# Using high octane (104+) gas in a regular gas engine **URL:** \ **Category:** Factual Questions\ **Created:** [September 9, 2001, 4:02pm UTC](https://boards.straightdope.com/t/using-high-octane-104-gas-in-a-regular-gas-engine/80521 "2001-09-09T16:02:23Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Mr.Blue\_Sky](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@Mr.Blue\_Sky](https://boards.straightdope.com/u/Mr.Blue_Sky)\ **Post date:** [September 9, 2001, 4:02pm UTC](https://boards.straightdope.com/t/using-high-octane-104-gas-in-a-regular-gas-engine/80521/1 "2001-09-09T16:02:23Z")
My car requires 87 octane fuel. What, if anything, would happen to it if I were to run 104+ octane fuel? ---
**Author:** ![Evnglion](https://avatars.discourse-cdn.com/v4/letter/e/f04885/32.png) [@Evnglion](https://boards.straightdope.com/u/Evnglion)\ **Post date:** [September 9, 2001, 4:06pm UTC](https://boards.straightdope.com/t/using-high-octane-104-gas-in-a-regular-gas-engine/80521/2 "2001-09-09T16:06:17Z")
well, I would think nothing. All these higher octane ratings are, are measurements of the combustion level, and for a higher-powered engine, it is more effective to have it at a higher level. I don’t remember the exact terminology, but I read an article about this a couple days ago. ---
**Author:** ![mojo\_filter](https://avatars.discourse-cdn.com/v4/letter/m/b5a626/32.png) [@mojo\_filter](https://boards.straightdope.com/u/mojo_filter)\ **Post date:** [September 9, 2001, 5:11pm UTC](https://boards.straightdope.com/t/using-high-octane-104-gas-in-a-regular-gas-engine/80521/3 "2001-09-09T17:11:48Z")
Hi Octane fuel should only be used if your engine _requires_ it. The octane level is the fuels resistance to detonation, or the combusting of the fuel by compression alone. If you have an engine with a high compression level of say 9:1 or higher the fuel can combust during the compression stroke and not when the sparkplug fires. If this happens, you can, over time, ruin your engine or immediately ruin it if you get a hole in your piston or bend a connecting rod. If your manufacturer suggests 87 octane, use it. Anything more will only be a waste of money because your engine won’t run any differently with a higher octane level fuel. I have a Ford Ranger that will rattle like a sack of marbles if I use any thing less than 92 octane, but the compression is a stout 11:1 due to shaving the head. ---
**Author:** ![DPWhite](https://avatars.discourse-cdn.com/v4/letter/d/d26b3c/32.png) [@DPWhite](https://boards.straightdope.com/u/DPWhite)\ **Post date:** [September 9, 2001, 5:50pm UTC](https://boards.straightdope.com/t/using-high-octane-104-gas-in-a-regular-gas-engine/80521/4 "2001-09-09T17:50:57Z")
Use what the manufacturer recommends, nothing else. Higher octane requires a higher temperature to ignite and an engine designed for lower octane may not burn the fuel correctly. Higher compression engines get much hotter on the compression stroke and require higher octane fuel so that the do not prematurely ignite, cause knocking and ruin the engine as described above. Under the gas laws, which I have conveniently forgotten, the temperature of the gas increases substantially when it is compressed (your air conditioner works on this principle). So in order to keep it from burning early, higher octane is required to retard the ignition. ---
**Author:** ![Tedster](https://avatars.discourse-cdn.com/v4/letter/t/4491bb/32.png) [@Tedster](https://boards.straightdope.com/u/Tedster)\ **Post date:** [September 9, 2001, 6:45pm UTC](https://boards.straightdope.com/t/using-high-octane-104-gas-in-a-regular-gas-engine/80521/5 "2001-09-09T18:45:27Z")
You need to advance the timing in order take advantage of a high octane fuel, typically. High compression engines can be harmed by running low octane stuff, but you’re unlikely to cause any harm to anything other than to your wallet. If the operators manual specifies premium gas, then by all means use it. Otherwise use the minimum octane fuel that results in good driveability and anti-knock. ---
