Thoughts on using 89 octane
 

So I mostly commute with my BRZ and lately rarely goose it over 3-4K. I have other cars to play with. So last time I got gas they did not have 93 and I out 89 in and the car felt exactly the same. No I did not log it to see if there was knock or anything funky going on. But if I did not know it was 89 I would never be able to tell for commuting. Thinking it would be great to do this and save some money burning gas on my long highway commute. Any thoughts? Is there and OFT OTS tune for 89?

take a ots tune for 91oct and open timing B map and reduce the timing, like -1/-2 degrees from 2k to redline
should be ok

will consume a little bit more fuel than usual, nothing extraordinary thought

Any Costco gas stations in your area? The price of Costco 91 for me is basically the same as 89 at other pumps (or even less).

Friends don't let friends RB

oh, wait

Really? You can't pump your own gas in OR and I have had them put 87 in my car. It feels so lazy every time. I can instantly tell.

What’s the $ spread on 89 to 93 or even 91 in your area? It’s maybe $.30 here or < $7 for a fill up. For $7 I’d just find a station with at least 91oct. These cars are knock sensitive enough without running the wrong gas long term.

The stock ECU uses a base ignition table in combination with a ignition advance table. The base ignition table is the minimum ignition advance that the engine will run. The ECU then adds a percentage of the advance table on top of the base table. This percentage depends on how much knock the engine is detecting. I believe every time you start the car the ECU is running 70% of the advance table on top of the base table and then increases that to 100% as long as there is no knock detected. On the flip side, if there is knock detected it would use less of the advance table down to a minimum of 0%, in which case it would just be running the base table.

For example: the base table says to run 10 degrees of ignition advance and the advance table is an additional 5 degrees. So the engine will be running anywhere from 10 to 15 degrees of advance depending on how much knock it sees.

When using lower octane fuel my guess is that you will be seeing more knock and therefore be running less ignition advance which ultimately means less torque. It's possible that the octane could be low enough that you are running only the base table (0% of advance table) and the engine is still knocking because it has no more room to retard the timing further. This is probably unlikely but would have to be verified with data logging.

How will this affect your engine long term? I couldn't tell you, but maybe this was still helpful...

Guess I will stick with 93, currently running a Wayne rom tune for 93. Data logged last summer without any knock. Maybe I will try 91 and datalog. My thought was that since the rpm’s rarely get above 4k in a highway commute that the ECU could easily compensate for lower octane. I need this car to last as my commuter, so better to be safe or prove it works.

This should throw a code and trigger a MIL (check engine light), no?

You could be right. From my understanding the ECU is only looking at knock and will adjust as needed to prevent it. It could also be adjusting the fuel trims, but I'm not sure on that (running richer AFR will help prevent knock). With ECUTEK the 0%-100% factor I talked about earlier is called the "ignition advance multiplier". It would be interesting to log on lower octane. If that multiplier is consistently staying at 100% then I would think the lower octane isn't making any difference.

My guess is that there's enough margin in the base ignition table to deal with the lowest octane fuel out there and this would probably never happen. Would be interesting to see some back to back testing comparing octane readings to see how much timing is getting pulled.

Really. Mine has only had Chevron Good Ole Premium 94 octane, its entire life. Well there was 89 octane crap in it when I bought it new, but that is the only alcohol my baby has ever seen.


Its for sale on Vancouver Island. It has 17,200 Km on it and is cherry, and red too. I'm trading it for a Solterra in a few weeks, so be quick. ;)

Weird. It's .65-.70 between 87 and 93 here in MD.

Here in Touristland the difference is about .40-.50 cents between 89 and 93. But gas is .40-.50 cents less at discount stores like Costco, Sam's or BJ's. I only use BJ's 93 octane because it's usually cheaper than regular anywhere else.

Or more. I've seen it as high as $0.90/gal difference.