Cammin' For Power

Running Big Cams On The Street

Mike Petralia Nov 3, 2005 0 Comment(s)

OEMs favored the hydraulic flat-tappet lifter design for years because it is the best compromise between power and cost. But hydraulic lifters are far from the best way to make max power. That's because, as engine speed increases, the hydraulic lifter's plunger depresses further into its body due to the increasing forces of trying to accelerate the valve off the seat. The cam begins to act like an aggressive solid design from the lash that's created, but now has a shorter duration as the result. The more aggressive the cam acts, the harder time the valvespring has controlling it. As it approaches max speed, the valve bounces off the seat and oil rushes into the lifter to take up the clearance that's now been created at the plunger. This effectively holds the valve off the seat, (valve float), bleeding off compression and power, and limits the high-rpm capacity of hydraulic designs.

Solid lobes, on the other hand, must have a lash or clearance ramp engineered into them. This is so that on the opening side of the cam the lifter does not smash against the lobe when it begins to move. The lash ramp is always a less aggressive portion of the lobe than the rest of its lift area. However, once lash is taken up--usually around 0.030-inch lift--the solid lobe design quickly becomes more aggressive than a hydraulic lobe while actually providing better control due to increased lifter stiffness. That's why a solid cam with the exact same lift and duration figures will usually make more power than a hydraulic cam. A hydraulic lobe design doesn't need a soft lash ramp because the lobes are always in contact with the lifter. Instead, hydraulic cams have a short transitional area on the lobe to get the valve moving in a stable manner. This transitional area actually makes a hydraulic lobe start lifting the tappet quicker, but slows down and tapers off much sooner so as not to toss the lifter off the top of the lobe.

With the advent of hydraulic roller cams, the question has arisen whether or not it's safe to run solid roller lifters on them. The answer is as readily disputed as most new forms of technology are today, but in a pinch you can do it. You must run very tight lash, .005 to .006, because of the lack of a clearance ramp on the hydraulic lobe design and running too much lash will quickly kill solid lifters on a hydraulic lobe.

Courtesy of CV Products
Titanium valves do not have as long of a service life as steel valves under similar conditions. Ti valves are used throughout the Pro Stock and Alcohol ranks of professional drag racing, however, Ti does not possess the same heat-resistant properties as other exotic high-temperature steel alloys. In extreme applications like Top Fuel, maximum combustion temperatures may exceed the capabilities of a Ti valve; this usually affects the exhaust valve first, since it sees considerably higher temperatures than the intake valve.

Titanium provides an approximate 40 percent weight savings over conventional steel alloys. The lower weight extends the service life of valvesprings and allows the engine to rev higher without risking valve float. Lower weight also reduces wear on related components like the valve seats and camshaft. It is true that titanium valves are expensive, but the extended service life of related valvetrain components, especially the valvesprings, can offset the higher initial investment.

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Valve angles are all the rage today. The valve angle, such as 23 degrees, is always in reference to the face of the valve relative to the flat portion of the piston. When you roll the valves away from the intake manifold, thereby decreasing valve angle, you can make more power, but will create all sorts of trouble for rocker arms and pistons.

One area of constant valvetrain discussions today has become valve angles. It seems that cylinder-head companies are changing their angles faster than the related technology can keep up, and people are finding out too late that their new multi-angle, splayed-valve heads are going to cost a lot more to run than they first thought. These changes refer to the angle of the valves in relation to the flat surface of the piston (imagine all pistons as flat-tops when talking about valve angles). In order to make the relatively wide 90-degree V-8 fit inside most engine compartments, engineers originally had to angle the valve stems in toward the intake manifold. Otherwise, the engine might not fit. Again, another area of compromise. Race engineers found out long ago that rolling the valve angles further out toward the exhaust side (by angle-milling the heads) puts the valve face at a better relation to the piston. This led to better-breathing cylinder heads and eventually, more power. But, as soon as you move the valve angles around, you must also move everything else to match, otherwise your intake ports might not work, and the pushrods, rocker arms, and sometimes even head bolts won't fit. The intake manifold and rocker-arm companies have been going crazy trying to keep up with all the new cylinder heads coming out with varying valve angles. And so far, we've just talked about rolling the valves towards the exhaust side of the motor. There are also canted valve heads, where the intake and exhaust valves are at different angles from each other. And most often, canted valve heads also have the valve centerlines pointing away from each other too, as viewed from the side of the head, to allow bigger valves room to open in a small cylinder. Actually, canted valve technology is nothing new. Big-block Fords and Chevys have had it since their inception, and that could be one reason why even an oval-port big-block head can be made to flow just as much or more cfm than many of the race-only small-block heads. The bottom lineis , if you're looking into a set of canted or splayed valve cylinder heads, be prepared to make a lot of changes and incur a huge additional expense.


Crane Cams
Daytona Beach, FL 32117
Comp Cams
Memphis, TN 38118
Crower Cams
San Diego, CA 92154
Powerhouse Products
Memphis, TN 38118
Erson Cams
Carson City, NV 89706
Lunati LLC
Olive Branch, MS
CV Products
Thomasville, NC 27360
Specialty Auto Parts (Proform)
Roseville, MI 48066
Reher-Morrison Racing Engines (RMRE)
Arlington, TX 76017
Lazer Cams
Memphis, TN 38118
Schubeck Racing Engines
Las Vegas, NV 89103


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