WantTwoRun?

Just wondering who uses octane boost.. if you do what kind of octane boost..and do you feel the diffrence?

WantTwoRun?

???

Jesstzn

No need to if your running the right octane fuel. Not enought bothers it but too much is just wasted.

Atrain

WantTwoRun?,

Octane boosters add a point to whatever you have in the tank. So if you have 91 octane and you add the expensive bottle of octane booster you end up with 91.1.

TIP: The higher the octane, the LESS volotile the gasoline.

Stick with what you need for your car. The rule of thumb is go as low as you can go before the engine knocks, then move up to the next octane. Since the TL has knock sensors, you must use 91 octane or you will lose some power when spark timing is retarded.

A-Train

u1arunit

iTrader: (1)

Run the specified octane.

Running too much octance can actually hurt performance since the additional octane slows down the combustion process flame travel to help eliminate pre ignition or detonation.

Atrain

u1arunit,

Octane does not control the burn rate of the Air Fuel mixture. Octane is added with Heptane to control the sensitivity to ignition. The higher the octane the less volitile the fuel. More octane means more resistance to knock and pre-ignition.

A-Train

Bearcat94

So, if my car is designed for 91 Octane Gasoline, but I choose to run (who knows why) 105 Octane Gasoline, the Air/Fuel burn will still occur "on time", not late?

SouthernBoy

The mixture will still ignite, but you will get less power and poorer fuel economy. Mr. atrain is correct. Higher octane-rate gasoline is less volatile than lower octane fuels. You get the best of what your engine can produce by using what is recommended by the manufacturer.. providing your engine is in a stock, or nearly stock, state of tune.

u1arunit

iTrader: (1)

You are correct in a sense.

I didn't say burn rate, but as you've said, volitility is a better way to explain it.

Either way we are saying the same thing to people who want an easy, quick answer. The rest is just semantics.

BTW, OT I see you are into Fords as well. I have a '04 SVT Cobra that puts down 663RWHP/665RWTQ that I tune myself with the SCT PRP software so this isn't my first rodeo either.

Jesstzn

Boyles Law
PV=nRT (pressure * volume = number of moles of gas present * universal gas constant * temperature)

As you compress a gas, it heats up. As the air/fuel mixture heats up, it tends to explode without a spark. With a forced induction system, you also have heat soak from the turbo. Intercoolers help with this, but they're not perfect.

Higher compression ratio cars create greater compression (and power) by taking a larger volume of air and compressing it into a smaller space.
Forced-induction system forces a greater mass of air but compresses it less. However, since there are a greater amount of air molecules to react with the gasoline (which there will be more of if the system is "healthy"), you get a more energetic reaction.

You can also look at it a different way.
A higher compression ratio (say, 11:1), normally aspirated system starts off at 1 atmosphere (air pressure at sea level; 14.6959488 psi or 1.01325 bar) and compresses the air to maybe 11 atmospheres (161.6554368 psi!).
A lower compression (say, 8:1), forced induction system might start off with an additional 1 atmosphere (29.3918976 psi, from the turbo- or supercharger pressurizing the intake system) and only compress it 8 times it's original pressure. Of course, that's 235.1351808 psi!

It's all about how much air you're forcing into how much space. The more air forced into a smaller space, the more heat is generated and the greater resistence to pre-ignition you need.

Atrain

u1arunit,

Yes, my tune was done by SCT, but it was soley done by Jerry W. That was in 2001 before SCT even was around. He was reading hexidecimal code...it's insane.


Almost half the rearwheel power your Cobra is making, my car still gets sideways at 30 mph. I don't know how you can drive that car on the street. Enjoy it, sounds like fun.

Agreed.

I still stand behind my statement though. Octane does not change the burn rate of the A/F mixture.

Jesstzn,

I agree with what you have said and I'd like to add, there are mainly 4 things that control knock sensitivity in an engine.

1) Fuel and Octane
2) Combustion chamber design
3) Cylinder pressure
4) Air Fuel mixture temperature

These are the 4 big hitters that control knock. From my experience all supercharged and turbocharged engines are knock constrained.

PV=nRT...BINGO...Whenever you compress air you heat it. I've been saying this for months and most people ignore what I'm saying.

I'll also try and explain to people that boost is made when you flow air through a restriction. The more boost you have, the more cork in the system. You don't want boost, you want air flow and c.f.m.

Open the exhaust and boost drops off, open the inlet to the supercharger and boost goes up. Port the heads and boost drops off.

Another hard concept for people to grasp is that boost does not equal power.

Bearcat94,

In theory it will occur slightly later, but once it starts...the rate is the same as 91 octane fuel.

SouthernBoy,

As always I concur.

A-Train

phee

iTrader: (5)

so lemme get this straight, does running 91 octane produce a cleaner burn=more power than 93 octane?

loulinjai

The idea is right but it's not a direct relationship. PV=nRT only applies to ideal mixtures, where molecular interactions are assumed to be ideal/perfect. You also have to consider thermodynamics, amount of liquid or vapor the fuel is it, and how far from ideality these molecules behave.
The relationship between octane is not the same either. The difference betwee 87 and 91 is not the same as the difference between 91 and 95. (although both have 4 octane points)

SouthernBoy

Exactly. The little fly in the ointment. And the faster it's compressed, the more it's compressed, the quicker and higher the heat generated. The essential principle behind the operation of diesel engines.

"Air", as in atmospheric air, acts funny under various conditions. In my teens, I used to scuba dive. One day after a dive and back home, I was checking the amount of air in my tank (71.2 cubic feet), and it still had around 500 pounds left. So like an idiot, I pressed my hand, the soft thumb base part, over the nozzle and started to open the tank valve. I was able to hold pressure until rather suddenly it overpowered my hold. For my stupid efforts, I received a nice little nasty burn.

u1arunit

iTrader: (1)

Taking aside all the technical stuff that confuses most normal folks:

Run what the manufacturer recommends for best results.

When we are speaking about running too much octane we are referring to using something like 100+ octane or more on a car with a mean best spark timing map that is optimized for 91 or 93 octane.

On most naturally aspirated engines the difference in power gain/loss that you can feel is so very minimal that it's a non-issue anyways.