As Lance points out, another problem inherent to the conventional single-throttle manifold in naturally aspirated applications is the pumping loss each cylinder incurs while 'pulling' air into combustion. "Collectively, ITBs offer a larger throttle area than would be practical to use in a single-throttle application," he explains. "So air can flow more easily through each throttle without having to be stretched through a smaller, shared single throttle, when 'pulled' into the engine."
Additionally, ITBs allow the user to custom-tailor power and torque production characteristics. Just as air most efficiently travels into an engine along a port-centerline, it is 'pulled' into an engine at the most efficient velocity through the smallest sufficient opening, at a specifically decreasing cross-sectional area. Without a lecture in fluid dynamics, this means that in custom applications, runners of ITB systems can be changed in length and width to maximize efficiency in power production for the desired part of the powerband, and to better complement the engine's intake ports. The ITB's horn-shaped runner design allows the desired cross-sectional area to be better realized than a plenum manifold could-very important to naturally aspirated engines.
Improvements in flow with ITBs also allow the addition of much more aggressive cams than can be used in conventional manifold situations. "Our ITB kits have made gains of as much as 40 whp to an otherwise stock S2000's mid-range, with over 20 whp top-end, and as much as 70 whp with cams," explains Lance. Forced-induction tuning experts, RB Motorsports, also know well of the advantages ITBs have in allowing the use of larger cams. "The reason that Nissan went with ITBs for the Skyline GT-R was so they could get the car to idle correctly with large duration cams," explains RB's Sean Morris. "With a common plenum there is a lot of reversion and crossover between cylinders."
An additional, well-documented advantage ITBs offer over the conventional single-throttle plenum manifold design, and possibly the most attractive to OEMs, is increased throttle response. A conventional manifold comes under vacuum when the throttle plate closes, and as Sean explains, "When the throttle opens again, the manifold needs to refill with air before the cylinders can. The area for vacuum to accumulate between a throttle and cylinder in an ITB situation is far smaller, which leads to quicker throttle response." Despite all apparent benefits, ITBs do have a particular drawback for forced-induction engines: The individual throttles can disrupt airflow in high-horsepower applications, if placed too close to the head. "The stock GT-R throttle blade and support is about 5mm thick," explains Sean, "but it creates a rough path about 5mm above and below itself, disrupting airflow and fuel atomization at high pressure. This is why a lot of the big power builds use a large single throttle manifold with a plenum."
Some mid-'80s Formula 1 teams actually fielded cars with turbocharged engines that combined ITBs near the port entrance, and a plenum-style single throttle to fine-tune manifold pressure and throttle response. After the organization's turbocharging ban in 1989, most F1 cars retained the use of ITBs for their tuning latitude and increased throttle response, but with a unique twist; in contrast to many production ITB designs that put the fuel injector in the runner between the throttle and head ports, F1 research teams found they could make significantly more power by placing the fuel injector outside the intake runner altogether for it to spray fuel through the throttle and into the engine. With the injector placed away from the throttle a bit, F1 researchers found airflow could be less obstructed and fuel could further expand on its way into the cylinders, dropping intake temperatures and mixing more minutely with the charge-a discovery the sportbike manufacturers were quick to adopt.
Another, more recent, fan of this stand-off injector placement technique is Honda all-motor champ Bisi Ezerioha. When his all-motor single-cam Insight dragger hit a plateau last season at about the 9.6-second quarter-mile range, Bisi decided that a switch from gasoline to methanol was necessary to stay competitive. The only problem was that the dual Weber carbs he was previously running wouldn't be able to keep up with the added flow demands meth required; Bisi decided to take a page from the F1 book and switch to electronic fuel injection, by way of Kinsler ITBs and a combination of stand-off and in-runner injectors. "The stand-offs come online as secondaries," he explains, "their small size, increased fuel pressure and out-board mounting position really atomize the fuel and cool down the charge. The difference in power was very noticeable." Bisi is quick to point out that while stand-off injector placement is optimum for performance, he wouldn't recommend a top-feed-only setup on the street, since a backfire and/or reversion backflow at camshaft overlap could cause an engine compartment fire. Still, the gears of the one-time street racer turned as we talked, "But if they were tuned secondary to in-line injectors, and only engaged at high rpm with an ideal EMS like an AEM, then the safety concerns can be eradicated. But of course, the tuner is a key component to this equation."