It has always been my understanding that higher octane gasoline is only needed in vehicles which have a higher compression engine, which allows them to get a more efficent burn with the lower flamability associated with the higher octane rating.

Many people believe that they are doing themselves a favor by putting premium fuel in their cars which do not require it.  It is always been my understanding that it wont hurt their car, but wont help either.  Just wasting money.

Well, awhile ago I mistakenly put 89 octane fuel into my '95 Tacoma 4X4, 2.7L 4cyl, 74,000 miles.  I didn't realize I hit the wrong button until the gas started flowing and just let it go.

Then, I forgot about it, and didn't drive my truck for a few days.

Well, over the past several days I had noticed that my truck was operating better.  A little more power and the engine is running much more smoothly.  It was strange and I wondered why my truck suddenly decided to run so much better....then I remembered.  The 89 octane.  It's not like I am imagining the better performance as I forgot I put it in there.

I verified in my manual today that it only requires 87 octane.  I did not hit the trip odometer when I filled up, so I wont be able to tell if I am getting better mileage with this fill up.

I am a bit amazed by this as it goes against everything I have learned about octane requirements.

Just for the hell of it, i am putting in 92 octane in the next time.

Comments?

Regardless of compression, if tuned for it, the gasoline powered car should get more power out of mid-high octane fuel than the regular swill.  

The trick is that the fuel injected vehicle, after 10-20 start-run-shutdown sequences will "learn" how to use the fuel efficently.  I can run my 5.9 dodge on 85, but I run mid level fuel in it, as it is a wee bit more powerful, and the vehicle runs smoother.  

You want the lowest octane rating without your engine knocking (detonation).  If your manual says it requires 87, use 87.  Using 89 or 92 will only cause fouling in your engine.  The higher the octane, the higher the temperature must be for the fuel to ignite.  Which is good for high compression/turbocharged engines to prevent detonation.

Quoted: You want the lowest octane rating without your engine knocking (detonation).  If your manual says it requires 87, use 87.  Using 89 or 92 will only cause fouling in your engine.  The higher the octane, the higher the temperature must be for the fuel to ignite.  Which is good for high compression/turbocharged engines to prevent detonation.

Only with a carb or shitty ancient EFI.  The higher quality gas will allow the engine to not retard the ignition as much.  

IIRC the OBD-II modules will only allow a certain level of advanced timing to match the higher octane before it will go no further.  This assumes you leave your air measurement and manifold pressure sensors stock.

At least that how I remember the guy at SHOShop explaining it to me.

You can, in theory, get a gain in performance by switching to higher octane gasoline -- but I'm pretty sure anything with an OBD-II computer will max out at some point.

Only 'tards put anything other than 87 in their vehicle, unless of course it says that it requires it.

Though, if you do put lower octane gas in your high-end sportscar, you're likely causing detonation, which is just about the worst thing you can do for an engine short of running out of oil.  Yes, it'll run fine after awhile, but it "learns" to run with retarded timing because you're feeding it shitty gas.  Basically, your car's engine detonates, screams "ow, shit, what the hell was that???" and the car's computer retards the timing to prevent further detonation, but any detonation causes engine damage.

Quoted: With more modern cars the engine control unit will actually adjust timing to match the octane of the fuel. My Nissan 350Z recommends +92 octane but runs just fine on 89 with probably the loss of a few ponies ... which it has a few extra to lose the way I drive!

Paul, I'm not sure that is true.  To be honest, it's contrary to things I have heard from guys who get their jollies off of building engines with boosted motors.

I believe the 350Z has a minimum advanced timing, and that timing requires a certain octane rating to keep from detonating.  FWIW, not all detonation is audible from under the hood.

They have some pretty sophisticated instruments to listen for detonation in an engine.

It's your car, but.....

Older engines can get carbon deposits in the combustion chambers that actually raises the compression ratio and causes hot spots and inefficient burning.

Higher octane gas will, in those cases, allow the engine computer to reduce the timing retard that was being used to prevent knock with the higher compression.  This results in more power.

Try running a good combustion chamber cleaner and see if you still notice a difference between 87 and 89.  I use Mopar Combustion Chamber Cleaner, but I've heard good things about Seafoam too.  Could be the same stuff for all I know.

NOT carb cleaner or any standard spray bottles of cleaner, but actual combustion chamber cleaner that you have to pour into the throttle body while the engine is running.  You'll gas the whole neighborhood with the cloud that comes out the tailpipe, but it's worth it.  I try to use it every year or two.

Side note, not all engines will adjust to higher octane gas.  All OBD-2 engines, and most computer controlled engines from the past fifteen or twenty years, will compensate some for LOWER than recommended octane gas.  For instance, the DOHC Neon engine is rated for a minimum 89 octane gas, but only makes the rated horsepower/torque on 92+ octane gas.  The SOHC Neon engine is rated for 87 octane, and gains nothing from higher octane gas.

Jim

Quoted: Only 'tards put anything other than 87 in their vehicle, unless of course it says that it requires it.

By Zippy Well, over the past several days I had noticed that my truck was operating better. A little more power and the engine is running much more smoothly.

I get the same results.  My friends get the same results.  Ever try it yourself?  




Shivan, very quick, OBD-II is the computer in my truck.  There is a limitation where timing advance does no good.  Beyond a certain degree of advance, it is pre-ignition.  I don't have any proof, but I think that the higher octane burns more consistently and more smoothly than the low octane stuff.  

Quoted: I don't have any proof, but I think that the higher octane burns more consistently and more smoothly than the low octane stuff.

That's my understanding as well, I'm just saying that with OBD-II computers the engine will only advance ignition so far before it's nto doing you any more good.  Which means I think we agree.

I've tried 89oct before in an 87 vehicle and didnt notice one bit of difference.  

I would not have expected to either...and I really did forget about it.

It wasnt until the next day or two that I noticed the smoothness and a little more power..and then remembered the different gasoline a couple of days after that.

The gas is the only thing I can think of.

Quoted: I've tried 89oct before in an 87 vehicle and didnt notice one bit of difference.

I had an 91 Caprice police model, with the cop v8...  it ran like SHIT on 87 or 89, and purred like a kitten with 92/93 octane.

Go figure, it already was expensive enough to feed.

Don't get me started on when I let my father take it for a spin and he filled it up with 87... damn thing would shut off at stoplights if I didn't rest my foot on the gas...  took a few days to get rid of the gas, even after putting in octane boosters.

Some vehicles require higher than 87. My dad's GMC 3500 8.1 needs 89 or the engine will run like crap.

I put 89 octane in my 03 tacoma v6 once just for kicks, got about 1/2 mpg more than usual wasnt worth the extra 10 cents a gallon