Extracting every last scrap of performance from a racing engine requires a detailed examination of each individual engine component to determine if there’s lost power due to undiscovered problems such as poor design, component compatibility issues or inaccurate tuning. Among the many critical components in your engine, the pushrod’s contribution is rarely considered as part of the overall power equation. For many builders, it’s simply a matter of selecting a hardened pushrod with the correct length and tip. Unfortunately that leaves a lot of untapped performance on the table, according to the experts at Manton Pushrods.
“The pushrod is the least considered part of any racing engine,” says Manton’s Al Perkins. “Pushrods are absolutely critical to valvetrain stability.”
Like every other part in your performance engine, the pushrod design is very much application specific. Back-to-back dyno tests have shown a degradation of power even on relatively mild engines when the pushrod does not suit the application. Once you move past 400 horsepower or so, cam profiles and spring pressures begin to escalate, placing increased stress on the rest of the valvetrain. Back in the day stiffer pushrods and better materials could sort of get the job done on most engines, but eventually more serious technical improvements became necessary.
“Incorrect pushrods are the number one cause of lifter failure,” confirms Perkins. “If the pushrod is flexing, it can beat up the lifter.”
As a result race and high performance engines now enjoy broad pushrod coverage with specific features tailored to suit every conceivable application. Larger diameter tapered pushrods improved the stiffness requirement to deal with deflection under rising spring pressures, but didn’t specifically address vibration frequency issues so cam timing variations still occurred. Manufacturers then determined that they could tune pushrods for specific applications via materials, wall thickness, taper, frequency and tip type. Pro Stock, Pro Mod, blown alcohol and fuel dragster classes drove the technology to where it is today.
Manton Rocker Shaft Assemblies
Manton also designs rocker-arm shaft assemblies for Chrysler Hemi engines used in Top Alcohol, Pro Mod or nostalgia racing. The geometry has been corrected to work specifically with boost, and the oiling strategy is also improved to increase oil pressure. Different exhaust ratios are available, and all rockers are billet, roller-tip construction.
Today, some engines require pushrods up to ¾-inch in diameter with various degrees of taper, including dual and single taper versions to deal with the tremendous cylinder pressures being generated by massive amounts of boost, nitrous oxide and fuel applications. A single tapered pushrods positions greater mass near the bottom where it is more easily controlled by the lifter and is usually needed to solve clearance issues in the cylinder head, because intake ports have reached extreme limits. Such designs are but one step in a calculated evolution of pushrod technology that now addresses issues seemingly far removed from the simple task from transferring lifter motion to the rocker arm.
Conventional wisdom used to dictate that the pushrod be light as possible, as most engine builders strived to decrease the weight of the entire valvetrain to improve the rev range while keeping the valves from floating. But new analysis treats the pushrod not as a weight on the valvetrain but as a suspended component during valvetrain motion. Current thinking now places more emphasis on valvetrain rigidity, especially with the pushrod. A light pushrod that flexes does more to upset the engine through unwanted harmonics and inconsistent valve timing than a heavier, sturdier pushrod that stabilizes the valvetrain.
Keeping heat out of the valve springs is critical to valvetrain performance and spring life.–Al Perkins, Manton Pushrods
Increasing power often means increased engine speed and more potential valvetrain distress, particularly with the valve springs. Many racers are now switching to dual springs instead of triples to help reduce friction and heat. They’re also incorporating springs with different tension on the intake and exhaust sides because the exhaust valves are generally heavier and operate against higher cylinder pressures. You can easily imagine the cylinder pressure generated by a blown alcohol dragster engine under 60 psi boost or Pro Stock engines running 16:1 or more compression. The exhaust valve springs are usually the first to suffer loss of tension and control due to heat and abuse.
Benefits of copper
To help combat spring issues Manton developed its Copper Radius Cup inserts for pushrod tips. These tips feature a larger loaded surface area with a very low coefficient of friction, particularly when used with the low viscosity oils favored by racers. Among the benefits is less heat. Because the valve lifters have a high sliding surface area, they generate considerable heat that can transfer through the valvetrain to the springs. The low heat coefficient of the Copper Radius Cup helps control the heat transfer while also providing longer wear via the reduced coefficient of friction.
“Keeping heat out of the valve springs is critical to valvetrain performance and spring life,” says Perkins. “Most tip problems are in the adjuster, either the radius or the adjuster angle or a material compatibility issue.”
Besides stroker crank clearance, one of the primary reasons that many racing blocks raise the cam position in the block is to shorten the pushrod length, thus allowing shorter, stiffer pushrods that vibrate at frequencies more favorable to high rpm operation. But when you shorten the pushrod you get more angle on the adjuster and more side loading. And because many cylinder heads require offset lifters to clear larger intake ports, you find that lifters can cock in the bore. Than can increase friction and add mechanical stress to the system. Manton recommends avoiding or minimizing lifter cup offset to minimize these problems.
To minimize flex in the valve train they generally favor a single tapered pushrod with the larger diameter toward the bottom where the energy transfers. This helps keep the lifter in contact with the lobe at all times and minimizes valvetrain instability because the additional mass down low prevents the lifter from bullying the pushrod and knocking it around too much.
You can buy standard high-performance pushrods for street engines from Manton, but for race engines they offer innovative solutions to critical valvetrain problems on an application specific basis. If your engine lives on the ragged edge, there is probably some power to be gained by optimizing the pushrod profile and that’s when advice from Manton can be most useful.