**Author:** ![Chas.E](https://avatars.discourse-cdn.com/v4/letter/c/e99b99/32.png) [@Chas.E](https://boards.straightdope.com/u/Chas.E)\ **Post date:** [September 10, 2001, 12:28am UTC](https://boards.straightdope.com/t/using-high-octane-104-gas-in-a-regular-gas-engine/80521/6 "2001-09-10T00:28:54Z")
104 is high octane? Ha. My old '65 Mustang GT’s owner’s manual specified high octane requirements for the engine. It suggested 126 octane but said 116 would suffice. ---
**Author:** ![ElwoodCuse](https://avatars.discourse-cdn.com/v4/letter/e/edb3f5/32.png) [@ElwoodCuse](https://boards.straightdope.com/u/ElwoodCuse)\ **Post date:** [September 11, 2001, 12:23am UTC](https://boards.straightdope.com/t/using-high-octane-104-gas-in-a-regular-gas-engine/80521/7 "2001-09-11T00:23:01Z")
This past summer I saw a gas station in Connecticut selling the stuff. I was really surprised they had it at all; it was a run-of-the-mill highway gas station. It was odd seeing the prices on the pump read 1.45, 1.55, 1.65, and then like 4.75 for this 104 octane gas. ---
**Author:** ![Mr.Blue\_Sky](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@Mr.Blue\_Sky](https://boards.straightdope.com/u/Mr.Blue_Sky)\ **Post date:** [September 11, 2001, 2:49am UTC](https://boards.straightdope.com/t/using-high-octane-104-gas-in-a-regular-gas-engine/80521/8 "2001-09-11T02:49:25Z")
Thanks for the info. I was just curious. I figured that, if nothing else, the onboard computer would go nuts. ---
**Author:** ![V6stang](https://avatars.discourse-cdn.com/v4/letter/v/8491ac/32.png) [@V6stang](https://boards.straightdope.com/u/V6stang)\ **Post date:** [September 11, 2001, 6:27am UTC](https://boards.straightdope.com/t/using-high-octane-104-gas-in-a-regular-gas-engine/80521/9 "2001-09-11T06:27:33Z")
I might add that typically 104+ octane gas contains lead that will wreak havoc with your catalytic converters and quite possibly destroy them. Only use an octane as high as it takes to prevent detonation (pinging). ---
**Author:** ![JD\_Fensty](https://avatars.discourse-cdn.com/v4/letter/j/b3f665/32.png) [@JD\_Fensty](https://boards.straightdope.com/u/JD_Fensty)\ **Post date:** [September 12, 2001, 4:59am UTC](https://boards.straightdope.com/t/using-high-octane-104-gas-in-a-regular-gas-engine/80521/10 "2001-09-12T04:59:36Z")
> [@](#): > > \*Originally posted by Chas.E \* > \*\*104 is high octane? Ha. My old '65 Mustang GT’s owner’s manual specified high octane requirements for the engine. It suggested 126 octane but said 116 would suffice. \*\* This, to me, sounded like utter BS. So I checked with RAMFM’s (rec.autos.makers.ford.mustang) resident experts, and garnered the following response: * * * Right out of the original 1966 owners manual (doesn’t everyone have one sitting around), page 38 sitting, as found on my desk: Engine Grade of Fuel Factory adjusted octane requirements 200 CID-6(120hp) Regular at least 94 octane\* 289 CID V8(200hp) Regular at least 94 octane\* 289 CID v8(225hp) Premium at least 99.8 octane\* 289 CID v8(271hp) Premium at least 99.8 octane\* - Octane as rated by the Research Method “Generally, the grade of fuel recommended will provide satisfactory engine performance. However, if “pining” or spark “knock” occurs and cannot be cured by spark timing or other engine adjustments, change to the next higher grade of fuel”. So, we can plainly see that the fellow on the forum in question is talking out of his rear end, and has no clue as to what he is posting about… Bill S. PS: Feel free to cut and paste the items above as needed * * * The older method of calculating octane, AFAIK, means those numbers (94 and 99.8) would be a bit LOWER by today’s standards. So…Chas…I dunno, if you were just “having fun” or intentionally one-upping, but either way… nope. \*Also the designation “GT” wasn’t used in '65. JD (Thanks to Bill S.)