Richard Holdener
July 29, 2011

It should be noted that throttle body sizing and maximum flow is less critical on a blow-through (centrifugal supercharger and turbo) application than a draw-through (positive displacement). Pressurizing the air before the throttle body artificially increases the flow rate of the throttle body. Obviously it is best to eliminate any pressure differential caused by the throttle body, but a stock throttle body will be less of a hindrance on a blow-through than a draw-through.

One of the areas often overlooked on a draw-through supercharged application is the induction system. The thought process seems to be that boost cures everything, and as long as there is boost, everything is working just fine. The reality is that the inlet system is a critical element on a twin-screw or Roots supercharged application.

Restrictions from the air-intake system (including the throttle body) into the blower result in a drop in flow, boost, and power production out of the blower. One of the confusing facts is that the power loss is present despite an increase in boost (and power). The consensus is that if the boost goes up, there must not be a restriction. The reality is actually that losses associated with a restrictive inlet system (throttle body, MAF, air intake, and even intake manifold between the throttle body and supercharger) increase with increased boost pressure and power output.

Testing on a Kenne Bell supercharged Three-Valve motor illustrate that a throttle body upgrade on a 500hp application (8 psi on stock motor) was worth 20 hp. Performing the same test at 615 hp was worth 35 hp and an amazing 61 hp at 20 psi. The greater the boost and power, the greater the losses associated with a restrictive throttle body. Remember, the same throttle body upgrade that was worth 61 hp on the supercharged application wasn't worth any power on the normally aspirated motor!

Uncovering all the valuable data on the correlation between airflow, horsepower, and associated losses is easy at the proper test facility. Equipped with a Dynojet chassis dyno, Superflow airflow bench, and extensive data-logging equipment, the Kenne Bell facility was ideally suited for gathering data, but what about the average Joe?

As luck would have it, testing for an inlet restriction can be performed with nothing more than a simple (and inexpensive) vacuum gauge. There is a direct correlation between the amount of vacuum present in the inlet system and the loss in boost and power offered by that restriction. Hooking up a vacuum gauge in front of the supercharger (in the inlet manifold on a Kenne Bell supercharged application) will immediately indicate any inherent restrictions. Vacuum present in the air intake system at WOT is an indication of a restriction.

According to (exhaustive) testing performed by Kenne Bell on the engine dyno, chassis dyno, and airflow bench, 1 inch HG of vacuum (vacuum is measured in inches; boost in pounds) is roughly equivalent to 0.5 psi of boost. On a high-horsepower Kenne Bell application, it is not uncommon to see 4-5 inches of vacuum using the factory air intake and throttle body. This restriction is limiting boost (and attending power output) by 2-2.5 psi. Curing the inlet restriction would add 2-2.5 psi of boost, and what supercharged Mustang owner wouldn't want an extra 2.5 psi of boost?

For the slide rule crowd, there is (naturally) a mathematical correlation between the restriction and power losses. Using a previous example, we saw that replacing the throttle body on a 500hp supercharged Three-Valve was worth 20 hp. Data logging indicated that the combination produced 502 hp at 8.5 psi with the stock throttle body. Upgrading to the Stage 2 air-intake kit, which included a 130mm, oval throttle body and revised air intake system (the combination increased the total airflow of the air intake system by 200 cfm), increased power output to 526 hp.

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