Harryman2002

No, Ive now had my XCal 2 for quite awhile now so I dont remember what the exact numbers were, but im pretty sure after I got the tune my other mods at the time were an x pipe and CAI and it was somewhere around 240RWHP.

I also had an 87 octane tune programmed into the car and the 91 still felt noticeably better.

Harryman2002

If it is just as good as 87 then why is it more expensive?

sonicx

Okay Okay here's what it boils down to: Premium gas really is a better fuel in terms of the power it provides in the right engine. Gasoline (all types) are brewed of many different hydrocarbon molecules, ranging from heptane (seven carbon atoms and 16 hydrogens) to decane (10 carbons and 22 hydrogens) and beyond. The hydrocarbon clearly identified on the pump is octane (eight carbon atoms and 18 hydrogens). This number isn't a measure of the percentage of octane actually in the gas itself. Instead, it is a measure of how that gasoline compares with a pure mixture of octane and heptane.

The octane rating of gasoline is a measure of how much you can compress the gasoline before it ignites, not how “good” it is. Cars that require higher octane gas actually need that higher priced gas because the engines compress the fuel more before it ignites it. If you put regular gas in a car that requires premium, the gas will prematurely ignite when it’s being compressed and the engine will give you a knocking sound. This is bad for your car.

AirBrontosaurus

Because it has different characteristics. When people say "it's better for the engine" they usually mean it's cleaner or burns better or treats the engine nicer or something like that. Most people think that 'octane' means 'purity' or 'quality' when in fact it doesn't mean that at all.

Octane is a scale of sorts. Pure octane has an octane rating of (get this) 100. So, if something has an octane rating of 100, then it burns at exactly the same speed of propagation and circumstances that pure octane does. If something has an octane rating of 50, then it burns at twice the speed and readiness of pure octane. If it has an octane rating of 200, then it burns at [i]half[/] the speed and readiness of octane. THAT IS THE ONLY THING OCTANE RATINGS MEASURE. They say nothing about the chemistry, the composition, the filtration, or the cleanliness of the fuel. Higher octane gas isn't more refined or better stored or cleaner burning or anything. The only only only only ONLY difference is that higher octane fuel burns more slowly than lower octane fuel.

HOWEVER, THIS IS WORTH PAYING MORE FOR IF YOU NEED IT. The general idea in a combustion chamber is that you want the fuel to ignite and burn in a wave, and you want that wave to impact the top of the piston at the instant that the piston is at the top of its travel. Anytime before that and the wave is pushing the piston down as the piston wants to go up, which causes damage and lost power. Anytime after that and the piston is already on its downward path and thus has taken away some of the engine's rotational power, which causes a loss of power. In most every stock engine the compression of the piston in the cylinder is not enough to spontaneously ignite 87 octane fuel. Thus, the spark plug is the sole decider as to when the pressure wave begins. HOWEVER, in aftermarket or modified engines the compression is enough to cause the cylinder temperature to spontaneously ignite before the spark plug has a chance to start the pressure wave. This means that the mixture can be ignited before the spark plug has a chance to start the wave, which means that instead of a measured pressure wave at a calculated time you get an erratic explosion at a random time. This causes immense pressures and forces that quickly tear an engine to shreds. Higher octane gas, because it burns slower and less readily, can withstand temperatures and pressures that lower octane gas cannot. That is why higher octane gas is necessary.

HOWEVER HOWEVER, the simple fact is that higher octane fuel has less energy stored in it. That's how it's able to burn less readily and survive in high-performance high pressure high temperature applications. What actually powers your engine is the propagating pressure wave inside the cylinder. The efficiency of this wave is decided by the spark plug's timing, but the raw power is decided by the amount of energy stored in the gas which is measured directly by the octane rating. It doesn't matter how great you are at timing the engine: given a perfectly timed 87 octane engine and an identical perfectly timed 91 octane engine the power created from the 87 octane gas will be numerically superior to the power created by the 91 octane gas, and thus the raw force pushing down on the piston will be greater, and thus the 87 octane engine will make more power (assuming a perfect tune and no detonation).

If you time your spark correctly you will make more power with 87 octane than with 91 octane. You should only go to higher octane fuels when you can no longer safely tune your car with your current octane.

AirBrontosaurus

That's because your 87 octane tune didn't push the timing as far as your 91 octane tune did. If you went back to an 87 octane tune and increased the timing your car would feel the same if not better. As long as you're not seeing detonation 87 octane gas has more energy in it than 91 octane gas, and thus all things being equal will give you more power.

Harryman2002

hmmm. that is interesting. Its kinda hard to completely understand, but ill take your word for it since im not by any means as experienced as you sound with this kind of stuff.

98SN95

Octane rating does not relate to the energy content of the fuel (see heating value). It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Where octane is raised by blending in ethanol, energy content per volume is reduced.

98SN95

WIKIPEDIA

AirBrontosaurus

I tend to get a bit long-winded with my replies, so thanks for bearing with me. However, I want to stress that you should do what works for you. If you tuned your stock car for 91 octane and it really felt better to you, then by all means keep it at 91 octane. If the extra happiness you get from it is worth the extra $0.20 per gallon price increase then there is no reason to stop using 91 octane. It certainly won't hurt your engine, and depending on how accurate your timing is you may well be making more power under 91 octane that you were under 87 octane.

My explanation was directed more towards the technical aspect of different fuel octanes. Technically speaking, you can make more power with a properly tuned 87 octane engine than with an equal engine running 91 octane. That being said, I ran 93 octane on my stock engine for almost 6 months before I installed my supercharger. I wanted to make sure I got into the habit of putting premium in before I got my supercharger so I didn't accidentally put regular in an get detonation. Was I getting the optimum power out of my engine? Probably not. But it worked for me.

There's nothing implicitly wrong with running a higher octane than you need. It just isn't technically the most optimal situation.

98SN95

yall keep comparing fuel octanes and timing, and the energy content and price, but the truth is if you want more power use higher octane, more timing and more fuel, its true that 87 does not have the enrgy that 93 has, but the difference is so small, all you do is add a little more fuel to componsate