Peter Duniho wrote:
"xerj" wrote in message
...

[...] what I don't understand immediately is why at altitude the MP
required is only 18.2 rather than 20.9.

Anyone have a simple explanation?


Su at the higher altitude, lower air density allows the necessary air to
go through the throttle at a lower MP.

simple but wrong explanation :-)

The important question is: what MP is neccessary to drive the required
amount of air through the engine. (assuming same air/fuel ratio for both
conditions) At lower ambient pressure slightly less MP is required to
get a given airflow into the cylinder, as there is less residual gas
left there. However this effect is rather slim.

Or: at the higher altitude, lower air temperature provides greater air
density at a given MP, allowing for more power at a lower MP.

Correct. This seems to me to be the more important explanation, if
standard altitude temperatures are is assumed.

regards,
Friedrich

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