Octane Question

[Tulo]

This might be stupid, but oh well. If you mix half a tank of 89 with half a tank of 93, will you have 91? Does it work like that?

[OGRE]

something like that, why does it matter, you spraying? advancing timing?

[Tulo]

lol na, just curious.

[OGRE]

if you aren't doing anything you should run the lowest octane without detonation.

[buelliganx1]

OMG thank you blackstang, I cant tell you how many people I hear that think you can put higher octane fuel in yur tank and just make more power.

[Dr. Pomade]

Originally posted by Blackstang:

if you aren't doing anything you should run the lowest octane without detonation.

Would you explain that a little more? I'm one of those who, as alluded to by buelliganx1, have been told that higher octane = more power.

[Renner]

It depends on the car really. Certain cars, such as the MK4 Supra, use the knock sensor for dynamic timing control all the way to redline. Therefore with the MK4, the maximum amount of timing advance is determined by how much the car knocks. So in this case, by putting in higher octance fuel and resetting the ecu, the car will make more power with only adding higher octane due to the timing advance.

 

For any boosted application, a higher octance fuel is almost always safer and therefore "better".

 

Many other cars however, only read the knock sensor in lower rpm ranges. They do not dynamically adjust timing throughout the rev range based on knock. In this case, running extra octane above what is required to stop detonation will not help you much, and may contribute to extra carbon build up.

[Putty]

I guess that would = 91

[Dr. Pomade]

Originally posted by Renner:

It depends on the car really.

Come on, hot shot, you of all people should know how easily it is to talk over my head on these things. tongue.gif So, be a pal and just tell me what octane I should be using.

[Tulo]

Originally posted by Rebel INS:

I guess that would = 91

lol thanks

[buelliganx1]

I guess someone could explain what "octane" is so it might be a little easier to understand. To the best of my knowledge octane is just a rateing of a fuels ability to resist detonation. So like blackstang said use the lowest possible without your engine detonating but like renner said be aware if your vehicle has a knock sensor that will pull back timeing in order to stop knocking. I've heard that useing to high of an octane cane actually make less power because it's a slower burning fuel.

[nurkvinny]

Some useful links -

 

www.racefuels.com/fuelterms.htm

 

Clicky

[OGRE]

Originally posted by buelliganx1:

To the best of my knowledge octane is just a rateing of a fuels ability to resist detonation.

Correct, but higher octane also burns slower, and cooler (althought very slightly) than lower octane gas, this is because it takes more energy to break down the molecules, all this combining to actually give you slightly less Horse Power in an engine ment to use lower octane fuel.

 

on an engine ment to run on 87 its really a waste of money to put more expencive gas in the car even occasionally unless you are spraying or advancing timing.

 

Nurk. . . that second tech page is a good one, I also have that one book marked to help explain things to people.

[gillbot]

One thing people overlook in the way an engine works is pressure. You need THREE things to burn fuel properly, spark, air and PRESSURE. This compresses the fuel/air so the spark can ignite it properly and efficiently. Basically, The higher the octane, the less combustible under pressure. That's why you wanna run the lowest octane possible, so that you get a better burn. Too high octane, the fuel burns slowly and most likely not completely.

[CobraKevin]

Originally posted by Venomss:

Come on, hot shot, you of all people should know how easily it is to talk over my head on these things. tongue.gif So, be a pal and just tell me what octane I should be using.

Many high performance cars like ours "require" premium fuel, so just use 93/94. That's what I use.

[Conesmasher]

I have always wondered how advanced my knock sensor is, and if it does compensate for higher octane fuel. I am gonna have to test this out on the dyno one day.

[desperado]

Yeah, it's a common misconception that more Octane equals more power, that is really not the case. It quite the opposite.

 

Here's why.

With the removal of TEL (tetraethel lead) from pump gas, the additives that raise the Octane rating of the fuel now are aromatic solvents, like touene, xylene, and acetone. These solvents have a very low boil point, low flash point and fast burn rate. The reason that we DON'T want this is simple, these additives burn fast, the more you compress them, the faster they burn. The motors we have, unless someone out there is running a F-1 motor in their car that turns 13 grand, have piston travel rates that are not fast enough to make a complete stroke before these additives are burnt off. At that point, the piston is not being pushed down by the expansion of the gases burning in the cylinder.

So, in order for these very high octane fuels to work, they either need to burn slower, or there needs to be alot more of it burnt so it will burn longer. This of course means drastic reduction in MPG.

Extreem example is nitromethane, while not an aromatic solvent it does show what I am talking about. Nitro in reality does not burn in a fuel car it litterally explodes, but there is SO much of it that the explosion is sustained for the entire downstroke of the piston and then some. If Nitro is looked at chemically it is 53% oxygen by content, this means that it WILL support it's own compustion it an oxygen free envrioment, that's why it was used as rocket fuel. Top fuel motors are run on the virge or hydrolock (hydrolocking is when a liquid, that is noncompressable fills the cylinder to the point that the piston can not reach top dead center because of the liquid. With running an 8 to 1 compression ratio or lower, that's a lot of Nitro,and it ruins the motor in short order.

 

Now the aromatic solvents are not nitro, not even close, but it reqilres alot more of them to sustain a burn for the entire stroke, and with the newer engines, that are EFI and computer controlled, they will never run that rich, so power suffers.

 

A motor will by design require a certain level of Octane to run at max power, and timing is directly proportional to power output at least to a point. But once that octane level is reached, maximum power and effency is reached, if higher octane fuel is introduced, it does MOTHING to the power at high RPM's and at low RPM torque suffers. ANd that is what we all want it torque, horsepower ratings are cool to brag about, but on the street, torque is king. All the horsepower at 7000 RPM you can imagine is going to do nothing for you from a standing start with street tires and a tight convertor, and 7000 RPM clutch dumps do nothing but make your street tires greasy and get you tickets.

 

My .02 on this.

[MrMeanGreen]

I'm gonna relocate this to Tech...

[Black ITR Guy]

Originally posted by Desperado:

ANd that is what we all want it torque, horsepower ratings are cool to brag about, but on the street, torque is king.

when you say that its sounds so much more convincing than when i say it. and all this time i thought i was an idiot for worrying more about tourque than my hp. thanks

[Putty]

Originally posted by Tulo:

lol thanks

Whatever I can do to help

[slow4now]

Originally posted by Desperado:

ANd that is what we all want it torque, horsepower ratings are cool to brag about, but on the street, torque is king. All the horsepower at 7000 RPM you can imagine is going to do nothing for you from a standing start with street tires and a tight convertor, and 7000 RPM clutch dumps do nothing but make your street tires greasy and get you tickets.

 

My .02 on this.

/singing/

Desperado

Why don't you come to your senses?

/singing/

[mrblunt]

there are a lot of variables that your overlooking. n/a intake temps and forced induction intake temps will be a huge determining factor. b/c i'm turbo there isn't a chance in hell i'm using 87 regardless of how cool and controlled people man think it burns. higher octane will burn in a more controlled manor b/c it will not pre-ignite/detonate as air intake temps rise.

 

now i'd agree with BlackStang in that if your running an econo 4 banger meant to run 87 your better off staying w/ the recommended. i have actually seen dyno tests of different octane in a n/a honda and it made more power on the lower octane.