When people put a turbo on an EJ25... it seems like they all switch to a higher octane. Why?

If the compression ratio is the same for the engine... why is there a need to switch when you add a turbo?

at 8 psi, your 10: 1 motor is close to 13:1 compression!

that makes sense! Thanks. :D

A little more detail:

A turbo forces a larger amount of air into the cylinders.

CR (compression ratio) is essentially a ratio of the volume of air before compression (end of the intake stroke) to the volume of air at the end of the compression stroke. Since the compression chamber on your heads does not change the final volume is the same. However, the initial volume of air is higher due to the turbo forcefeeding it into the engine.

It's hard to explain on paper.

Octane is basically a fuel's ability to resist combustion. The higher the octane the higher the pressure needed to cause combustion. Diesel has a VERY high octane and thus diesel engines has very high cylinder pressures. Ever tried to light diesel fuel?

So when you increase cylinder pressures (with a turbo or supercharger) the lower octane fuel has a tendency to ignite from a combination of the residual heat in the head and spark plug and the higher pressures. This premature ignition happens on the compression stroke PRIOR to the spark plug firing. This is often called detonation. IT's very harmful to an engine because the piston is still traveling upwards and the combustion is trying to push it back down.

Raising the octane of the fuel prevents this premature ignition because it takes the spark plug firing to ignite the fuel, not just the residual heat.

Ya dig?

good info.