In article . com,
wrote:

On Apr 27, 6:34 pm, "Morgans" wrote:
"JGalban via AviationKB.com" wrote

That's what I figured. Turbocharged engines are a different kettle of
fish.
The main reason is the higher temperatures in the induction system lowers
the
detonation margin considerably.

Not just the temp, but the pressures involved.

In a turbo, not producing boost, a low compression ratio is fine with low
octane gas. When the boost is putting all of that extra air and gas into
the combustion chamber, it is still compressing at the same ratio. You end
up with the normal internal pressure, plus the extra pressure the turbo
boost shoved in there. Then detonation becomes a big problem, without the
extra octane.

But you knew all of that, already. g
--
Jim in NC

If the engine is turbo "normalized", it never increases the boost
above what the engine would see at sea level power, right? That's why
turbo aircarft engines are rated at the same max power as non turbo
engines. If the engine doesn't need high octane gas at sea level, why
would it need it at altitude where the cylinder pressures are no
higher (merely boosted back to sea level MP). Am I confused?

Regards,
Bud

Even turbo normalizing increases the temperature of the fuel/air mix
entering the cylinders, due to compression of the air to achieve
"normal" pressures. The increased temperature increases the octane
requirements, all by itself.