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View Full Version : Mixing Gas to Get Higher Octane Thread


uwhusky
07-26-2012, 09:16 AM
I've noticed a couple of threads, mostly around Cobb and JB4 tunes, that have been side-tracked in to talking about how to get the octane gas needed to run various maps.

So instead of hijacking those threads, I thought it worthwhile to have just a thread about how people are attaining their octane needs.

I'll start first... :)

Where I live, the best you can get at 99% of gas stations is 91 octane. I'm a little leary of mixing E85 and 91 together to achieve the desired results but today I found a gas station near where I work that carries Unleaded 100 octane at the pump.

My plan is to do a 50/50 split of 91 and 100, which would give me roughly 95. This is simple and easy from a math perspective when I fill up, if I put 8 gallons in of one I just do 8 gallons of the other or whatever.

Alternatively, I could mix 88 octane with 100 and still be at 94 or
do a different ratio to try and get closer to the 93 I'm looking for. My thinking is higher octane is probably fine but anything lower than 93 (when running the aggressive maps) is bad.

Any thoughts on any of these approaches or please share your stories on how you're getting higher octane gas, > 91.

Oceans10
07-26-2012, 04:13 PM
You should always run at least 93 octane with a tune to avoid early detonation at high compression rates. This is known as knock, very bad for the engine and the better tunes will have built in prevention against this. But it then affects performance adversely.
As a rule use the highest octane you can find. Also an 87 or 91 may not actually be what it claims to be. There have been lots of exposes of scams where gas stations mix inferior gas with what you think you are buying. BMW recommends minimum 91 octane as well, so why mess with that.

I am running 93 octane Shell with 40% E85. That gives me an octane rating in the 98-100 range. I am guessing it adds 20-30 hp, but the car is much more responsive and rev happy. Higher octanes burn cooler as well, so it is a win-win for the engine.
I also experimented with nearly pure Shell URT Advanced, which is 110-115 octane. Pure crack! But damn expensive. $70 for a 5 gallon drum. I was actually concerned about my transmission holding up.

PolkNole
07-26-2012, 04:21 PM
Yes, it's that simple…blending 91 with say 101 50/50 will give you 96 octane.

TLO03
07-26-2012, 06:06 PM
If you have a Cobb the make maps for lower octane, guys a the west coast that are tuned run 91. With all the major tunes.. Back in the jb3 days I wasted money on 100 oct with no mods besides a tune , I wasn't logging back then but I didnt notice any difference. 100 oct is around $10 gal here last time I checked.

If it was me I'd run e85 it's been tested for awhile now, and a tech told me its cool if blended same affect ,higher octane and it's way cheaper than 100oct.. No matter what you do log. And that will tell you how your engines doing. In Ct your 50/50 blend would be $120 with e85 it would be $56 gas milage would suffer but it's still less than 16 gal of 93oct. Take the extra cash and save for down pipes.. Juss my thoughts.

GOR777
07-27-2012, 01:26 AM
Once I mixed 10 gallons of 100 with 6 gallons of 91. Very noticeable power, way higher range. Sometimes its worth it ;)

MachtSchnell
07-27-2012, 05:37 AM
So can we add to this post mixing of E85 to get desired Octane. I know Peder is an advocate. What has been other peoples experience, long term is it going to effect seals and gaskets it comes in touch with in a mix ratio of 40/60 or 50/50? I live in the south some what in the country so lots of grain fed ponies but is it okay to feed the ponies under the hood?

uwhusky
07-27-2012, 08:32 AM
So can we add to this post mixing of E85 to get desired Octane. I know Peder is an advocate. What has been other peoples experience, long term is it going to effect seals and gaskets it comes in touch with in a mix ratio of 40/60 or 50/50? I live in the south some what in the country so lots of grain fed ponies but is it okay to feed the ponies under the hood?

Yes, mixing E85 with 91 is a perfectly legitmate way of getting to your desired octane level, IMO.

I personally haven't tried it and if that was my only option I would try it. From what I've read to achieve a good amount of octane 94+ you should mix about 40% E85 and 60% Oct91 but I don't have the charts or the formulas.

wyb
07-27-2012, 10:21 AM
Yes, mixing E85 with 91 is a perfectly legitmate way of getting to your desired octane level, IMO.

I personally haven't tried it and if that was my only option I would try it. From what I've read to achieve a good amount of octane 94+ you should mix about 40% E85 and 60% Oct91 but I don't have the charts or the formulas.

You don't need a chart - but here is how to work out the relative octane...

