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Boosting Your Horsepower - Gettin’ Air

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

By Richard Holdener, Photography by Richard Holdener

For the power hungry, the only thing better than a modified LS engine is a supercharged LS motor. Nothing wakes up a Gen 3 or Gen 4 small-block like a little boost. As cool and powerful as the supercharger kits are, they are only the beginning. You see, a supercharger is only as good as its induction system. Nothing kills the power potential of a supercharger faster than a restrictive throttle body, mass airflow (MAF), or air intake system.

How much power is a throttle body, MAF, or air intake upgrade worth? Not surprisingly, the power gains offered by the flow restriction inherent in the stock inlet system depend on the modified power output. Technically speaking, the more powerful the engine, the more restrictive the stock components become. This should not come as a big surprise, since the factory inlet system and throttle body were never designed for these elevated power levels. The system GM designed to support 426 hp on a stock Camaro LS3 has no business on a supercharged motor making 600 or more horsepower.

Have you ever read a dyno test on a throttle body that offered no power gains? How about one that offered 10 hp, 20 hp or even 30 hp? Just how much is a simple component like a throttle body (or air intake system) worth? The question seems simple enough, but the answer is somewhat less so. Power gains seem to range from as little as zero horsepower to as much as 25-30. Why such a broad range? Is the testing flawed or otherwise biased? In point of fact, we have seen throttle body upgrades that netted over 60 hp on a Kenne Bell supercharged application. The key to the power gains offered by a throttle body upgrade is the application. In reality, the question is not so much can a throttle body swap net a sizable jump in power, but rather does such an upgrade always results in said gains? The quick and dirty answer to that question is a resounding no, but the reason behind it is certainly worth a closer look.

In its simplest form, the throttle body is nothing more than an air valve. There is no magic to the workings of a throttle body, though there is some magic to maximizing the flow rate through it. A given opening will flow a certain amount of air, but radiused entries, thin throttle blades and the elimination of hardware in the air stream all combine to further increase the airflow of a given bore size. It stands to reason that a 90mm throttle body should outflow a 80mm throttle body, but it is possible for a well-designed 80mm throttle body to outflow a poorly designed 90mm unit. Back in the early days of the L98 small-blocks, we marveled at the ability to upgrade the factory throttle bodies to dual 58mm units. Modern LS3 motors are equipped with massive 90mm throttle bodies flowing over 1,100 cfm, but even these monstrosities are not enough to support a high-horsepower, supercharged application.

It stands to reason that a larger throttle body will outflow a smaller version (assuming equal design quality), but now we can take a look at why the installation of a larger throttle body may or may not improve power. According to testing performed at Kenne Bell, the stock ’10-up Camaro SS 90mm throttle body flows 1,147 cfm. From testing on the airflow bench and DynoJet, we know the formula to convert hp from airflow. Using the formula that 1 hp requires roughly 1.5 cfm, we see that this stock throttle body might support as much as 765 hp—way more than the factory-rated 426 hp. The first obstacle in terms of power production is the fact that the throttle body is not the only component in the induction system. The flow rate of the throttle body is only as good as the supporting components. In the case of the Camaro, the flow rate of the complete induction system is only 964 cfm, a significant drop from the 1,147 cfm offered by the throttle body alone. According to our formula, the stock air intake system is capable of supporting 643 hp, considerably more than the stock power output, but things quickly go south once you add a supercharger.

Before going on, it is imperative that we backtrack a bit on our power/airflow formula. We indicated that it takes roughly 1.5 cfm to make 1 horsepower, correct? While this calculation has been proven accurate time and time again, it is by no means an absolute. Just because the induction system flows 964 cfm, doesn’t mean that it is only capable of supporting 643 hp, or that there won’t be any power losses up to that point. The formula is a rough estimation, but our testing has shown that even at lower power levels, the components in question start posing a restriction. The level of restriction and associated power losses increase with the power level. The throttle body or air intake system doesn’t all of a sudden become a restriction; it is a little restrictive at lower power levels, becoming more restrictive at higher power levels. Just because the component flows enough to support 650 hp, 700 hp, or even 800 hp doesn’t mean there aren’t additional power gains to be had below these points. On a positive displacement supercharged application, maximum airflow on the inlet side is the key.

An obvious factor when it comes to the power gains offered by the throttle body or air intake is engine combination. From a most basic standpoint, the higher the power output of the test engine, the larger the throttle body required. Upgrading a throttle body already capable of supporting 765 hp with a larger version capable of supporting 1,000 hp on a 425hp motor will have predictable results.

The 765hp throttle body is already oversized for the application, so there is no need to upgrade on the normally aspirated motor. This is especially critical on supercharged applications, where elevated power levels are more commonplace. While 600 hp, normally aspirated Camaro motors are less common, supercharged LS3s exceeding 600 hp are everywhere.

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, like Roots- or Screw-type). 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 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
10743 Bell Ct.
Rancho Cucamonga
CA  91730
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By Richard Holdener
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