FIRST PRINCIPLES: FORCE EQUALS MASS TIMES ACCELERATION

By 460-BBF-Turbo-In-CC (from the legendary Car Craft turbo thread**)

"Force Equals Mass Times Acceleration (F=MxA)"

The first step in developing a PLAN for where you want to end up is determining how much power you're going to need to get there. For most Car Crafters, the "holy grail" of high-performance is power-to-weight ratio. It's the most fundamental relationship.

And for an overwhelming number of Car Crafters. power-to-weight is best benchmarked by standing-start acceleration over a quarter mile. Decades ago, Chrysler engineers worked out the math for how much power it takes to hit a target trap speed in a production-based vehicle at the end of a quarter mile.

Based on their formula:

M.P.H. Lbs Per Horsepower

100 m.p.h 12.82 lbs/hp

110 m.p.h 9.61 lbs/hp

120 m.p.h. 7.40 lbs/hp

130 m.p.h. 5.38 lbs/hp

140 m.p.h. 4.67 lbs/hp

150 m.p.h. 3.79 lbs/hp

Now to avoid disappointment and account for differences in measuring horsepower, mechanical efficiencies, and other factors, I'd add in 10-15 percent more horsepower, just to be safe.

Notice that these results don't mention engine size or overall weight. A big, heavy car with ~ 5 lbs/h.p. should have roughly the same trap speed as a small, light car with ~ 5 lbs/h.p. (excluding aerodynamic effects). It just takes more "engine" (and budget) to make a heavy car as fast as a light one.

The results don't mention [elapsed time] (E.T.) because "quickness" tends to be influenced by more factors than effective h.p. More often than not, E.T. is a measure of how effective a car is at relatively low speeds in the first third of the quarter mile. But unless you're doing something really wrong (i.e. running FWD, horrible weight distribution, "no traction" tires, "peg-leg" differential . . . .) you should be able to obtain at least the following under proper conditions on the street:

Lbs/Hp E.T. Zone

12.82 14s

9.61 13s

7.40 12s

5.38 11s

4.67 10s

Obviously, a well-sorted drag-oriented supsension and better tires will produce quicker E.T.s, but for conservative PLAN purposes, these numbers will get you "in the ballpark" on how much power to build for.

**Reposted with permission of the author.