How To Increase Engine RPM - How It Works

Modern race engines turn 9,500-plus rpm with ease, and here’s how they do it

Stephen Kim Sep 23, 2011 0 Comment(s)
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Darin Morgan: Every component in the valvetrain has a natural resonant frequency, so you have to design a motor to avoid those points. Sometimes, the only way to do that is through trial and error. One example that comes to mind is a particular valvespring we used in one of our crate motors that worked great in drag cars. When those same motors were used in boats, however, the springs started breaking. What we discovered was that the springs had a natural resonance at 7,400 rpm, and if held there long enough, they would get excited and eventually break. That wasn’t an issue in a drag application, but became a problem in boat motors that ran at sustained engine speeds. The adverse effects of resonance are also why it’s so important to use the stiffest pushrods possible. When a pushrod flexes, it stores energy and then releases it later in the lift curve, causing resonance. Reducing weight isn’t as important on the pushrod side of a rocker as on the valve side, so we now use large 9/16- and 3/4-inch diameter pushrods on high-rpm race motors.

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Where to Cut Weight

COMP Cams: Reducing mass, or weight, is more critical on the valve side of the rocker arm than on the pushrod side. From the rocker arm to the lifter, increasing stiffness will be more advantageous than reducing mass every time. The goal here isn’t to go after the lightest weight parts when considering a lifter or pushrod. The top priority is increasing stiffness and reducing flex. With regards to pushrods, I would recommend going with the largest diameter, thickest-wall pushrods you can fit in an engine. On the valve side of the rocker arm, weight is much more important. Here, it’s critical to get the locks, retainers, and springs as light as possible to reduce inertia.

Reducing Mass

Judson Massingill: After a valve is accelerated to maximum lift, it comes to a stop and then completely reverses direction as it closes. This makes it difficult to stabilize the valvetrain and keep it out of float since it is constantly fighting this inertia. That’s why reducing valvetrain mass is so important. Titanium engine parts have been around since the ’80s, but now they’re more readily available. To shave every last gram possible, modern race engines have titanium valves, retainers, and locks. Engineers are literally looking everywhere to reduce mass. It wasn’t enough to just make a valve out of titanium. Valve manufacturers started reducing the diameter of the stem, down the 7mm in some cases, and now they’re hollowing out the stems as well. That might seem extreme, but a motor doesn’t know what cam it has in it. All it knows is valve movement, and reducing mass and inertia is critical to achieving valvetrain stability. To put things into perspective, there’s a story of a motor Richard Childress Racing built for its NASCAR Sprint Cup cars several years ago. It had a cam that was worth 8-10 hp more than the grinds they were using before, but the motor would only last 300 miles before the valvetrain broke. By simply removing three grams off the valve side of the rockers, the motors lasted the full 500-mile race distance.

Phil Elliot: Every time a rocker arm moves, it has to start, stop, and then change directions. Naturally, at T&D we’re always trying to reduce mass as much as possible to cut down on inertia. When you remove mass, it’s easier to keep the valvetrain in control. In addition to working closely with race teams, we perform extensive stress tests to see how much we can get away with. We put our rockers through fracture tests that bend the rockers until they break. Likewise, we put our parts in Sprintrons and really try to wreck them. An analogy we like to use is that if a 2x6 is too big, then we use a 2x4 instead. Even so, you can’t take things too far and compromise durability. Lighter is better to point, but parts can’t get too light.


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