TPI Induction System Dyno Test - Inches And Pounds

How many times have you heard that a crankshaft, connecting rod, or even forged piston is good for 13 psi of boost? On the surface, this seems reasonable, but the reality is that boost is far from being any type of reasonable yardstick. In the example above, boost is being used as a measurement of power or at the very least cylinder pressure. While it is true that power (and average cylinder pressure) increases with boost, the mere fact that a blower or turbo supplies a given boost level does not equate to any given power or (cylinder) pressure level. Having built more than my fair share of forced-induction motors, I can say that I've run motors that produced as little as 365 hp at 13 psi and as much as 1,040 hp at the same boost level. Using the boost method for measurement, would the crank, rods, and/or pistons be strong enough for the 365hp motor or the one exceeding 1,000 hp? It should be obvious from this example that it takes much more than boost to determine the power output of any combination, and the strength of the components therein.

While boost certainly plays a part (which we will cover in a moment), so too does the power output of the combination it is applied to. The best route to an exceptional forced-induction motor is to start with a powerful normally aspirated combination. Building power in the normally aspirated combination can be accomplished by something we like to call shifting the torque curve. It is a basic law of physics that for any given torque output, the horsepower production is a simple matter of the engine speed at which the torque is produced. An example works well here. Suppose we have a 350 small-block that puts out 350 lb-ft of torque, not an unusual amount given the power potential of even a mild small-block Chevy. If the motor produced 350 lb-ft of torque at 2,000 rpm (an impressive amount given the minimal engine speed), this would correspond to a horsepower output (at 2,000 rpm) of 133.28hp. The formula we use is HP=TQ x RPM/5252. Using this formula, we see that shifting the 350 lb-ft of torque to 3,000 rpm equates to a hair under 200 hp, while 4,000 rpm will up the power ante to 266 hp. A further shift to 5,000 rpm means the torque numbers are nearly matched by the horsepower numbers since the mathematical equation relies on 5,252 rpm as the constant. This means that the horsepower and torque curves (for any motor ever produced) will always cross at 5,252 rpm. At 5,000 rpm our 350 lb-ft will equate to 333 lb-ft and the same torque output at 6,000 rpm will allow our small-block to produce 400 hp. Obviously, the higher the engine speed of a given torque output, the greater the horsepower production.