Quentin has it right. To put it in other words, higher octane gas simply has a higher temperature at which it ignites. I'm about to write a book here, so get comfortable. For those of you who don't really understand how motors and octane work, it might be worth the read.
It helps if you imagine (in slow motion) how the combustion stroke of an engine works. As the piston rises and starts to compress the air and fuel mixture that is in the cylinder chamber, that mixture starts to get hotter and hotter. This happens any time you compress something; its temperature rises. So that air-fuel mixture gets hotter and hotter as it's getting compressed more and more. Normally, when the piston finally gets near the top of the stroke and the air fuel mixture is compressed just the right amount, the spark plug fires and ignites that compressed air-fuel mixture, causing a really powerful explosion that forces the piston back down. That's where the motor's power comes from.
Knocking (pre-detonation) occurs when the air-fuel mixtures gets so hot while it's being compressed that it spontaneously ignites *before* the spark plug fires. This means it explodes while the piston is still going up, so you suddenly have this huge explosive force pushing down on the piston while the rest of the motor is trying to push the piston up. That's why pre-detonation can destroy your motor. It can bend and break all sorts of stuff.
So if your motor is starting to pre-detonate, that means the air-fuel mixture is reaching its ignition temperature too soon, and thus you need to somehow make it so that the the mixture doesn't explode so early. If you are running 87 octane fuel, then you can try using a higher octane fuel. The higher octane (like 91) will require a higher temperature before it explodes. Hopefully, that higher octane will be enough for the piston to compress all the way up so that the spark plug fires *before* the fuel mixture spontaneously explodes. You never want the fuel mixture to explode on its own. You always want the spark plug to cause the explosion.
When the fuel finally does ignite, though, it has the same explosive force no matter what octane you are using. Whether it's 87 or 93 octane, the explosive force is the same. The only difference is the temperature at which the fuel would explode on its own if given the chance. Like I said, though, you never want that to happen. You always want the spark plug causing the explosion.
So given all that information, you can see that if your engine is not pre-detonating with 87 octane, then using higher octane won't give you more power. In that circumstance, no matter which octane you use, the spark plug is causing the explosion. And that's a good thing! That means you can run 87 and not worry about it. Of course, you could run 93 also, and it wouldn't hurt anything, but it's unnecessary.
Now, there is a bit more to the story. In a motor with electronic timing control and knock sensors, it can detect when that pre-detonation is happening. When it detects it, it can very quickly back off the timing of the motor to prevent that pre-detonation. That's great and your engine is once again running perfectly safely, but the downside is that you lose a bit of power when it backs off the timing. So, to get that timing advanced again, you can put in higher octane. The timing system in the computer is always trying to advance the timing a little bit at a time to figure out just where the limit is before pre-detonation happens. So if you put higher octane in, the computer will keep advancing the timing a little at a time, giving you a little more power.
It's important to remember that the computer can only advance the timing so much, though. Once it has reached the extent of its adjustment range, it can go no further, and that's as much power as you'll get from that motor no matter how much higher of an octane you put in it. The real question for everyone here is: Which octane of fuel allows the computer to advance the timing all the way to its limit in the 5th-gen 4runner? If maximum timing advance can be reached with 87 octane, then anything higher is a waste. If maximum advance can be reached with 91 octane, then running 91 will give you a little more power.
I don't know the answer to that, but anecdotal evidence here in this thread suggests that some people are indeed seeing increases in power with higher octanes. Other people aren't seeing any difference at all. It will depend on a lot of factors, such as:
- Your elevation (do you live near the coast or way up in the mountains?)
- How hot it is outside?
- In what environmental factors did the Toyota engineers design the motor to work at its maximum?
So you can see there is a lot to it, and arguing on this forum simply will never answer the question. Every person will have a different experience since everyone lives in a slightly different environment. One person could go out and do measurements and find that he gets no performance increase with 91 octane over 87, but some other person in a different part of the country can do the exact same test and find that he does see a difference.
I suggest you all simply try running different octanes and see if you can tell a difference. If you cannot tell a different, that's great. Run 87 and save yourself some money. If you think you can tell a difference, feel free to run higher octane. Or maybe you can feel a difference, but it's so small it's not worth the extra money. In that case, be happy with 87 and once again save yourself some money.
Regardless, you won't hurt the engine. And remember that higher octane does not "burn more cleanly." The "cleanliness" of fuel has to do with the various additives different companies put in the fuel. That is something I cannot comment on, since those mixtures are usually a secret, and I don't know of any extensive, long term tests that've been done on different brands of fuel.
