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.
Being a mathematical equation, the reverse is also true. If we produced 400 hp at 6,000 rpm, this would equate to 350 lb-ft. Dropping the 400 hp number down to 5,000 rpm would yield 420 lb-ft, while dropping it further to 4,000 rpm would produce 525 lb-ft. Combining a given horsepower with lower engine speeds will yield greater torque numbers. The same 400 hp produced at just 3,000 rpm would unearth 700 lb-ft of torque and an astounding (and probably rod bending and piston smashing) 1,050 lb-ft down at 2,000 rpm. This is, of course, modified turbo diesel territory, but it is important to show the relationship between horsepower and torque as maximizing the horsepower or torque outputs may require rethinking where the motor makes power. This shifting of the torque curve can be accomplished with the installation of a wilder cam, a different intake design, or even a set of ported heads. Our pair of turbo test motors demonstrated this fact perfectly, as the L98 TPI motor was clearly designed with low-speed torque production in mind. In stock trim, the TPI motor produced peak power at just 4,400 rpm and peak torque at just 3,200 rpm. Not surprisingly, having the motor produce peak power at such a low engine speed resulted in huge torque numbers. The L98 TPI small-block in the Corvette produced 100 lb-ft of torque more than it produced horsepower. Such was the benefit (or curse) of the TPI system. By contrast, the 383 from Pro Comp shifted the torque curve higher in the rev range, resulting in more peak power (the increase in displacement further increased torque production).
Boost from either a turbo or supercharger is a wonderful thing. It acts as a multiplier of the power output of the original normally aspirated combination. The reason this is possible is that your normally aspirated combination is running under pressure already. It is the atmospheric pressure (14.7 psi at sea level and a given temperature) that literally forces the air into your motor to fill the low-pressure area created by the downward moving piston. A turbo or blower simply adds to this pressure differential. Using the power/boost formula, it is possible to predict the power output of any given combination with reasonable accuracy. If we take a 350 hp normally aspirated motor and add 14.7 psi of boost (basically doubling the current atmospheric pressure) we should (in theory) be able to double the power output to 700 hp. Adding 7.35 psi (half atmosphere) we should see an increase of 50 percent to 525 hp, while 10 psi will increase the power output of our 350 hp motor by 68 percent to 588 hp. Basically, the power output of the boosted motor can be calculated by multiplying the NA power output by the percentage of atmospheric change (14.7 psi equals 1 bar).
Sharp-eyed readers should now be seeing the potential gains offered by this formula and the reason for this article. If we have a 350hp normally aspirated motor and add 7.35 psi, we wind up with 525 hp. If we increase the power output of the normally aspirated combination from 350 hp up to 400 hp (with a cam change and ported heads for instance) and then add the same 7.35 psi, we wind up with 600 hp. Improving the power output of the normally aspirated combination by 50 hp resulted in a gain of 75 hp once we added .5 bar (7.35 psi) of boost. The gains increase even more as we further increase the boost. That same 50hp gain (going from 350 hp to 400 hp NA) jumps to an even 100 hp if we add 1 bar (14.7 psi). Adding 14.7 psi to the 350hp NA motor will result in 700 hp while adding the same amount of boost to the 400hp motor will produce 800 hp. You see, the power gains on the NA combination are actually multiplied by the boost pressure, so it is easy to see why starting with a powerful normally aspirated combination is so important.
When it comes to boost, the general thinking is that more is better. While there is some truth to the fact that a motor will make more power at 10 psi than it does at 7 psi, there is more to the equation than this simplistic model. The problem associated with more boost is that boost pressure brings with it another set of problems. One of the basic laws of physics is that compression (we see as boost) causes heat. What this means is that higher boost levels bring with it an increase in inlet air temperature. With that increase in inlet air temperature comes the increased likelihood of harmful detonation. Increased boost pressure amplifies the need for precise tuning. The higher the boost pressure, the less forgiving it is to mistakes in timing and air/fuel ratio. Miss the air/fuel ratio by half-a-point on a motor running 7 psi and there probably won't be any issues. Do the same thing at double the boost and you're much more likely to put a hole in a piston. Ignition timing is even more critical, as a mis-tune by just a degree or two means saying goodbye to those expensive forged pistons or head gaskets. The problems associated with increasing the boost pressure further points to the importance of running less boost on a more powerful normally aspirated combination to reach your intended power goal.
While theories are all well and good, we decided to put our money where our mouth is by applying boost to two different engine combinations. These combinations were chosen to demonstrate both the effect of boost on different power levels as well as the effect of shifting the torque curve. Thus, the test covers both aspects discussed in the text. The first motor is an L98 TPI pirated from a 1988 Corvette. The only upgrade to the TPI mill was the installation of a mild Xtreme Energy cam from Comp Cams. Equipped with the cam, headers, and run with a FAST XFI management system, the injected 5.7L (350) produced 331 horsepower and (a very TPI-like) 394 lb-ft of torque. As we have come to expect of the long-runner TPI motors, peak power occurred at just 4,800 rpm while the peak torque value came at 4,000 rpm. Next, we added a single-turbo kit from HP Performance in Roswell, New Mexico (designed for the C4 TPI Vette). The kit included a single 60mm turbo and air-to-air intercooler. Running right at 7 psi of boost, the peak power numbers jumped to 481 hp and 579 lb-ft of torque. These came after changing the air/fuel ratio from 13.0:1 (with the NA motor) to 11.5:1 and decreasing the total ignition timing from 34 degrees down to 20 degrees. We hedged or bets by adding a can of octane booster from Lucas Oil to the 91-octane pump gas.
Test motor number two is a 383 crate motor recently made available from Pro Comp. The 383 featured 10.0:1 compression, a healthy hydraulic roller cam (0.545/0.565-inch lift split and 248/254 duration split at 0.050), and a Holley Stealth Ram EFI intake manifold. Also present was a set of CNC-ported aluminum heads. The 210cc intake ports offered nearly 300 cfm of airflow, or a ton more than the factory L98 heads on the TPI motor. Compared to the L98, the Pro Comp 383 offered more displacement, compression, and cam timing, not to mention an intake that allowed the motor to make peak power up near 6,000 rpm rather than below 5,000 rpm like the TPI. In effect, the 383 had both elevated and shifted the torque curve. Run with a Holley Pro Commander management system, the Pro Comp 383 produced peak numbers of 491 hp and 457 lb-ft of torque. The 383 offered a broad torque curve, with torque production exceeding 450 lb-ft from 3,800 rpm to 5,600 rpm. Adding the single-turbo kit to the 383 produced some rather dramatic results. Running 6.9 psi of boost, the Pro Comp 383 produced 712 hp and 673 lb-ft of torque. Now the question is--which one represents the real 7 psi of boost? The answer is of course--they both do, but which 7 psi would you rather have?