# Gasoline Octane and Requirements (yet more) **URL:** \ **Category:** Factual Questions\ **Created:** [July 29, 2002, 10:57am UTC](https://boards.straightdope.com/t/gasoline-octane-and-requirements-yet-more/121355 "2002-07-29T10:57:32Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Balthisar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/balthisar/32/2825_2.png) [@Balthisar](https://boards.straightdope.com/u/Balthisar)\ **Post date:** [July 29, 2002, 10:57am UTC](https://boards.straightdope.com/t/gasoline-octane-and-requirements-yet-more/121355/1 "2002-07-29T10:57:32Z")
We’ve all seen the octane posts here several times – does putting in higher octane gas really help? Etc., etc. _My_ question is just about the opposite… My new car (a Ford product) “recommends” the use of 91 octane gas or better. All of the reviews online state that back in 2001 or so, Ford added an anti-knock sensor that allows the use of 87 octane gas – we all know that using _lower grade_ gas can be a bad thing because of knocking. So, it appears it’s _safe_ to use lower grade gas in this vehicle, even though it’s not _recommended_. So far, I can see only one consequence of using cheaper gas: less power. But can anyone tell me _how much_ less? With 91 octane, it’s rated 275 horse/275 ft-lb torque. What would happen with 87 octane? How about fuel efficiency? I average 21.5mpg (that’s with a heavy foot) with the “good” gas. Would that drop, too? I guess the anti-knock sensor feeds into the computer and the timing or valve-timing or something is changed based on this. Does that guarantee no engine damage, or just no knocking? It only costs about $4 extra to fill up with 91 instead of 87, and I bought the car for its power, so I probably won’t downgrade – but I’m curious as to the consequences. Thanks! ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [July 29, 2002, 12:19pm UTC](https://boards.straightdope.com/t/gasoline-octane-and-requirements-yet-more/121355/2 "2002-07-29T12:19:59Z")
Is your car a turbo or supercharged? Lowering octane issue: engine knock, which causes premature engine wear. Ford’d electronics can prob compensate though, unless it’s a turbo, or supercharged or truck for towing. In that case, use 91 as recommended, since the engine comp is set for max fuel/power/engine knock balance. ---
**Author:** ![Balthisar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/balthisar/32/2825_2.png) [@Balthisar](https://boards.straightdope.com/u/Balthisar)\ **Post date:** [July 29, 2002, 4:15pm UTC](https://boards.straightdope.com/t/gasoline-octane-and-requirements-yet-more/121355/3 "2002-07-29T16:15:30Z")
Nope, naturally aspirated. I don’t think Ford makes anything turbocharged or supercharged, which is really too bad. Nor is it a truck. I’ve tinkered with the idea of filling up with 87 just to see what happens to the horsepower, but if it’s a dog on the cheap gas, I didn’t want to have to wait 400 miles to fill it up again with the good stuff. On the other hand, filling it up with the cheap stuff COULD result in lower mileage maybe? In which case it costs the same as the 91 in the long run… Oh well. ---
**Author:** ![zwede](https://avatars.discourse-cdn.com/v4/letter/z/ac91a4/32.png) [@zwede](https://boards.straightdope.com/u/zwede)\ **Post date:** [July 29, 2002, 4:31pm UTC](https://boards.straightdope.com/t/gasoline-octane-and-requirements-yet-more/121355/4 "2002-07-29T16:31:40Z")
Yes, you will see reduced power and mileage. How much depends on the individual engine and conditions. For instance, higher ambient temperature makes it knock more, and the ECU will have to pull more timing out to make it stop. I did a somewhat un-scientific test 2 years ago. We rented a Buick park avenue for a trip from Dallas to California. It had a supercharged V6 making 240 hp. Recommended octane was 91 or 92 (can’t remember). The first tank I filled it with 87. After all, it’s a rental… It had a trip computer and driving 80 mph it got 23 mpg, average. Next tank I used 93 octane, and that tank it made 27 mpg! Encouraged by my success I used the highest rated gas for the rest of the trip and averaged between 27 and 29 mpg all the way to CA. Speeds were between 75 and 85 mph, with a short blast to 100 where the computer decided to cut the fuel off. Atleast on this particular engine using high octane made a big difference. Markus ---
**Author:** ![handy](https://avatars.discourse-cdn.com/v4/letter/h/b5a626/32.png) [@handy](https://boards.straightdope.com/u/handy)\ **Post date:** [July 29, 2002, 10:35pm UTC](https://boards.straightdope.com/t/gasoline-octane-and-requirements-yet-more/121355/5 "2002-07-29T22:35:47Z")
"The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like “regular” 87-octane gasoline) can handle the least amount of compression before igniting. " > **[What Does Octane Mean?](https://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/question90.htm)** > > Adding a chemical called tetraethyl to gasoline can significantly improve the gasoline's octane rating. So what does octane mean?