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does anyone know how much pressure is needed to turn horsepower
I'm having a hard time understanding your question. Are you asking what is generally a good launch nitrous pressure?
Generally speaking 900-950 is a good starting pressure as you won't see a large pressure drop as you do above 950. The lower the bottle pressure the greater the density as well. However, your brand of kit as well as your n/f ratio (jetting) will play a role in launch pressure.