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Discussion Starter · #1 ·

Here's a really cool online calculator to determine absolute air pressure and horsepower. Remember, absolute pressure is only the base for determining static compression and boost levels.

Horsepower calculator

There are many other variables that determine what timing and compression is 'safe' such as octane rating, thermal load (a biggie), gear ratios, gross vehicle weight, driving style, etc.

Often, raising compression at sea level will gain you next to nil
as any increase in torque is offset by having to back off ignition timing to avoid detonation.

However, at altitude, increasing compression 'compensates' for reduced absolute air pressures and merely puts cylinder pressures back in line where they would normally be at sea level.

Hpro, thinner air in and of itself won't cause detonation, but

a) On warm/hot days, many vehicles run hotter at altitude due to the reduced cooling effect of thinner air passing through the radiator. The Infiniti cooling system is massively over-engineered, however

b) Aluminum cylinder heads cannot transfer/radiate (heat loss) as much heat to the surrounding thinner air. The result is higher combustion chamber and exhaust valve temperatures - this normally isn't significant unless you are already on the edge

c) some ECUs adjust for absolute air pressure by sensing air flow during WOT. If Nissan does, your ECU is already leaning out the mixture and advancing the timing at altitude.

d) and if you don't switch gasoline grades, you will be filling up with lower octane fuel

hope this helps
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