I've got a non-turbo 2.2 in a 99 Impreza. What's the highest octane rating that is safe to occasionally run in this engine?

I don't think there is a max high octane number that would be unsafe that you can purchase at a gas station. Your engine might run just slightly better, but not much at any higher octane. Yea, aviation gas is super high octane, and could cause trouble, but I doubt that you are referring to that. What is unsafe, is how much extra $$ you will be spending for minimal performance increase.

 

On the other hand, using fuel rated below 87 octane would be unsafe because of probable fuel detonation (pinging). I have seen where real bad pinging can blow a hole in the top of a piston. However, I think heavy carbon deposits helped cause the detonation.

Octane is a resistance to detonation, not an increase in "power".

 

Higher octane gives you more power because it allows you to run higher compression, earlier ignition timing, and more boost if thats the way you go.

 

 

99.9% of the time increasing your octane above factory reccomendations will not increase power or economy, unless you've done something to the engine.

 

It costs more and does nothing, why use it?

Octane is a resistance to detonation, not an increase in "power".

 

Higher octane gives you more power because it allows you to run higher compression, earlier ignition timing, and more boost if thats the way you go.

 

 

99.9% of the time increasing your octane above factory reccomendations will not increase power or economy, unless you've done something to the engine.

 

It costs more and does nothing, why use it?

+1 finally a voice of reason on hi octane

Yea, aviation gas is super high octane, and could cause trouble

 

I believe aviation gas still contains lead. (not good for our engines)

I believe aviation gas still contains lead. (not good for our engines)

Lead is very good for engines, that's one reason why planes still use it. It's just not so good for the environment.

Its not good for Cat converters, if thats a 1000 dollar mistake you want to make go for it.

 

Also with those new fangled hardened valve seats, its kind of redundant

actually, it's not the lead that's the problem - it's the phosphorous contained in the lead scavengers. Trouble is that you won't get one without the other. TEL lead will cause a combustion chamber build up by itself - the PHOSPHOROUS just combines with it and flushes it thru the chamber (and poisons catalysts).

 

The lead atmospheric emissions reduction was a "side benefit" With the stellited hardened valve seats, there isn't a need for the lubrication that the lead provided between the valve faces and the seats.

 

As to running 115/145 (super hi octane aromatic av-gas) - one problem is that it runs so hot (combustion temperature) it tends to burn the valves. Another is that 115/145 is VERY unfriendly to most of the gasketry in the fuel system (but since I've been away from it for years now, it might have changed)