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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Fighting Flex

Judson Massingill: You can spend all kinds of time and money designing the best cam in the world, but unless you can get the valve to properly follow the lobe profile, all that R&D work is worthless. Any flex in the valvetrain means that the valves are not doing what the cam wants them to do. The goal is to have extremely stiff parts, and this is particularly true with the pushrods. Up until the early ’90s, engine builders thought that as long as the pushrod didn’t bend, everything was OK. Now we’ve learned that even if a pushrod doesn’t bend, it can still flex tremendously. To combat this, the trend these days is to use giant diameter pushrods. In racing classes that allow it, using a shorter deck height block is also common. This allows using shorter pushrods, which reduces both pushrod flex and mass. In fact, GM Performance Parts sells low deck height small-block Chevy blocks that have an 8.325-inch-tall deck opposed to a standard 9.025-inch tall deck.

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Spintron Testing

Darin Morgan: The Spintron is a great tool that helps simulate valvetrain dynamics, but it by no means has the final answer for everything. Interestingly, a loft curve that looks great on the Spintron does not necessarily correlate to good numbers on the dyno and at the track numbers. That’s because a Spintron can’t simulate the crankshaft pulsations that are transmitted into the cam belt and valvetrain. It’s just another example of why there is no substitute for real-world testing. Only after dyno testing can you begin fine-tuning the loft curve.

Big Journal Cams

Judson Massingill: Camshafts with larger journal diameters definitely decrease how much the cam flexes, but one of the biggest advantages of larger journals is much simpler to understand. When you’re sliding a cam into a block, the amount of lift you can pack into the lobes is limited by the size of the cam bores. If you make the lobes too big, the cam won’t physically fit inside the block. That’s where big journal cams come into play. Many aftermarket blocks are available with larger diameter cam bores. This allows installing a cam with larger, more aggressive lobes. To achieve any given amount of valve lift, you generally want the most lobe lift as possible with the lowest rocker arm ratio possible to help stabilize the valvetrain. The reason why NASCAR Sprint Cup teams use 2.4:1 rockers is because they have to run flat-tappet cams that can’t accelerate the lifters as fast as a roller lifter motor. Since they can’t run as much lobe lift as they’d like to, they have to make up for it with higher-ratio rockers. If you’re racing in a class that allows it, using a larger journal cam with bigger lobes and a lower rocker ratio is a better way of achieving high valve lifts. With a 50mm cam, about .440-inch lobe lift is the limit and with a 60mm cam you can get about 0.590-inch lobe lift.

COMP Cams: The barrel diameter of a camshaft plays a big role in the overall stiffness of the valvetrain. In high-rpm applications, it’s best to opt for the largest journal diameter you can get for a specific engine. The base circle of a cam lobe is determined by journal diameter and lobe lift. When starting from a standard journal, whether small- or big-block, going to a larger journal will increase your barrel diameter and base circle size. There are blocks available now that feature raised cam locations. Furthermore, I would advise anyone not to let the stroke or rods dictate your base circle size. When designing the 400 small-block, there is a reason why Chevrolet made changes to the connecting rod in order to clear the cam instead of using a smaller base circle cam. Doing so would have been cheaper than designing a new connecting rod, but bigger is always better with regard to the size of the base circle.

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