There is only one way you can force cbr, that is with Q settings to drive up the bitrate and set a frame limit on it. But it needs measuring exactly, else at one end of the scale you get poor Q in fine detail scenes (poor coarseness) and at the other end with less detail scenes you get exceptionally good Q, amazing psnr (not that its a measure of human perceptual quality), and large transform coeffecient percentages with definition but with a much wider range of Q. Perceptually it looks great, but on the charts its strange to see a line of the picture coded with great looking macroblocks and alternate rows coded with poor Q mbs. Somewhere in the middle is always our goal.