Explain this

[stevekx]

When I use 93 octane gas (use 87 most of the time) my engine seems to runs a lot more quiet and smoother, however my gas mileage drops around 2 MPG.

[mytundrarocks]

I've read in many places to use the same grade of gas all the time, even the same station and pump if possable. IDK but if I was paying more and getting less I wouldn't let it happen twice...

[nhdubfan]

I am going to try and explain this one, Octane Rating numbers describe a fuels resistance to detonation , a higher octane fuel can withstand higher compression and more advanced spark curve before it is ignited by the spark plug at the proper moment. A lower octane fuel will cause the knock sensor circuitry to retard the advance curve to prevent detonation (fuel exploding before the spark comes just from heat and compression "diesel style" ) this causes more noise potentially (there has to be some "knock" for the knock sensor to sense) and less power (due to the more conservative spark timing) and better mileage. So in a nutshell, cheap gas= more "bang for your buck" literally although at the cost of lower performance (and additive packages etc.) high octane fuel=maximum power potential especially under heat/heavy loads etc. at the expense of economy. Anyone with any better answer please chime in, this is my understanding of how it all sorts out, I'm no chemist , and I didn't even stay at a Holiday Inn last night either!

[bill3508]

Originally Posted by nhdubfan

I am going to try and explain this one, Octane Rating numbers describe a fuels resistance to detonation , a higher octane fuel can withstand higher compression and more advanced spark curve before it is ignited by the spark plug at the proper moment. A lower octane fuel will cause the knock sensor circuitry to retard the advance curve to prevent detonation (fuel exploding before the spark comes just from heat and compression "diesel style" ) this causes more noise potentially (there has to be some "knock" for the knock sensor to sense) and less power (due to the more conservative spark timing) and better mileage. So in a nutshell, cheap gas= more "bang for your buck" literally although at the cost of lower performance (and additive packages etc.) high octane fuel=maximum power potential especially under heat/heavy loads etc. at the expense of economy. Anyone with any better answer please chime in, this is my understanding of how it all sorts out, I'm no chemist , and I didn't even stay at a Holiday Inn last night either!

You should not get lower mileage using higher octane. You may very well not get better but it should not drop. In reality using the highest octane assuming your vehicle can adapt to it should provide the best possible power and fuel economy. Our trucks do not utilize high octane timing curves so it doesn't help much. Also using lower octane and having knock(which can occur during spirited driving or towing) just pulls timing and won't give you good gas mileage. Your best bet is to use the lowest octane you can as long as you don't have detonation. If you tow or run your truck hard you should not be using the lowest octane available. I use 89 pretty much exclusively in my truck as it shows no detonation during towing which I do a lot and 93 has showed no benefits to me.

[Mr. Creosote]

I agree, and I tried both regular and premium at the dragstrip and there was no improvement in ET. I will note, however, that I can feel smoother operation with the premium. It might just be in my head though. My brain has been damaged by years of espresso.

['00 Tundy]

same here with my truck. when i put in super (93) you cant feel it running at all, but with regular, it has a slight vibration to it at idle. im going to run midgrade/super from now on..

[NoKz]

Shell gas is the only gas that lasts long enough in my truck. I always fill up at the same station.

[GTABurnout]

93 octane is not needed unless something is wrong with your truck. The octane that prevents detonation in a high output system will hamper a lower performance ignition system causing lack of complete detonation. This can have the effect of the truck thinking its running lean hence more fuel. I run 87 all the time I had a cherokee that would get pissed and run like crap if I put in 91.

Haven't you guys heard of all the sories of guys putting race gas in to stock cars and blowing them up. Stock ignitions systems are not designed to woth with harder to detonate fuels.

[bill3508]

Originally Posted by GTABurnout

93 octane is not needed unless something is wrong with your truck. The octane that prevents detonation in a high output system will hamper a lower performance ignition system causing lack of complete detonation. This can have the effect of the truck thinking its running lean hence more fuel. I run 87 all the time I had a cherokee that would get pissed and run like crap if I put in 91.

Haven't you guys heard of all the sories of guys putting race gas in to stock cars and blowing them up. Stock ignitions systems are not designed to woth with harder to detonate fuels.

All those those folks blowing up their cars probably don't have them tuned properly and it can happen with any grade gasoline. Higher octane fuel has nothing to do with it and you won't run lean because of it. You would be much more likely to destroy your engine racing running low octane fuel. Higher octane fuel prevents detonation during compression, not the spark. It is very typical to this day on most vehicles to get pinging when using low octane fuel under hard acceleration and towing.

[bill3508]

Here is a pretty good write up. Some posted if off the GTO forums regarding octane and how it affects your engine.




I'll be the first to admit, I stole this from the Honda Shadow forum 'hondashadow.net'. But it is one of the best write-ups I've seen on the topic.

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<< Originally posted by 'litnin' >>

Octane affects the flash point of gasoline, period.

The flash point is the point at which the vapors from the fuel will ignite by means of heat, instead of spark.
It's also known as 'dieseling'.

Just to help clarify some terms also:
Pre-ignition, detonation, pinging, are not interchangable terms.
Pre-ignition is when the vapors ignite from heat, not spark.
Detonation is what happens AFTER pre-igntion occurs.
Detonation is the result of two flame fronts (one by pre-ignition, the second by spark) colliding in a massive release of heat energy.
Pinging/Spark Knock is the noise you hear as a result of the two flame fronts colliding.

