Thanks, but if I am running a higher compression engine, would it not then
require a lower allowable boost than a lower compression engine?
>It's a bit more complicated than that... for normal running, compression
>in a supercharged engine has to be lowered to decrease the chance of
>detonation--where the mixture explodes from heat of compression, rather
>than being burned, which is when the spark plug ignites the mixture and
>the flame front progresses smoothly through the mixture.
>
>The second reason is for power production. Power is determined in large
>part by the amount of fuel-air mixture drawn into the cylinders. In a
>normally-aspirated engine, peak torque occurs at peak volumetric
>efficiency (this is the rpm where the most mixture is drawn into the
>engine, and is determined by cam timing and lift, and intake and exhaust
>lengths). In an efficient, normally aspirated engine, the peak
>volumetric efficiency is somewhere around 1 (meaning that if the volume
>of the cylinder is 30 cubic inches, 30 cubic inches of mixture (at
>standard atmospheric pressure) will fill the cylinder at the peak torque
>rpm).
>
>A supercharger essentially stuffs more mixture into the cylinders by
>increasing the pressure. At lower speeds, it may be producing only 1
>psi, but that is still 5-6 psi over the partial vacuum without
>supercharging. At high speeds, without any air control, boost can be
>very high. By weight, 15 psi boost will charge the cylinder with more
>than double the mixture possible without supercharging.
>
>For a given stroke, a lower compression ratio also means a larger volume
>in the cylinder when the piston is fully down, even though the swept
>volume is still the same (therefore the displacement is the same). The
>loss of compression is more than offset by the entering pressure of the
>mixture, and the larger initial volume means more power.
>
>Cheers.
>
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