Fuel 1:

Octane = 91
Gallons = 10

Fuel 2:

Octane = 101
Gallons = 6

Mixed Octane = ((fuel1Octane x fuel1Gallons) + (fuel2Octane x fuel2Gallons) ) / (total number of gallons)

Example above:

Mixed Octane = ((91 x 10) + (101 x 6)) / (10 + 6)

or (910 + 606) / 16

or 1516 / 16 = 94.75

If you want to get fancy - sign up for a Google docs account (free) = put the formula into a spreadsheet and save the location (url) as a shortcut on your smartphone - you can then use it "on the go" but entering 4 numbers - saving the sheet and hitting refresh (the doc recalculates when you do this).

Oceans10
07-27-2012, 10:32 AM
The higher the octane the better. Provided the tune can handle the octane increase and make hay with it.
The reason for limiting the mix to 40% is to minimize corrosive impact on seals and gaskets. Regular gas has 15% ethanol. The 40/60 mix will raise that to around 30%.
There are no readily available studies indicating that 30% ethanol wears down seals faster, it's just a rule of thumb that has emerged from hundreds of users in the tuner community over the past couple of years. There have been no reports of fuel pump failures or any other negative events associated with higher ethanol content. We are pushing the performance envelope a bit while paying heed to BMW's concerns regarding ethanol and their long history of HPFP problems.

Seeking more power responsibly would be the motto.

uwhusky
07-27-2012, 12:41 PM
The higher the octane the better. Provided the tune can handle the octane increase and make hay with it.
The reason for limiting the mix to 40% is to minimize corrosive impact on seals and gaskets. Regular gas has 15% ethanol. The 40/60 mix will raise that to around 30%.
There are no readily available studies indicating that 30% ethanol wears down seals faster, it's just a rule of thumb that has emerged from hundreds of users in the tuner community over the past couple of years. There have been no reports of fuel pump failures or any other negative events associated with higher ethanol content. We are pushing the performance envelope a bit while paying heed to BMW's concerns regarding ethanol and their long history of HPFP problems.

Seeking more power responsibly would be the motto.

This is if you're mixing with E85, correct?

When you say regular gas has 15% ethanol, where are you getting that? Most of my local gas stations have a sticker on the pump saying this gas "may contain up to 10% ethanol", which to me means 10% max and it may contain less.

wyb
07-27-2012, 01:21 PM
This is if you're mixing with E85, correct?

When you say regular gas has 15% ethanol, where are you getting that? Most of my local gas stations have a sticker on the pump saying this gas "may contain up to 10% ethanol", which to me means 10% max and it may contain less.

You are in a mountain state, the same as me - I believe we get less ethanol for a reason...

EPA rules allow UP TO 15%, but only for late model cars:

http://content.usatoday.com/communities/driveon/post/2010/10/epa-to-allow-15-ethanol-in-gasoline-up-from-10-now-/1#.UBL3xfXNm4Y

booyaazaa
07-27-2012, 06:22 PM
You are in a mountain state, the same as me - I believe we get less ethanol for a reason...

EPA rules allow UP TO 15%, but only for late model cars:

http://content.usatoday.com/communities/driveon/post/2010/10/epa-to-allow-15-ethanol-in-gasoline-up-from-10-now-/1#.UBL3xfXNm4Y

Yep. The ethanol content varies by area so you just need to pay attention to the sticker if you are trying to be exact. Also there are tons of calculators out there that help you with calculating how much e85 to mix with gas to achieve the desired octane rating. http://www.intercepteft.com/calc.html

MachtSchnell
07-27-2012, 06:27 PM
What about the long term affects of the ethanol running through engine and components.

wyb
07-27-2012, 08:39 PM
I know it is trivial to find an octane calculator but that won't teach him the simple mathematics which would be far preferable to using something prepared by someone else! :tsk:

Oceans10
07-28-2012, 05:26 AM
What about the long term affects of the ethanol running through engine and components.

Ethanol is corrosive and will weaken seals and neoprene hoses faster than gasoline. However, regular gas already has up to 15% ethanol, so obviously all engines can handle some ethanol. The GM Flex Fuel engines can handle straight E85!
The question is at what rate does it cause damage? I have not seen any definitive studies on this, but many tuned BMWs have been running a 40-50% mix for several years without any discernible impact to fuel pump, injectors or lines. On the combustion side there are several benefits. It burns cooler and cleans better. That means more deposits in the gas tank are broken up and get trapped in the fuel filter. It is therefore advisable to periodically change it. ECS recommends every 60k miles w regular gas, BMW says 80-100k. Here's a link with picture, I believe the 5 series is identical, just has a different part number. http://www.ecstuning.com/BMW-E90-335i-N54_3.0L/ES912/ES36230/
(I am showing this since the 535 filter has no photo.)

The US seems destined to raise the ethanol content in pump gasoline, see the following congressional study: http://www.fas.org/sgp/crs/misc/R40445.pdf