What octane do you run?

Daryl · Aug 31, 2022

Owner’s manual says 87 octane. Same for a 347 stroker??

Chip66 · Aug 31, 2022

The owner's manual is referring to a stock engine.

You're no longer stock, so it's based on the build. The right octane for the jobs becomes a complex question real fast.

In most cases, a normally aspirated engine can always be tuned to run 87. However, that isn't necessarily going to give the best performance or economy. 87 octane has the most energy per gallon, so set up properly can make the most economy. (I'm guessing that SoCal gas prices lead you to this question.)

Dynamic compression ratio is what ultimately dictates your octane need. There are many calculators out there. If you have enough information about your build, you can get an accurate enough estimate.

joemomma · Aug 31, 2022

Yeah it's likely going to be different for everyone, depending on their specific build. What compression ratio is your build? Higher compression ratios are going to require higher octane.

Personally, I ran 87 until I advanced my stock timing to around 13 degrees. Now I run 93. I may actually bump the timing back down to 12 degrees or so, the benefit is minor compared to the penalty for running premium gas currently. 93 was about $1/gal higher than 87 at my last fill up. Granted, I don't drive it that much, but I don't see the benefit honestly, especially on an otherwise stock engine.

Daryl · Aug 31, 2022

No 93 octane in CA. I started out running 91(highest available), then switched to 87(after reading in the Owner’s Manual). I’m getting better mpg with the 87 actually. But I may run the 91 again to double check.

My build: Scat rotating assembly, Crane cam and AFR 165 heads. I don’t remember what they told me the compression ratio is, but maybe you can ascertain by the components??

RAU03MACH · Aug 31, 2022

start stocking up on octain boost

RAU03MACH · Aug 31, 2022

i would move out of California myself , no gas power anything

Daryl · Aug 31, 2022

RAU03MACH said:
start stocking up on octain boost

I hear ya, but with the price of gas here in SoCal, the last thing in mind is to add to the cost! Like the idea, wish I was rich or won the lottery, but I’m not and I didn’t!

badass98svt · Aug 31, 2022

I run 93 here in MA
If that were my 347 it would have the highest octane available at the pump. Is there no new tune on your car?

J_lope82 · Aug 31, 2022

Bald eagles straight out of the oil well

joemomma · Aug 31, 2022

Do you notice any ping/spark knock on the 87?

Daryl · Aug 31, 2022

joemomma said:
Do you notice any ping/spark knock on the 87?

No. No knocks, pings, etc.
But still will resort back to the 91 octane and see if I notice any difference

Adfalenski · Sep 1, 2022

I am running a stock engine and have a noticeable improvement in performance from running high octane rather than 87 octane. It took a while for the ECU to remap but that is what the sensors are for.

Connie · Sep 6, 2022

We use 93 Octane in our 'Stangs. There is NO Ethanol in 93, as there is in 87, and I think 91(up to 10%) that evaporates out before the next Fill-up. Especially if you don't drive as much as we once did. I also believe Ethanol invites moisture into the Fuel System. My beliefs on this may be wrong, I may need to be corrected about this, but I don't want anything in my Fuel, but Fuel. And we also have a Tune in both our cars. BOB.

badass98svt · Sep 6, 2022

Connie said:
We use 93 Octane in our 'Stangs. There is NO Ethanol in 93, as there is in 87, and I think 91(up to 10%) that evaporates out before the next Fill-up. Especially if you don't drive as much as we once did. I also believe Ethanol invites moisture into the Fuel System. My beliefs on this may be wrong, I may need to be corrected about this, but I don't want anything in my Fuel, but Fuel. And we also have a Tune in both our cars. BOB.

Where are you buying 93 with 0 ethanol?

duh09 · Sep 6, 2022

Realistically, there's not much reason to run higher octanes unless you're facing detonation issues. Corn gas vs non corn gas is another discussion altogether but if the car is running fine on 87, then I wouldn't sweat the extra money for 91. Higher compression, bumped timing, etc could create the need for higher octane but if 87 works, it works.

