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Performance Lifters, Pushrods and Valve Springs
A high performance valvetrain includes many components, but in this article we’re focusing on three essential parts: the lifters, pushrods and valve springs. The first two work with the camshaft to provide valve lift, while the springs control the motion and push the valves shut.
By Larry Carley
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Low friction roller lifters replaced flat tappet lifters many years ago in stock production engines, but there are still plenty of hot flat tappet cams bumping the valves open in vintage muscle cars, street rods and race cars. Even NASCAR is still using flat tappet cams to actuate the valves.
The advantage of flat tappet lifters is that they are relatively simple and cheap compared to roller lifters. With solid lifters, there’s not much that can go wrong. The lifter is nothing more than a hollow steel bucket with a slightly convex bottom. The lifter rides on the cam lobe and transfers the rotary motion of the lobe into vertical lift. That motion passes up through the pushrods to operate the rocker arms and open the valves.
Solid lifters work well in high-revving engines, but do require constant valve lash adjustments. A solid lifter valvetrain is also quite noisy because of the clearances between the rocker arms and valves. That’s not an issue in a race car, but for a street car it may be a consideration. However, solid lifters do provide the ability to change the valve lash by adjusting the rocker arms or pushrods. This allows cam lift and duration to be tuned slightly to adapt to changing track conditions.
With hydraulic lifters, there’s an oil-filled bucket inside the lifter. Oil pressure enters the lifter through a small hole in the side and fills the piston cavity. A check ball and spring in the lifter trap the oil temporarily, allowing the piston to move upward and take up slack in the valvetrain. This eliminates valve lash, noise and the need for frequent adjustments. But the trade off is added complexity, cost and a tendency to “pump up” at higher rpms. Hydraulic lifters are usually the best choice for stock engines, street performance applications and low rpm torque motors that don’t rev much beyond 5,500 to 6,000 rpm.
The taper on the cam lobe and the curvature of the bottom of a flat tappet lifter cause it to rotate as it rides on the lobe. This reduces friction, but still creates a lot of pressure between the cam lobe and the bottom of the lifter. Proper lubrication is absolutely essential to prevent cam and lifter failure.
Never reuse old lifters if you are replacing a cam. A new camshaft requires a new set of lifters.
To prevent rapid camshaft and lifter wear, the oil must contain adequate levels of anti-wear additive, such as ZDDP (zinc dialkyl dithiophosphate). ZDDP dates back to the 1950s, and has long been used to protect flat tappet cams in pushrod engines. But with overhead cam engines and pushrod roller cam engines, valvetrain friction has been greatly reduced. Consequently, there is less need for ZDDP in today’s motor oils.
The oil companies have been gradually reducing ZDDP levels in recent years to prolong the life of catalytic converters. The problem is that ZDDP can contaminate the catalyst and reduce its efficiency if an engine is using oil. “SM” rated motor oils and newer oils (including both conventional and synthetic) have much less ZDDP than they once had.
There is still enough of the anti-wear additive to provide marginal protection for older pushrod engines with stock flat tappet cams and valve springs, but not enough to protect the cam and lifters in modified or high performance engines with a flat tappet cam (especially if the engine has stiffer-than-stock valve springs). For these applications, a ZDDP supplement or other anti-wear oil additive must be used. Or, you can fill the crankcase with a racing oil that is formulated with adequate levels of ZDDP or another anti-wear additive.
Something else every engine builder has to keep in mind with flat tappet cams and lifters is the need to properly lubricate and protect a new camshaft and lifters when an engine is assembled, then started. All cam lobes and lifter bottoms must be pre-coated with high pressure assembly lube to provide protection during the first critical moments following the engine’s initial start up.
Pressurizing the engine’s oil system prior to cranking it over for the first time is also recommended. Once the engine starts, keep it above 2,000 rpm for 20 to 30 minutes. Don’t let it idle or you run the risk of wiping out the new cam. Also, advise your customer of the importance of using a ZDDP additive or racing oil.
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