The top row in each TPS range are the custom tune values (changes values are highlighted):



You can see that at about 55% throttle the custom tuned has richened the mix for more "tip-in/mid-range" power, but then at 70% it starts to pull fuel at higher revs. This is to correct the overly rich stock settings.

I have never seen a canned or mail order tune that did not do this to some extent...
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Old 01-30-2014, 08:00 PM   #19
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Ok i knew i would ask a dumb question sooner or later but here goes, I am not understanding the table and the relationship between the tp voltage and rpm, i'm assuming tp is throttle position?
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Old 01-30-2014, 08:20 PM   #20
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The Stabilised Open Loop Fuel table provides the AFR that the PCM will command, in open loop mode¹, for a given throttle position sensor voltage and at the indicated RPM. The PCM does this by looking up the necessary value in the table (say 13.21:1 at 3.42 V TP and 4000 rpm) and then finding out from the MAF how many pounds of air are being drawn in to the engine.

It then calculates how long the fuel injector needs to be open (the fuel injector pulse width) to inject the proper amount of fuel to make that AFR a reality. IAT, ECT and a number of other inputs are also part of this calculation.

The TPS and MAF values (and as always some other stuff) are also used to calculate engine load.

Next the PCM is calculates the appropriate ignition timing for the calculated engine load, tps, air flow, AFR, etc. Once it calculates a number that value is compared to the value in the Spark Borderline Table--a table listing the maximum allowed spark advance for a given load and rpm. The lessor of the calculated or table value is used, then the Global Spark Adder and Global Spark Multiplier values are applied to arrive at a final ignition timing value.

It's not pure rocket science, but it is also not too far removed. It is also basically how all modern EFI systems work when the engine is running too fast and/or at higher loads when the O2 sensor feedback is too slow to be of any use...

------------------------------------------
¹ - As opposed to closed loop mode where the PCM uses feedback from the OO2 sensors to keep AFR at an average of 14.7:1.
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Old 02-01-2014, 08:14 PM   #21
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Ok thanks i kind of understand, I've search fuel tables and have been read a little about it, it's pretty cool how all of this works and how reliable it is. a long way from mechanical fuel injectors.
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