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?
 The highlight of the 383 was...  The highlight of the 383 was a set of CNC-ported aluminum heads (also from Pro Comp). Testing on the flow bench indicated that the heads could easily support 600 hp (in normally aspirated trim). |  These 210cc heads started...  These 210cc heads started out life with as-cast, 190 cc intake ports. The CNC-porting unleashed over 40 cfm compared to the as-cast heads. The displacement and wild cam timing could take full advantage of the flow offered by the aluminum heads. |  The hot hydraulic roller cam...  The hot hydraulic roller cam necessitated a valvespring upgrade. We installed a set of PN 26120 beehive springs combined with PN 795-16 retainers, 4704-16 spring cups and 613-16 super locks, all from Comp Cams. |
 While the crate motor is available...  While the crate motor is available with a simple carburetor, we elected to install something a bit more sophisticated in the form of this Holley Stealth ram intake. The impressive EFI intake was basically a tunnel ram with a common plenum and dual-blade throttle body inlet. |  The turbo manifolds used to...  The turbo manifolds used to run the 383 were slightly different than those on the L98, but each set was used to feed the same 60mm single turbo. |  The timing and fuel curve...  The timing and fuel curve were optimized using a Holley Commander 950 management system. Like the L98, we ran the motor at 11.5:1 and with 20 degrees of total timing (though we also ran a splash of race fuel on the higher-compression 383). |