just out of curiosity..if a car requires lets say 89 octane gas...any plus or minuses to using 93 octane in that car...i drive a 96 se and have used nothing but super 93 octane in it....friend tells me i dont need super that i should go with premium 89 octane instead but had no reasons as to why other than ( u dont need 93 octane in a ls/se)...any one with 2 cents on this...thankz

 

I have a Civic VX, and when I first got it I tried every grade of gas I could find for one tank each. I found that the engine ran smoother, and I certainly got better gas mileage, on 87 octane vs. 91 octane (which is the highest I can get in California). 89 octane exactly split the difference between the two in terms of performance and mileage. Also, on the higher octanes, I was getting spark-detonation (a quick "tick-tick-tick" with the engine at load) that doesn't occur on 87 octane. My $0.02, stick with the cheaper gas. I could not find any advantage to the higher octane at all.

 

On a relatively stock N/A motor you will get more power with lower octane. If you have higher compression, FI, advanced timing, etc. higher octane will help to keep you from detonating. By keeping your pistons from melting, it will give you more power under those conditions. Higher octane fuel will also cause the vehicle to emit more HC.

 

isnt super (93) octane supposed to burn cleaner or some **** like that...and also is there any truth to this statement ive heard also...switching octane grades as well as gas brands isnt good for the ecu...as to it being confused all the time.... i"ll take one to the head if this is hilarious....

as for me i like to stick to exxon or mobil and always used super (93) octane...depending on the feedback from this post i"ll try (89) or maybe even as low as (87)... so your responses is highly valued....thanks


[Modified by teg-your-it, 2:53 PM 1/14/2003]

 

Go ahead and take one to the head then

The detergents in each grade of gasoline are largely a marketing ploy, and vary seasonally and by manufacturer anyway (so how can you draw conclusions of their effectiveness?).

Blanket statement concerning the effect of high or low octane on an engine, in respect to anything other than detonation tolerance, are not valid. Higher octane gas will not necessarily burn faster, slower, cleaner or create more power than a lower octane. Gasoline isn't simply just one chemical fuel you pour into the tank, it's a witch's brew of several compounds that varies from suppplier to supplier, season to season and state to state. No assumptions can be made concerning it's performance based on octane rating OTHER THAN it's overall detonation tolerance, and even that's subject to scrutiny due to the method of averaging RON and MON octane numbers.

 

so in lamens terms....even using the same brand of gasoline and same octane at every fill up your still getting a gasoline that varies from fill up to fill up and higher octane is not better for anything other than preventing detonation..so by me switching to 89 octane from previously using 93 i should see no losses as to maybe a slight gain due to it being cheaper for my pocket..

 

octane measures a fuels resistance to detonation. It has nothing to do with how fast it burns. This will vary even at the same station. Remember that the components in gasoline are volatile and evaporate readily. Try running the lower grade, your vehicles knock sensor will most likely make adjustments (retard timing) to keep your car from detonating, if you hear pinging, try higher octane and see if it's better.

 

No, what I'm saying is that you can't draw conclusions about any aspect of fuel performance other than detonation tolerance from the fuel's octane rating. Burn rate is a different property and should be discussed as such; so is BTU generation per lb (aka energy density).

For example, aromatics (a family of chemicals which generally have high octane ratings) tend to burn faster than gasoline. A common additive such as lead doesn't burn at all and can actually slow burn rate, while oxygenates like MTBE may slow or increase burn rate depending upon type and concentration.

So switching from one gas to another, or one octane to another, will probably produce changes in how it burns. But what changes those are can't be predicted reliably with just the fuel's octane rating, except for detonation tolerance. In other words you need to either experiment with different fuels in a controlled setting or get more information about the fuels you intend to use before making assumptions as to what will happen when you put Brand Y in your tank instead of Brand X.


[Modified by texan, 12:28 PM 1/14/2003]

 

gotcha....

i dont wanna make to dramatic of a change and drop right to 87 being the lowest octane rating around here i"l drop to 89 which should'nt make to much a difference...cause after 89 only thing higher is the 93....which will save me damn near a quarter a gallon=about 3 bucks a fill up..give or take a few cents...thanks again for positive replies...


