Boosting Your Horsepower - Gettin’ Air

Understanding the finer points of larger throttle bodies, boost and cold air.

Richard Holdener Sep 23, 2013 0 Comment(s)
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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 Kenne Bell twin-screw or any positive displacement supercharged application. Restrictions in the air intake system (including the throttle body) into the blower result in a drop in flow into and 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 seems to be 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 Camaro illustrated that a throttle body upgrade on a 600hp application (9.3 psi on stock motor) was worth 8 hp. Performing the same test at 13 psi (678 hp) was worth 26 hp (up to 702 hp) and an amazing 34 hp at 17 psi (from 755 hp to 789 hp). 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 34 hp on the supercharged application wasn’t worth any power on the normally aspirated engine.

For the geek squad, 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 600hp supercharged Camaro was worth 8 hp. Data logging indicated that the combination produced 609 hp at 9.0 psi with the stock throttle body.

The 102mm throttle body upgrade dropped the vacuum present in the inlet tract by 0.29 inches and increased boost pressure by 0.3 psi which equated to an extra 8 hp. Testing at higher boost level (12.8 psi) resulted in and increase to 13.2 psi, a drop in vacuum of 0.31 inches and 26 additional horsepower. The final test at 17.3 psi netted an additional 1.0 psi of boost, a drop in vacuum of 0.45 inches and 34 hp. It should be noted that even the 102-mm throttle body had become a restriction above 18 psi, as there was still 1.30 inches of vacuum present with the 102mm throttle body. What this supercharged LS3 needed was the 2,350 cfm, single-oval, throttle body offered by Kenne Bell for their supercharger. Easily capable of supporting over 1,200 hp, the single-oval throttle body maximizes flow to the supercharger. Just make sure the rest of the inlet system flows enough to support the massive throttle body.

We have concentrated our efforts primarily on the throttle body, but the rest of the air intake system also bears mentioning. Through exhaustive testing on the flow bench and DynoJet, Kenne Bell has designed an intake system to keep pace with the flow needs of a supercharged motor. According to Kenne Bell, the stock air intake system on the ’10-up Camaro SS flowed 964 cfm, roughly enough to support 643 hp. The 4.5-inch upgrade offered by Kenne Bell flows an amazing 1,800 cfm, enough to support over 1,000.

Why such a large inlet system you ask? Flow bench and chassis dyno testing has shown that a restrictive 4.0-inch system flowed just 1,380 cfm, partly because it did not allow for the radiused entry on the 102mm or 110mm throttle body. The radiused entry on the 102mm throttle body tested improved the flow rate by 400 cfm! Running a filter with a radiused entry on the 4.5-inch air intake improved the flow rate of by nearly the same amount. That extra flow equates to an extra 266 hp. For maximizing flow, radiused entries are not just a good idea, they should be considered mandatory.

Given the fact that the stock air box flows so poorly, upgrades on supercharged applications are commonplace. The upgrades include everything from a simple K&N filter (good for an extra 35 cfm), to the complete Kenne Bell air intake system. Positioned between these extremes are lesser 4.0-inch systems, and even cutting openings in the factory air box. While extra (or larger) inlet openings will improve the flow rate, they do so at the expense of inlet temperature. The gains offered through flow improvements are nullified by the increase in inlet air temps that accompany grabbing air from the heated engine compartment.

Kenne Bell SC Camaro-Stock vs. 102mm TB (9, 13, 18 psi)

Description Boost Boost Drop Vacuum (Ins) RWHP
4.0 Pulley-Stk TB 9.0 0.50 psi 1.02 ins 609 hp
4.0 Pulley 102mm TB 9.3 0.36 psi 0.73 ins 617 hp
Difference 0.30 psi 0.14 psi 0.29 ins 8 hp
3.5 Pulley Stk TB 12.8 0.61 1.24 ins 678 hp
3.5 Pulley 102mm TB 13.2 0.46 0.93 ins 702 hp
Difference 0.4 psi 0.15 psi 0.31 ins 24 hp
3.0 Pulley Stk TB 17.3 0.86 1.75 ins 755 hp
3.0 Pulley 102mm TB 18.3 0.64 1.30 ins 789 hp
Difference 1.0 psi 0.22 psi 0.45 ins 34 hp

We all know that cold, dense air offers more power-producing oxygen molecules. In additional to sufficient airflow, a dedicated cold air intake must also provide cold air. Tested on the chassis dyno with the hood open, these air box mods might look appealing. But close the hood and run the test the way the car drives down the street and watch the power levels plummet. Despite the extra airflow offered by the modifications to the stock air box, the increase in inlet air temps reduced power by as much as 30 hp compared to stock air box. Supercharged engines do not like hot air.

The supplied chart illustrates just how much a throttle body upgrade can be worth at various power/boost levels. The supplied data also includes the drop in vacuum and resulting increase in boost that accompanied each power gain. Tested at 9.0 psi, the throttle body was worth just 8 hp, but running the same test at 12.8 psi resulted in a jump in boost of 0.4 psi and 26 extra hp. The final test run at 17.3 psi resulted in an increase of 1.0 psi of boost and 34 extra hp, though even the 102mm throttle body was now a restriction at nearly 800 hp.


Kenne Bell
Rancho Cucamonga, CA 91730




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