The amount of pressure present in the cylinder will actually surpress spark combustion. One of the many reasons that high boost engines require very small spark gaps and very high ignition voltage.
The more compression you have, the higher the electrical resistance is present and the harder the plug is to fire.
However, the more compression you have, the greater the change of pre-ignition there is... that's why high compression engines detonate so easily.
Ever see a Top Fuel motor? They run DUAL 44amp magnetos putting out about 70,000 volts, each... and spark plugs in the 10 and 11 heat range with a gap of only .015-0.017".

All you are doing by increasing octane, is increasing the point at which the gasoline vapors will ignite by heat. When you change the compression ratio, you change the heat level. High compression generally generates higher combustion temperatures.

Now, if you take a 12.0:1 compression engine at 90 degree ambient air temp, and run it on 87 gasoline... will it ping? Possibly.
Compression is NOT the only thing that makes an engine ping.
Take that same 12.0:1 compression engine and take it where it can run in 40 degree ambient air... Likely, it will not ping. You could likely even increase the compression ratio to 12.5:1 and still not have it ping.

Just another thing to throw in to the mix.
In general, the slower a fuel burns, the more power it can produce.
That is generally the case in most all fuels... even gun powder.

It's all a balancing act, really. You can't just say that high compression engines need higher octane gas. You HAVE to factor in the enviroment(termperature and altitude), the cylinder head design, etc.. There are MANY factors that determine whether or not an engine will detonate.

But, as it's been said, the Shadow's do not generally require high octane gasoline. However, if you live somewhere that has extreme heat (such as parts of California, Arizona, New Mexico, etc...) a higher octane gas may be needed as your outside air temp rises.

So, why do some people notice a difference in power when switching to high octane gas? Well, likely, they live in an environment that makes it necessary. If you have aftermarket exhaust on your ride, you may not hear the pinging.. Switching to a higher octane gas IF your engine is pinging can and most likely will result in better gas mileage and more power. Why? Well, let's look at the cycle of an engine.
On the intake stroke, the cylinder takes in an intake charge that consists of gasoline and air. As the piston moves up on the compression stroke, that mixture starts to heat up. If it gets too hot and flashes before the spark ignites the fuel at the appropriate time, the flash point flame starts burning bottom to top. Not the way you want the flame to burn. You want a downward expansion. Now, as the bottom flame is burning upwards, the spark plug ignites the mixture and starts burning top to bottom.
The two flame fronts collide and all that energy that would normally be used to push the piston down has exerted much of itself upwards and outwards as the flame rises and collides. This force is also trying to push the piston back down as it's trying to come up in the hole.
Now, switch to a higher octane gas and the gas is resistant to the heat generated by compression and outside air so that the plug ignites the mixture at the appropriate time. Now, the expansion of the exhaust gases are expanding correctly in time with the piston. As the piston travels over TDC, the expanding gas can push the piston back down in the hole with the majority of it's energy. So, does pre-ignition cause power loss? Absolutely. Will higher octane fuel help? Absolutely. Will you get more power by running higher octane fuel? Yes, in a case where pre-ignition is robbing the engine of power. Will it always give more power? Nope. If it's not detonating, the higher octane is not doing anything.

And no, higher octane doesn't burn any cleaner or dirtier than low octane gas.
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[bill3508]

And if anyone is interested here is a very detailed writeup of fuel to pick through.

About.com: http://www.turborick.com/gsxr1127/gasoline.html

[Sandinmytires]

Is everybody confused now or is it just me?

The following is based on a million years of driving experience (OK, not quite a million): Higher octane = better performance. Better performance = heavier right foot.

And that right foot has more to do with MPG than any other factor.

[bill3508]

Originally Posted by Sandinmytires

Is everybody confused now or is it just me?

The following is based on a million years of driving experience (OK, not quite a million): Higher octane = better performance. Better performance = heavier right foot.

And that right foot has more to do with MPG than any other factor.

More of an octane discussion than gas mileage.

A little change to your idea:

Highest octane that does not produce knock=better performance. The rest is totally true.

I doubt a lot of people running low octane realize knock or pinging is there and just believe its normal valve train noise.

[stevekx]

Thanks for the responses, just got back from a 3 day trip, with getting around 18 mpg on 89 octane. I don't use nothing but regular.


I just tried the regular vs premium gas test a while ago (when I used to commute), same route, same speed (cruise control), same gas station, using regular and 93 octane (3 tank full in a row) and noticed the MPG drop with 93 ocatne.. It was a semi scientific test.

[Rrumbler]

All valid points, except for the part about using the same pump at the same station all the time. It does not make a whit of difference which pump you use; they all draw from the same tank of a specific octane rating, and should all be calibrated to pump the same volume, certified and sealed by the State Department of Weights and Measures. The only difference between the various brands of fuels is the additive package that is put into the mix at the time the load is pumped into the truck at the loading rack; Chevron specifies a certain additive mix, and Shell another, for example. Odd brand or independent stations may have a different spec, as well, but most use a generally standard mix. We would load out five or six compartments, and drop half at "Joe Blow's Place", and the other at "Billybob's" clear on the other side of town; same gas same additives; then go back to the rack and load out for Chevron, or Shell, or Sinclair, or Costco, etc. So, brand loyalty and a certain amount of personal consistency may be good, but in the greater scheme of things, it really doesn't matter very much.