Let's talk octane...

Jordo!

Octane refers to how easily the fuel will spontaneously ignite, as in from heat or pressure (which also increases heat); the higher the number, the less likely to auto-ignite.

Ideally, we want the fuel mixture to only ignite when there is a timed spark event. If it ignites at the wrong time, the force can turn the crank the wrong way (bad), and otherwise put tremenous shock and stress on the engine internals -- this is essentially what knock (preignition or detonantion) refers to: unplanned auto ignition of the fuel that can damage the motor from the stresses it creates.

Higher heat and cylinder pressure (affected by boost and piston CR, as well as timing advance -- spark events tend to have flame kernels that keep high heat on the piston, thus more timing, also means more in-clyinder heat that doesn't have as much time to dissipate before the next rotation) increase the likelihood that the fuel will auto ignite, so for applciations with higher CR's, boost (or even excessive heat under load), a higher octane is generally necessary (DI sytems can get around this somewhat, thanks to the super fine atomized high pressure fueling mix it creates and the ability to spray directly and precisely into the cylinder -- but that's another story).

There is a bit of a down side -- really high octane fuels are harder to ignite even when you want to -- thus a 120 octane fuel in a DD might be hard to turn over on a cold morning, and be more prone to misfires, whereas on a boosted, high CR track car that might work very well.

In general, there are no direct benefits running a hgiher than needed octane -- again, octane is all about preventing knock. The extra power is all based on cylinder pressure in the piston or engine speed, so unless you are running more timing, boost, higher CR pistons, etc, that might require a higher octane to run safely, it's not needed.

As a precaution on a very hot day (again, in-cylinder temps increase the chance of knock), running the car at constant high load on the track, the extra octane might be a reasonable precaution, in that it will prevent power loss/damage from possible knock events, but it will not give you any additonal power than you were already capable of making given your tune.

Incidentially, that is also why tuners will find better power on boosted cars by running a bit richer a mixture -- the extra fuel is not fully burnt, rather it just cools the piston, reducing heat, and therefore quenching possible knock.

Anybody want to add to/amend that?

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Methodical4u

good post... very informative. I started this thread because it seems like the exhaust/intake part of our engines is pretty well maxed out, so perhaps we could look to fuel to somehow add more power. I personally don't want to go FI... I also don't have the money to do so right now and if I did have it I wouldn't spend 10 grand to do it. The intakes are about as good as they are going to get. Motordyne's manifold, while very well made is sort of hit and miss from one car to the next... no cams because there is no VVEL crack... in general there isn't much left to do I guess.

Red__Zed

good post, good info. Octane is always one of those confusing specs for most people. Maybe we should try and get a reference thread together for info on fuels if people would find that helpful?

MightyBobo

If you cant get more air into the motor, adding "better" fuel is a waste of time anyway.

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Methodical4u

so any idea on how to get more air?

Red__Zed

Honestly, doing lots of "little things" to add power is just not the smartest path to take. You spend almost as much as you would to go FI, and it leaves the car just as (if not more so) unreliable as it would be with boost.

370guy

Ok, I had a GT500 before my Z and have tried every fuel possible. Heres What ive found.

Basics of higher octane, allows you to safely run more advanced timing without the fear of detonation. So it is more valuable on forced induction cars than NA builds because the FI have more to gain off timing changes. So now to the options

1. E85. corn fuel. Its cheap and has and advanced level of octane far exceeding regular pump gas. Advantages = more timing, more power, CHEAP! Disadvantages = very VERY corrosive, upgraded fuel lines, pumps fitting, and injectors required. hard on motor, hard to come by in some places

2. C16. 116 octane. come in 2 variants, leaded and non-leaded. non leaded required by cars WITH catalytic convertors. Advantages = BIG power, smells awesome (opinion i guess), basic fuel system upgrades required Disadvantages = Very Expensive, Terrible economy

3. 100 octane. sold in some gas stations same advantges as above but with basic tuning can be run on stock fuel system.

Know for the curve ball!!!
Methanol Injection! allows you to run advanced timing on pump gas! basically increases the effective of octane of what ever is in the tank. Advantages = Its washer fluid so its cheap! Horsepower. Disadvantages = Seperate methanol system required for us (pumps, lines, tanks etc...) when you run out car can bnecome unsafe (if its tuned for the Meth and it doesnt get it you have problems)

So in conclusion (sorry fo rthe long post) there are advantages and disadvantages for all fuels. You gotta choose whats right for you. But as a rule of thumb, NA cars dont need to super high octane 91-93 is generally good enough

Methodical4u

There are a lot of people who will tell you methanol injection on an NA motor isn't going to do a whole lot... i've done a lot of looking into it myself.

370guy

It wont for the sheer fact that the timing adjustments where the enhanced ocatane levels that methanol will let you achieve wont make a big difference on an NA car. you could pick up maybe 5-10 WHP MAYBE but with the price of the kit and all the tuning required to make it run well wouldnt be worth it at all. your looking at 500 for a basic Kit then install then who knnows how many hours of tuning. Id never put it on an NA car

Red__Zed

maybe with ITBs

370guy

ITBs and a Methanol on a 370...hmmmmm....this could be interesting. Im going to look into this. sounds like it could be pretty radical

Methodical4u

Right... that's my point... it's really not worth all of the hassle. However, I have heard people say they have gotten better mileage which might make it worth it if you were willing to keep it for the long haul.

Methodical4u

what the hell is an IBT?

370guy

ITB 350. Im liking this more and more!
http://www.google.com/imgres?q=itbs+...1t:429,r:2,s:0

Red__Zed

individual throttle bodies