VOdoc

Every time someone on any Acura forum brings up the topic of using gasoline grades less than premium, there is massive flameage. "If you can afford this car, you can afford premium gas! Why would you put inferior fuel in a superior vehicle!! Cheapskate!!! You'll pay in reduced gas mileage more than you gain in lower gas prices!!!!" Etc., etc.

With 91 octane premium spiking above $3.50 a gallon in SoCal, I naturally wondered if the mid-grade might be an alternative. My RL had never tasted anything other than Chevron Supreme until 3 tankfuls ago. My TL never cared for anything below premium grade. Should I chance it?

So...I've fueled up 3 times with 89 octane and guess what?? No reduction in gas mileage (actually, a slight increase). No noticeable reduction in acceleration or power. No hint of knock or hesitation. No freaking out of the engine-management computer. No warning lights. No lightning bolts from above. Nothing.

Yes, I realize I'm only saving about 3%. But hey, the entire grocery industry survives on smaller margins than that.

Let the flaming begin!

sotiri

the higher the octane the slower the gas burns.
Now how you got a better MPG with lower octane is the key.
On a 3.0 or larger... with the compression and other performance issues we are setup for, if you are reporting better MPG, I would hope that you can verify this with more than just one or 2 tanks and with very similar driving routines

kirbyflorida

The computer adjusts, however HP will be lower.

Mike_TX

The difference between 89 and 91 octane is about 2.2%, so I doubt that's enough to bring forth those lightning bolts from above (or the other potential dire consequences you cited).

Your car will run on 87 octane "regular", too, but I figure there's enough difference there that you'd begin to see some adverse effects. And of course, it's not recommended.

If you want to use mid-grade, I say go for it.

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Touge

The ecu will retard ignition timing to prevent the engine from knocking. There is a knock sensor that tells it to.

Rich in NC

Just my opinion.
Running with a slightly retarded engine is not a great thing. The engine is less efficent and combustion is not as complete. It used to be you could (pre knock sensors) drop the octane until you heard a ping and just adjust from there. But knock sensing will mask the retard.

My guess is until it gets hot you can run with a lower octane, but the Acura has a big compression ratio, and I would guess, and agressive advance program.
290 HP out of a 3.5 liter engine is impressive but demanding.

kirbyflorida

To save $100 per year on 12,000 miles is not worth it in any way possible IMO.
This car was designed for 93 octane.
You could also save more than that with really cheap tires, also not recommended.

Touge

Rich I totally agree, I would never run anything less than premium in my RL. The stock CR is def very high on this motor. With the timing retarded you will definitely be loosing some power and the ecu ignition multiplier table will not be happy.

VOdoc

Slightly retarded? I resemble that remark!

BTW in California premium is 91 octane; there is no 93 octane available here to my knowledge. The three grades are 91, 89 and 87. No, I wouldn't stoop so low as to try 87. Even I have my standards!

The lack of any detectable decrement in performance, gas mileage, engine temperature, etc. could lead the paranoid mind to imagine that certain gas stations might be selling mid-grade as premium to increase their margins!?

loulinjai

is 11:1 really that high? it certainity is not low. but even the new civics that run on regular run a CR of 10.5:1. For reference the V6 found in IS 250 runs 12:1. If you want to go to the extreme end a M5 also runs 12:1. I believe the highest CR a production honda model motor in stock form is 11.1:1 found in S2K. I hope honda can up the CR a bit and give us better torque, like the motors used for the JDM market

Rich in NC

To get the whole picture you have to see the advance tables, inlet temperatures, stratified fuel charge shape, type of head heat flow and, head cooling. Along with a zillion other things I am too old to remember.

I do remember when GM finally had a good technique to make aluminum heads that resisted warping (sort of). It allowed an increase in compression (compared to similar iron heads) because the heads dumped heat faster.

Anyway that is all to say CR is just one part of quite large formula.

WtaggiN

Yeah, I was thinking about it, but just no point. It saves me 3 bucks every fill up, but I would rather run my car right and save off on some Cheetos than chance it.

The only time I could tell a difference was when I put 87 in my 94 3kgt. It ran like complete shit. Put 91 in it, ran smoother...I don't know shit about the logistics and reasonings to why the 2.2% difference is so important, but I didn't design the car, just following the rules.

sotiri

car runs better on non-ethanol blended gas too.
everytime I leave IL, and head out west, boy does it run better and with more mpg.

catsailr

One good thing - Back when 87 octane was $1.00, 93 octane was $1.20, or 20% higher. Now with 87 octance at $2.60 (around here anyway) the 93 ocatane is $2.80, or just 7.7% higher.

kirbyflorida

Sorry to rant but to save $100 per year on a $45,000 car designed for 91 octane is really not right, to be polite. What are you thinking??????????

VOdoc

I'm finding that it probably isn't really designed for 91 octane. Engine temp is slightly cooler, gas mileage is slightly better, acceleration is the same, and $100 is enough to take my wife out to dinner and hopefully get lucky!

I knew I would get flamed. YMMV... :surrender

Rich in NC

If the engine is running cooler it means you are running less advance or have less complete combustion.

If it wasn't designed for 91 they would spec a lower octane. I would withhold judgement for hotter days and different blends which will show up all year.

jhummer

I rented an Acura TSX for a week and treated it to 87 octane the for several tank fulls. It was the 3rd tank of gas that the engine started pinging very badly. I am glad I got that test out of the way on a rental

Rich in NC

Maybe incomplete combustion build carbon deposits. The deposts will cause pinging.

gavine

My 06 RL pngs on 93 octane. Maybe my knock sensors are bad?

Low octane is more volatile, thus more easily ignited. In a high compression engine, the heat generated during compression can cause low octane gas to detonate prior to the spark which causes the pinging sound.

Advancing the timing in any car will yield more power and better throttle response. So, the ECU will advance the timing as much as possible, until it "hears" pinging and then it will retard the timing accordingly.

Therefore, lower octane fuel will "force" timing retardation, which is not good for performance.