octane tunes

[jchev502]

im trying to figure out how you change your octane rating the motor needs or "likes" with a tune. im used to tinkering with older vehicles and the only way i know to really jump up octane is more compression. so i dont under stand how your supposed to jump up with just tuning it.

[joecar]

You can't...

To change the compression ratio, you have to change internal "rotating" parts (crank, rods, pistons)...
To fine tune the CR, you change the heads and head gaskets.

Increased CR requires petrol with a higher octane number, otherwise your motor will chronically knock.

You can't do this by tuning the PCM.

[swingtan]

Joecar is correct, as usual......

there are a couple of points in the original post that are simply incorrect.



jump up octane: The octane rating is a measure of how well the fuel controls it's "burn" once ignited. It also describes how well the fuel prevents false ignition, or detonation. "Octane" does not describe anything about the motor, just the fuel.

The only way to change the octane rating of a fuel, is to add "stuff" to the fuel to increase it's octane rating. This "stuff" can be found at most auto parts stores and fuel outlets. Making changes to the motor will not change the octane rating of the fuel.


more compression: Increasing compression not only allows for more power to be made, it also heats the intake charge more on the compression stroke. If the octane rating of the fuel is not sufficient, this heating effect may cause the mixture to ignite before the normal "spark event". This is called detonation and has the effect of trying to stop the motor from tuning. It can result in a poorly running engine at the very least, or broken pistons / bent rods in the extreme.



I guess though, you could adjust things like spark advance and AFR's to "suit" the octane rating of the petrol you use, for a given motor. For example, a fuel with a higher octane rating will be able to sustain more spark advance than a fuel with a lower octane rating. You may also be able to run slightly leaner with a higher octane fuel. In the end though, it will come down to more factors than just compression ratio and octane rating.

Simon