Turbo Cams

Chris Mays: Turbo cars act very differently than supercharged or nitrous applications. For any given power level, a turbo motor doesn’t need much duration at all. Compared to a nitrous or blower car, the camshafts in turbo motors are much smaller. Since turbos operate off exhaust pressures, the main objective is to minimize overlap to prevent disrupting the exhaust pulses going to the turbo. If you go from blower to turbo but don’t change cam, the engine would have so much overlap that it couldn’t build boost like it needs to. In an effort to reduce turbo lag in street cars, we often use single-pattern or reverse-split cams that have 3-4 degrees less duration on the exhaust side than on the intake side. That’s because you don’t need much intake duration to fill the cylinder, but if you don’t have enough exhaust duration, the turbo will hit a wall at a certain rpm and run out of steam. What we’re doing is changing the exhaust duration to control the rpm range of the engine. Exhaust duration tremendously affects the operating rpm range of a turbo motor. Let’s say you have a mild turbo combo that turns 7,000 rpm with twin 67mm turbos, but then decide to get more aggressive with better heads, twin 88mm turbos, and a 9,200-rpm peak engine speed. To adjust the powerband, you would leave the intake duration the same while increasing exhaust duration.

Valvespring Longevity

Chris Mays: Modern cylinder heads flow astounding volumes of air, and it’s not uncommon for big-block heads to achieve peak flow above 0.800-inch valve lift. A cam with that much lift would have spelled doom for the valvesprings many years ago, but that’s not the case anymore. Not only has valvetrain technology improved, through extensive testing we’ve learned various methods to increase spring longevity. With cylinder heads that flow to the 0.800- or 0.900-inch range, we tend to accelerate the valves more with rocker ratio than with the cam lobe. This allows backing off the acceleration rate of the cam. Stability in rocker arms and the rest of valvetrain is then crucial to prevent valve float. You need very light valves and retainers, and even smaller-diameter springs. This reduction in mass greatly reduces the stress on the valvetrain. The end result is a lot of lift with springs that last two to three times longer.

Nolan Jamora: With as good as cylinder heads are these days, 0.800-inch lift isn’t that much for a big-block anymore. Some high-end big-blocks are now running 1.000- to 1.500-inch lift. At the Pro Stock level, valvespring pressure is now 1,200-1,400 pounds. This puts lots of stress on the valvetrain, and every piece of the puzzle is important in extending parts longevity. One of the biggest problems is trying to get the springs to live, and that requires a stable valvetrain. To accomplish this, we have developed our EZ roller lifter that utilizes bushings instead of needle bearings. This eliminates the lifter bounce associated with high spring pressure and rpm. Consequently, it has enabled us to design cam profiles that take advantage of this roller lifter. In the past, we used to employ very aggressive opening ramps, and match them up with gentle closing ramps to extend valvetrain life. The goal is to close the valve as smoothly as possible. Now we can design lobes in nine different sections. This allows us to design really short and steep ramps going up, but long, smooth ramps on closing side. These efforts dramatically improve the longevity of the valvetrain.