How to blend 91 Octane in 87/89/93 Octane pump

[pacem]

I am getting a PCM tune for 91 Octane, the engine is a high-compression LT1 in 96 Buick. Problem is, in my state, all you usually see is 87/89/93 Octane gas. I understand that 50/50 mix of 89 and 93 will get you 91 Octane. Problem is, how do you do it? You have to estimate how much gas you need, then divide that by 2. Furthermore, once you pump the first half of 93, you have to hang the pump, authorize your CC or DB again at the pump and pump the remaining half. I use DB so it gets very annoying having enter your PIN twice, etc. Is there any way I can not hang up the first nozzle, just lay it aside and remove the second (i.e. 89 Oct.) nozzle and use that? Or do you wait until the tank is half empty and just keep alternating 89 and 93. Or do you mix 89 and 93 at home? Seems like a PITA.

 

[expat]

How much more $ would 100% 93 cost you?

 

[pacem]

It's not that it would cost more but that it wouldn't do anything, if PCM is tuned for 91. It would advance or retard timing based on the gas it gets. I thought about getting a tune for 93 but the problem with that is that 93 Oct is not available everywhere. CA being one example, with 91 the highest you can get in that state.

 

[CivicFan]

I used to alternate 93 and 89 with a quarter of a tank at a time. But filling up every other day and remembering which car had what gas in it previously got burdensome very quickly. Now I pump 93 only.

 

[lexus114]

Sunoco still sells 91 octane dont they ?

 

[XS650]

Sounds like more trouble that it's worth, but since it's your labor and not mine... There is no simple formula that tells you the octane of a combination of two octane fuels, it depends on the chemistry of the fuels and it isn't usually a simple mathematical average. Good news is that the resultant octane is usually a little bit higher than the simple mathematical average. If you want somethng half way close to an average, you will need to fill the tank 1/2 and 1/2 each time. If you run the tank down to 1/2 and top it off with one of them, you won't be any where close to an average.

 

[pacem]

I wonder if it's possible to get a dual tune, 91/93, if it would detect which one is being used, or get a 93 tune and run 91 in it. LT1 is a high-compression engine and while you can run 87 in it, if you tune the PCM so it doesn't knock, given the knock sensor, it's safer to run at least 91. 87 retards timing and you actually lose power and MPG.

 

[Quattro Pete]

Originally Posted By: pacem

It's not that it would cost more but that it wouldn't do anything,

Sure it would. It would save you the trouble of having to blend two types of gasoline. Personally, I'd just run 93 and not worry about it.

 

[getnpsi]

using 93 wont hurt a thing, my car is tuned from factory assuming premium is 91. If you get a batch of worse quality gas then you have more of a safety buffer than us californians do. enjoy your new tune(s).

 

[willix]

Over 2.65 for reg the prices graduate about 10 cents up for mid grade and another 10 cents for prem. 20 cents X 20 gallon = 4.00

 

[JHZR2]

I just generally swap between the two, or just use 89 octane in my application, which is designed to advance up to 90 octane.

 

[pacem]

Originally Posted By: Quattro Pete

Originally Posted By: pacem

It's not that it would cost more but that it wouldn't do anything,

Sure it would. It would save you the trouble of having to blend two types of gasoline. Personally, I'd just run 93 and not worry about it.

The problem with that is there are vast areas that don't have higher than 91 Octane, CA or AZ being prime examples, or Nebraska. Plus some mountain states where "Regular" is 85 Octane. I am not in CA so it's not an issue for me - yet, as the trends usually spread from CA to the rest of the states. I wouldn't be all that surprised to see 93 disappear in MD, too. Then I would have to reflash my PCM. What happens if your PCM person is unavailable? DIY? Plus if you travel and are unable to find 93, you are in trouble. The easiest thing to do would probably to keep 2 PCMs, one tuned for 93 Octane and the other for 91. Swapping them is trivial.

 

[willix]

You need to find the balance between the per gallon saving- loss of mpg cost- swiping the debit card twice transaction fee. see what is most reasonable of the 3.

 

[JustinH]

Gas stations only carry two grades of gas, 87 and Premium (91, or 93). The mid grade gas is a blend of the two grades. Here in NY there are no debit card transaction fees, but in some states there are. We just pay more for our gas!

 

[Shannow]

Originally Posted By: XS650

Good news is that the resultant octane is usually a little bit higher than the simple mathematical average.

That was the good ting when Oz was converting to unleaded. The lead in premium, spread across itself and high octane unleaded gave significantly more than the average, nigh on 100 (RON)

 

[pacem]

I hear reports of people with high compression engines tuned for 93 Octane running 87 on a daily basis and putting in the expensive performance stuff when needed. Maybe I can have it tuned for 93 and put in 89 on a semi-regular basis.

 

[Scott_Tucker]

That depends on whether you have a closed loop ignition system. If you have knock sensors you likely can but it won't be optimal.

 

[pacem]

Yeah, I have a knock sensor but I would rather not rely on it. I would be happy with 91 tune but the key problem is, I cannot find it anywhere in MD. It's 87/89/93.

 

[Scott_Tucker]

You could buy a drum of it from out of state.

 

[sxg6]

If you're getting it tuned for 91, I would use 93 on a regular basis and not worry as others have stated. What surprises me is that you're reluctant to do that, but entertaining the idea of having it tuned for 93, and running 89 on a semi-regular basis?! Yeah it might work for some, but as you said you don't want to rely on your knock sensor and that is playing with fire if you ask me. Tuning for 91 and running 93 will be just fine.