Propagational delay in circuits

Question

which is better for accurate proportional delay: spice simulation method or calculation using elmores delay (RC delay modeling)

Answer 1

Spice simulation is more accurate than elmore delay modelling. This is mentioned in the book CMOS VLSI Design by Weste Harris on page 93, Section 2.6

Blindly trusting one’s models Models should be viewed as only approximations to reality, not reality itself, and used within their limitations. In particular, simple models like the Shockley or RC models aren’t even close to accurate fits for the I-V characteristics of a modern transistor. They are valuable for the insight they give on trends (i.e., making a transistor wider increases its gate capacitance and decreases its ON resistance), not for the absolute values they predict. Cutting-edge projects often target processes that are still under development, so these models should only be viewed as speculative. Finally, processes may not be fully characterized over all operating regimes; for example, don’t assume that your models are accurate in the subthreshold region unless your vendor tells you so. Having said this, modern SPICE models do an extremely good job of predicting performance well into the GHz range for well-characterized processes and models when using proper design practices (such as accounting for temperature, voltage, and process variation).