[Modified by teg-your-it, 4:06 PM 1/14/2003]

 

Octane is 2 things. A type or form of fuel; Used as a rate at which the fuel burns. So.... The pump uses the second definition, a rate at which the fuel burns. The higher the octane, the slower the gasoline burns and the more thoroughly it burns. Thouroughly burned fuel means it is broken down and very few hydro carbons are left. So from an enviromental stand point, high octane is good because it burns up all the hydro carbons and produces stuff like CO (carbon Monoxide), CO2 (carbon dyeoxide) H2O (Water) and a whole mess of other stuff including SO2 (sulfer dyoxide) and even formaldahyde.

So what the hell does all that have to do with power? Not much. If your octane rating is to low you may get "pinging" or detonation. This is caused by the fuel detonating to much Before Top Dead Center (BTDC) It burns faster and as the piston is traveling up it is fighting the explosion that happened to soon. PING PING PING. BAD! So if you can run the cheap stuff (87 octane run it).

Some things to consider, if you say add a turbo, super charger, mill your head, increase lenghth of connectiong rods, increas stroke of crank, increase depth of valve opening, etc, you will need to increase your octane. Why, each of those things has increased the compression, or added aditional gases to the combustion chamber. And there fore your spark will be to soon BTDC and cause detonation. In these cases, add octane.

Another thing to consider, we are lucky as Honda owners. Our Hondas do allot of this thinking for us so we can be fat dumb and happy. Thanks Honda for letting us be lazy and not have to think of this until we modify things

 

That is not true. A fuel has a higher octane rating primarily because, after being heated (added to the presense of end gasses from the last power stroke that didn't make it out of the chamber) it's consituent parts don't break down into more easily combusted chemicals. By more easily combusted I mean lower auto-ignition point, NOT faster burn rate. As I stated before, some fuels by nature are both high in auto-ignition point AND high in burn rate. For others it's the exact opposite, or for still others you can mix and match those properties. Hence a higher octane gasoline mixture does not mean it burns slower than another, lower octane mix. It also doesn't mean it burns more or less completely, as that has to do with burn rate, a/f ratio, mixture homegeny and the specific engine's combustion dynamics.

So once again, octane rating speaks of the fuel's ability to resist detonation through high auto-ignition point (which is attributable to molecules that don't break down so easily or immediately when heated), and NOTHING ELSE.


[Modified by texan, 4:01 PM 1/14/2003]

 

Make me bust out the Thermo book why don't ya

Octane is C8H18. This is an idividual molecule that is added to Gasoline.

I may have bee a touch premature on stating that High Octane fuel takes longer to burn. I more acurate statement would be High octane fuel takes longer to completely burn all of it's constituents. Whether or not a portion of the exhuast gasses are recirculated into the combustion chamber for the second stroke is not the issue, and has no effect on octane.

What I gather from your comment is the octane rating in your opinion is BS because of different constuents added to the gasoline.

I am no historian, but you may be right that the octane rating was developed as a way to define/stop detonation. But define detonation and then tell me why Higher octane fuel stops it.

If I wasn't going home right now I would love to dive into Enthalpy of reaction.

O well maybe tomorrow.

The orginal question is Do I need to run 93 octane. The answer is in the owners manual. And typically in 2-3 tanks of gas your computer will have adjusted the timing just right. Granted you burn the same octane purchased at the same gas station.

 

Then you've misunderstood me.

My point is simply that no inferences should be made about a fuel other than detonation resistance when looking only at the octane rating. This is what the octane rating describes... nothing else.

You may bust out the thermo book and get into enthalpy if you like, but realize we are not talking about reactions in a book or on some scratch paper. The octane rating is derived from an true running engine with real fuel that is actually burned and whose detonation limitations are thus found. End gasses do matter because in the real world, the unburnt chemicals left over from prior combustion help affect how the next combustion cycle occurs. These molecules interact with the new mixture and help to define how resistant the resulting mix will be to heat induced ignition, which is exactly what we are talking about. Octane rating is the combined effect of fresh charge dynamics and the interaction of residual end gasses from the last power stroke. Of course that's excluding all the other engine specific variables, but you get my point.

I would never say octane rating is BS, I was simply trying to show that it matters only in terms of auto-ignition point. It is the most important property of fuel as far as I am concerned, as energy density and the like are nearly identical in all but the strangest mixes of gasoline.

 

I use 94

 

ok so by me always putting 93 and dropping to 89 ....what can i expect using the 89 octane now...expect from the car and not what goes on in the process of burning the gas as previously was broken down to me...i simply was wondering what to expect from the car by going from 93 octane to 89 ,if i should expect anthing...thanks to all for the breakdown of combustion and what goes on inside the engine....