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On Apr 28, 9:09 am, Orval Fairbairn wrote:
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.- Hide quoted text - - Show quoted text - Never disagreed with this. It is the assertion that the turbo "is putting all of that extra air and gas into the combustion chamber" that is of contention. The total mass of fuel (air + gas) the cylinder injests is the same as a normally aspirated engine at sea level. That is why having a good fuel flow guage is such an important factor in running lean of peak, and is a major part of TAT turbo installations. Heck, when doing the lean test they recommend, monitoring fuel flow is the primary issue. The poster said it is "not just the temp". It is. It is also why intercoolers are so nice. They reduce the work the turbo has to do, and increase the detonation margin without lowering power output. Regards, Bud |
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