You might indeed be mistaken. With 91 octane the ECU will pull timing to ensure no detonation and will also probably run a richer mixture (not sure about the A/F ratio). With 93 or higher octane the engine will happily run more advanced timing and/or higher boost before the knock sensor makes the ECU pull timing. You all probably know very well how much performance (power) can be lost or gained by retarding or advancing the timing....

The answer would seem to be clear: higher octane= higher performance (up to a point)

 

Correct, a higher octane gas does NOT mean that it has more explosive power or that it will provide better performance or more horsepower. To be honest, octane actually LOWERS the gasoline flammability.

Basically, higher octane = more timing = more power because of less retarded timing. Lower octane = less timing = less power due to retarded timing. In high performance engines (e.g Evo), you get very hot cylinder temperatures. This is especially true when dealing with forced induction (e.g. turbos, superchargers) and nitrous. Lower octane gasolines (e.g. 87) are bad for performance engines because they can pre-ignite as soon as the fuel enters the chamber instead of waiting for the spark plug to fire. This is very bad. Higher octane gasolines don't ignite as easily, and they better resist this pre-ignition (also called knocking, pinging, or premature detonation) and can therefore wait until further in the compression stroke (i.e. deeper timing) for the spark plug to fire and ignite the charge (which pushes the cylinder down, which turns the crank). The further down in the compression stroke before the spark plug fires, the more power you make.

If your car runs 93 octane gasoline and the computer is not pulling ANY timing at all, then you have found your peak octane for use. Adding 94 octane or even 104 octane will not provide any additional power or performance to your car at this point. This is also why it is REALLY dumb to use Super Unleaded (e.g. 91 or higher octane) fuels in cars that only require regular Unleaded gasoline. Since they only require 87 octane to run normally (i.e. without retarding timing), running 89 or 91 or 93 octane is a total waste...you are just pi$$ing your money away and not gaining ANY performance boost.

So no, higher octane does not actually "add" or "create" power in your engine, but it CAN GAIN ("unlock") horsepower in your engine and let the engine realize its full HP potential IF your car was having to retard timing on the lower octane fuel.

If you are running 91 octane, my guess is that your Evo is probably retarding timing to some extent (maybe not much, but it could be a few horsepower). Upgrading to 93 octane (or higher) should restore some of this timing (and thus power), up to the point that no timing is being pulled. Make sense?

 

Itll make less power because you will be operating under a little bit of knock ... the computer will retard the timing a little to keep your motor from blowing up, and thus, you lose power.

I live in Cali also, and my Evo runs like crap on 91 octane. You can feel the sudden loss of power as the computer retards timing .. and it happens quite frequently.

What i started doing, and it works great, is always going to 76 station, putting in 4 gallons of 100 octane and the rest 91. The car makes noticeable more power, and i havent felt any timing retard on the last 3 tanks of gas ive blown thru

 

Mobil 93 ONLY...!

i don't have anything with a higher octane in my area...

 

One more fact...
Higher octane burns slower.
That equals more time "pushing" the piston.

P.S. Pumps here have 100 octane for $3.49/gal.

 

87 octane

 

Okay I was expecting massive flames for my post.

So i had my flame-retardant suit on for no apparant reason?

In actuality, I use 93 octane, ALWAYS.

 

maytar good info, that helped a lot.

 

i know this is a really dumb question but.....what is knocking?

 

"Knocking" (also called pinging or detonation) is caused by small explosions in your engine cylinders that were NOT caused by your spark plug. Knocking is caused by using fuel that contains too low an octane number for the amount of timing (i.e. spark plug firing) in your engine.

Basically, in each cylinder, when gasoline and air are sprayed into the cylinder, the spark plug will fire ONCE (don't worry about MSD for now) at the end of the compression stroke. This causes a single spark that ignites the fuel/air mixture in the cylinder, causing a single explosion in an even pattern (i.e. flame front) that hits the top of the cylinder evenly and pushes the cylinder back down to turn the crank. This is the ideal situation when the engine is using gasoline with a high enough octane number to support its timing.

For "knocking", when you use a gasoline with too low of an octane number (e.g. 87) in a high performance engine (e.g. Evo), the gas is MORE flammable and ignites more easily. This can cause "hot spots" on your cylinder walls and the top of the piston. These hot spots are like glowing embers or coals (but very small). When the low octane gasoline and air enters the cylinder, it will ignite as soon as it hits one of these hot spots (instead of waiting until the end of the compression stroke for the spark plug to fire). This means that the piston is on it's upward stroke towards the spark plug when the detonation occurs TOO EARLY, thus exerting all of that explosive force against the piston while the crank is still PUSHING the piston. This is VERY VERY bad and creates ALOT of stress on your piston seals, connecting rods, and drivetrain.

Most modern cars today (e.g. Evo) have computer "anti-knock" sensors that are tuned to listen for this knocking (i.e. premature detonations), and as soon as they detect it they will retard the engine timing (i.e. spark plug firing) deter the knocking. This retarding of timing (also called "backing off" or "pulling" timing) is usually felt as decreased horsepower and acceleration performance. Make sense?

Note that "knocking" (detonation) is frequently heard in discussion with people who have modded their cars using forced induction (turbos, superchargers) and/or nitrous. Many hot rod owners will spend alot of time tuning their car's fuel mappings and timing trying to achieve the optimal balance between being too rich (i.e. more fuel than air in the intake charge) or too lean (i.e. more air than fuel in the intake charge). In general, more fuel + more air = more power, but as the pressure increases in your cylinder, heat also increases, and lower octane fuels detonate earlier than high octane fuels. Many guys who try to squeeze the last ounce of timing and performance out of their cars frequently end up "blowing" their engines because of detonation including burnt piston tops, broken piston rings and seals, snapped connecting rods, etc.

 

thanks that helped ALOT

 

Nice job explaining that

 

Need to add a "100 octane" option to the poll for us hardcore types.

At least a "mixture" option.

 

Well, I use 92 because that's the best that's available in the whole state of Washington as far as I can tell (with the exception of the occasional 100 racing fuel for $6/gal). I have to say I'm a bit jealous of you 93/94 people being only a small little step better than Cali gas. BTW, why isn't 92 on the poll?

 

So if you want to use racing gas, do you just get a container of it at the gas station and mix it with the 93 pump gas??

How much of each??