To test the flow limits of a 90-mm throttle body, the first thing you need is a powerful test engine, something considerably stronger than a stock 5.0L. The reason for this is that a 75-mm throttle body flows well enough to support somewhere over 400 hp. Were we to test the throttle bodies on a stock engine, there might be very little power gain because the engine would not take advantage of the larger throttle body’s extra capacity. On the other hand, if we were to perform the test on a radical EFI race engine, something capable of producing 600-plus hp, the power gains might be huge, on the order of 30 hp or more. Then everyone reading this test might jump to the conclusion that throttle-body upgrades are all worth 30 hp. The reality is that no throttle body test is an absolute. The gains experienced on this test are indicative of what will happen at this power level, but results will most definitely change with milder or wilder combinations. The same can be said of supercharged, especially with positive-displacement blowers, and turbocharged engines, as these applications have specific throttle-body needs.
Our 331 test engine was the same CHP stroker used in test 4. The 331 was comprised of a 3.25-inch stroker crank, forged rods and pistons and topped off with a set of TFS CNC-ported Twisted Wedge heads with 185-cc ports. Though designed with forced induction in mind, and as such equipped with 9.0:1 compression, the 331 stroker was nonetheless powerful even in naturally aspirated trim thanks to the free-flowing TFS heads, XE274HR cam, and TFS Box R intake. Whether on the dyno or in the car, a good indication that the throttle body sizing is too small is the presence of vacuum at wide-open throttle. During our dyno session, the data logging indicated a solid 1 inch of vacuum present at wide-open throttle with the 75-mm throttle body. Installation of the larger 90-mm throttle body dropped the vacuum present down to just 3/10 inch. Vaccuum is measured in 10ths/inch. It’s the way the dyno provides the information. Naturally, removing the inherent air flow restriction had a positive effect on power as well, with the larger throttle body moving peak power output from 441 hp to 454 hp. As expected of this type of change, the power gains increased with engine speed. That the Box R upper intake was designed with a 90-mm opening to accept the larger throttle body was the deciding factor—this 331 combination needs the 90-mm.