93 octane tune vs 87

[Jagerbeast]

I mentioned to my boss (who knows way more about cars than i do) that i got a tuner and im going to start putting 93 octane in my car because thats what the tune requires. then he asked me something i really couldnt answer.... I thought the octane had something to do with compression, now if so.... what is the 93 tune going to do that i should use 93 instead of 87 or 89 octane?


On a side note im bitter as hell .... i thought there was going to be a tune for 87, 89, and 93 octane. Ive been putting 89 in my 98 gt, and just refilled yesterday, went to instal the 89 tune and there wasnt one. Now i have to wait till this tank runs out to put 93 in

 

[Stark77]

The tune will advance your timing and add some fuel. I'm not too sure if jumping from 87 to 93 would provide an increase in compression big enough to mention.

 

[ItsStock02]

The 93 tune will just advance your timing more than the 87 tune. Higher octane means less detonation.

 

[ChillPhatCat]

When the spark fires from a spark plug it takes a certain amount of time for the air/fuel mixture to ignite and explode. When this "timing" it is advanced, the spark lights off before the full compression of the mixture explodes in order to have the explosion occur at the optimal point. When the timing is retarded, that means that the spark occurs too late and some of the fuel doesn't burn and the engine runs rich.

By advancing the timing you can increase the amount of fuel that can be burned in the explosion (and thus power), but without the extra fuel you risk "pre-ignition" or detonation/pinging as it is more commonly known in the car culture. If you increasing the octane of the fuel, which is the fuel's resistance to detonation, you can get the most out of the air/fuel mixture in the combustion chamber. The gains are minimal, but they are there, and you might as well set it up to make the most power... where you will appreciate a custom tune the most is once you've added significant mods like cams, heads, blowers etc.

 

[bhuff30]

Wow, interesting ways of explaining this so far. hehe In short, just like above, the tuner will advance the ignition timing (fire the spark plug earlier) which increases the cylinder pressures and makes more power.

You want to avoid auto-ignition of the fuel mixture (detonation). At high pressures and temperatures, the fuel can self-ignite, which is a cause of detonation (very bad to engines, and can be heard as an audible knock or rattle).

The spark plug fires long before the piston reaches the top of the cylinder. If it is fired too early, the pressures and temperatures will become so great that the rest of the mixture self ignites, resulting in detonation. The higher octane fuel is required to prevent this auto-ignition. As I said earlier, it is the increased cylinder pressures that produce more power.

Just to clarify, advancing the timing does not increase the compression. It increases cylinder pressures.

Also, the ignition timing has little effect on how much fuel is burnt, but rather when it is burnt. The earlier combustion is started, the higher your cylinder pressures can be, which makes more power (more pressure pushing the piston down).

There are volumes and volumes written about this stuff, and it is very interesting to me. I've spent quite a while in the engineering library reading about flame fronts and the effect of turbulence on the burn process. That little wall on the inside of the PI head is there for a reason.

 

[ChillPhatCat]

Also, the ignition timing has little effect on how much fuel is burnt, but rather when it is burnt. The earlier combustion is started, the higher your cylinder pressures can be, which makes more power (more pressure pushing the piston down).

Ahh. Gotcha, that makes more sense now. That helps me understand my thermodynamics lecture from this afternoon a little better.

 

[Jagerbeast]

Thanks everyone for making some sense of this to me.