Hey guys i'm at that point where i have to decide what fuel. Here is Sa we have 93 and 95 octane pump gas. And i wondered what about methanol? :P

it all depends on your C/R...
and how deep your pockets are!

i agree on the pockets
but what is the difference
in here we use unleaded and aviation

aviation is for engines with really high compression...
if it isn't detonating you don't need more octane, its just wasting money.

who uses his engine for racing they use aviation in here. what u mean by high compression. any compression ratio suggested???

over 8.5:1 would be higher than stock

again, if you aren't detonating you don't need higher octane.

10:1 engines start to need aviation gas, 12:1 for sure, but then it still what octane...

more octane doesn't magically make more power...

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I don't agree. The GX engine is standard 10:1 and it uses normal 93 here in ZA.
My VW 1.8i engine is 10.5 : 1.Actually all VW engines are 10.5 : 1 and it runs fine on 93 and 95 octane.
I think it is from 11:1 that one must go 95 and higher octane to prevent detonation.
I know of a guy that build a performance A15.He used a GX head that was skimmed to the max. No detonation and I think he is running 95 octane.

See one needs to know what is the increase in power with a higher compression ratio, say 12 : 1 or higher. And fueling.

I disagree with b210sleeper, and agree with 1200gxman on most, but not even at 11:1 do you really need high octane, the 20V blacktop 4AGE motors run 93 and/or 95 happily, and they are 11:1 standard.

Clive, I used to run my 4AGE that's turboed on 95 just to make sure, but sometimes they ran out and I used 93 and it made no difference, no pinging at all.

Octane can help prevent knock, but if you fuel your engine properly with the right timing settings, you don't need such a high octane number most of the times. Some engines are also less prone to knock than other, due to combustion chamber, piston and valve design, and with them you would be able to use far lower octane and run more timing than most others.

I ran my 11:1 road car engine for years on 96 with no issues. Cam timing does come into it too though (wilder cams can tolerate higher calculated compression ratio). I wouldn't be running much higher than this on pump gas though.

Modern engines are able to stand lower octane vs high compression due to electronic timing trickery.