Quote:
Originally Posted by
Jackknife
Makes sense, Dave.
Next question: I assume a PD supercharger coming "on line" will have about the same boost
at any RPM (since the belt and pulley system is a constant)...but, the volume of air traveling
through the cylinders will vary with RPM, will it not?
If this assumption is correct, then for most of the engine's RPM range the "restrictiveness"
of the exhaust (within reason) should have only minimal effect on power...only at high RPM,
for example, should headers make any difference. Is this so?
Jackknife
A S/C is always spinning, and you have I think correctly identified that at idle or low torque requirement it does not come "on line" I notice this in my own S/C'd GP. The dash indicates boost in what I presume to be psi. It would be the simplest measurement, and if I am not mistaken, psi can build with rpms, but is not directly related to it. For example, when riding with my wife, the boost indicator never lights up (its a horizontal series of bars, numerically ignorant) she rarely punches teh accelerator. When I drive and ask for a little more aggression from the engine - I get it back with a varying bright light display. So I am assuming that psi will vary as well with a S/C, yet the underlying difference between s/c and t is that teh s/c usually comes "on line" quicker at any rpm.
The volume of air traveling through the cylinders at any specific steady state rpm will be almost identical among 3 engines of equal displacement; n/a, s/c, and turbo. What boost does is compress the incoming air charge, making it more dense, eliminating free space between the atoms. Since combustion process is based on a close to nominal air/fuel ratio by mass of 14.7 at normal atmospheric pressure (coincidentally identical to standard bar air pressure at sea level 14.7 = 1 atm) any increase in mass of the air must, to complete an efficient combustion process be balanced by an increase in mass of fuel. When on boost, you will obviously use more fuel per rpm than when not on boost. MPG savings on boosted vehicles come about because of teh use of a smaller displacement engine for the times when extra hp is not needed, such as steady state highway speeds.