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Date: Tue, 01 Feb 2005 04:27:45 GMT
From: "James Sweet" <jamessweetnospamail.com>
Subject: Re: Saab newbie


"Valued Customer" <nonospaml.com> wrote in message news:r10uv0pvf5fvif55kbeaabap3rp85orvavnospamcom... > Yes, a turbo charger is indeed a compressor... > > However, I still fail to see the logic in lower octane at altitude is > okay for naturally aspirated engines but not for turbo charged ones > (suggested by Zon and I guess now Graham). > > Octane is a measurement of how high the ignition point of gasoline > is... nothing more. Altitude is not an "excuse" to use lower octane. > I'm not a refiner but I doubt octane is directly related to the cost > of refining the oil. Because of the altitude, the air pressure is > lower and therefore things heat up at a lower temp -- where I live at > 6,200' water boils at 209 deg F and hence gasoline ignites at a lower > temp too -- which leads to the 85 - 91 ratings. > > Turbos are actually more an asset at high altitude than sea level... > and this is mutually exclusive of octane level. For example, why do > you think commuter aircraft are turbo charged? Because they do make > already thin air denser for combustion... but again, it is a stretch > of deductive reasoning to say because things get hotter at lower > pressures that octane is therefore a derivative pressure. > There's still something you're missing here. Turbochargers effectively raise the compression ratio when under boost, higher compression engines require higher octane ratings to prevent detonation due to heated air from the compression, in a turbocharged engine the intake air temperature is higher by nature and it gets hotter still when compressed in the cylinder. This remains fairly constant regardless of atmospheric pressure, while in a N/A engine the compression is effectively lowered with the thinner air. Turbos are more susceptible to detonation under any circumstances and are not affected much by altitude, N/A engines are less prone to detonation at higher altitudes so lower octane is permissable. That said I have yet to come across a production N/A engine that wouldn't run just fine on plain old 87 octane, you can't advance the timing as far and won't get as much power but in engines with a knock sensor there's no fiddling, just less power produced on lower grade fuel.

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