RAU03MACH · Sep 6, 2022

Just depends on what beans are on the shelf
Refried beans gives me boost
Red kendny beans don't do much

All kidding aside I do stock up on octane and mix
We have 91 and that the high

TrickVert · Sep 7, 2022

I use E0 91 in mine almost exclusively because it sets more than it's driven, and the E10 fuels aren't as stable. The 91 octane is required for my boosted tune. As stated above, though, if your car is tock and/or doesn't ping on 87, you're wasting money buying anything higher. "Running a tank of the good stuff" to keep things clean, etc. is a myth.

b1pig · Sep 9, 2022

Connie said:
We use 93 Octane in our 'Stangs. There is NO Ethanol in 93, as there is in 87, and I think 91(up to 10%) that evaporates out before the next Fill-up. Especially if you don't drive as much as we once did. I also believe Ethanol invites moisture into the Fuel System. My beliefs on this may be wrong, I may need to be corrected about this, but I don't want anything in my Fuel, but Fuel. And we also have a Tune in both our cars. BOB.

You might want to test your fuel. Unless your are buying fuel from a very specific supplier, pretty much every fuel from mainline (Shell, BP, Circle-K, etc) has up to 10% ethanol no matter the octane. The only exception will be pumps marked as "Non Ethanol / Marine".

When ethanol blends first hit the streets, 87 octane got ethanol first. Later on premium fuels got it. (mid-grades are usually blended in the pump from the 87 and 91/93 available from the in-ground tanks)

You can check it using a clear plastic cup that is uniform in shape. (cylinder) Pour some water in the cup. Mark the water level. Add fuel... at least 3x the amount of water. More fuel will allow for more separation. (Example, 1/4 cup of water, 3/4 cup of gasoline. Make sure you thoroughly wash out your wife's measuring cup)
If there is ethanol in the fuel, the ethanol will separate from the gasoline. You will see the water separation in the fuel.... If there is no ethanol, the line you marked for the water you added will be roughly the same. If it's ethanol fuel, there will be a noticeable difference in the height of the water above the line you marked. I tested it at the two stations local to me, as we have motorcycles that sit for months at the time. There is ethanol in all fuel selections in my small town.
If you want a graphic representation, search "testing ethanol content in gasoline" on youtube... just pick a video.....

There are some non-ethanol sources out there. A blanket statement of "all premium fuels are ethanol free" is inaccurate.

In regard to the octane rating, Higher octanes resist detonation. Older engines that have a lot of carbon buildup or even defects in the head will get detonation knock or pinging at certain throttle settings when hot. When a more modern engine senses knocking/pinging, the computer will retard the ignition timing to compensate. Retarding the ignition timing will result in a minor-to-moderate loss of power. So if you have a 87 octane engine and experience improved performance when running higher octane fuel... that MIGHT be the cause. I run 87 octane in everything I have owned that wasn't turbo charged. I don't recommend it, but I have also ran 87 in the turbo cars from time to time. So long as you are not running it hard (ie: making lots of boost) you're "probably" not going to hurt the engine. Again, I don't recommend it. The only exception for me, is that in the summer, I run premium fuels in the motorcycles we have owned which are air cooled during the summer. Not for performance, but to reduce knock/ping. The old Harley we recently sold was REAL bad about it, but H-D recommended 87 octane fuels. When we ran premium or added 104+, the knock/ping stopped.

Consider this. Modern Ford EcoBoost engines provide tuning to run on 87 octane fuels. If you read the brochures carefully, you will see "All horsepower/performance ratings are obtained using 91 octane fuel." Ford says your EcoBoost will run fine on 87... and it will... but you will not be making 400 horsepower with it.

You mileage may vary. Choose wisely.

For those of you who run turbos and superchargers. Why do some folks go to E-85? For more power. Why? Because ethanol INCREASES octane. Higher ethanol percentage fuels will resist detonation which will DESTROY a boosted engine. Thats just one of many reasons why you'll see E-85 in boosted cars pushing over 800hp. It allows for more aggressive timing. The tradeoff is that E85 is not as efficient, so you have to use more fuel. You can also get E-85 at lots of gas stations. You can't get 100LL aviation fuel or 100+ race fuels at your local 7-11.