There are many misconceptions about spacer plates, so I am gonna tell you the way it is /forums/images/graemlins/thumb.gif. 4-Hole Spacers: The 4-hole spacers for 4-barrel carbs will increase low end power(same with TBI spacers), top end may be effected. The 4-holes will increase the velocity at lower RPMs therefore increasing the intake charge creating more power. Open Spacers: The open spacer for 4-barrel carbs will increase power at high RPMs sacrificing your low end. The open space under the carb increases the plenum size which increases the top end, but the larger plenum lowers velocity at low RPMs losing power. Another benefit of the open spacer is to help prevent fuel seperation because the A/F mixture can turn towards the port sooner and not smash into the bottom of the plenum. Also if you have a carb that is running to rich, raising it up with an open spacer will lean out the mixture. Combination spacers: These integrate both the 4-hole and open spacer qualitys.