OHC vs. OHV

Valvetrains come in two basic configurations: Overhead Cam and Overhead Valve. Here’s a look at the differences and the advantages to both setups.

Valvetrain configurations take two forms, overhead cam (OHC) and overhead valve (OHV). In an OHC layout, the camshaft is installed in the cylinder head and the lobes push down on rocker arms, or the top of the valves, to open the valves. In OHV engines, the camshaft is located in the engine block and valve lifters, sitting on the cam lobes, push up on pushrods, activating the rocker arms to open the engine valves.

While OHV designs dominated engine layouts for decades, the OHC engine gained popularity in the late 1970s and early ’80s. This resulted from automakers’ desire to make more compact and efficient engines. OHC engines offer many advantages over OHV counterparts and these advantages were utilized fully.

The OHV engine, though, is making a comeback. And the best example of this is Chrysler’s latest reincarnation of the classic HEMI® engine. It has it all: Camshaft in the block, hydraulic valve lifters, pushrods and rocker arms.

In addition to the new HEMI engine, there are several other Chrysler engines that utilize the OHV design. Probably the most popular of the pushrod engines is the 3.3/3.8L V6 that has been, and is still, used in the minivans. The 2.5L inline four-cylinder and 4.0L inline six-cylinder engines have been used extensively over the years in various Jeep® models. And the Ram truck line uses the 5.9L V8 and the 8.0L V10 OHV engines. All of these engines, regardless of displacement and configuration (inline versus V-layout), utilize hydraulic lifters, or tappets, that sit on the lobes of an in-the-block camshaft (see Figure 1).

OHV Engine Operation

Hollow, steel pushrods are positioned between the hydraulic lifter (often referred to as a tappet) and the rocker arm, sitting in the recess in the top of the lifter. As the camshaft rotates, the pushrods are forced upward, pressing on one end of the rocker arm. This upward motion causes the rocker arm to pivot, translating the up and down motions provided by the camshaft, hydraulic lifter, and pushrod, into the down and up motions on the valve stem. Also, pressurized engine oil from the hydraulic lifter is fed up through the hollow pushrod, providing lubrication for the rocker arm and valve.

In addition to acting as the seat for the pushrod, the hydraulic valve lifter eliminates valve lash, or clearance, between the pushrod, rocker arm and valve tip. As pressure from the pushrod is applied to the spring-loaded inner plunger, the plunger presses down on the pressurized oil in the lifter body. This action forces the oil out of the bottom of the lifter body between the plunger and the inner wall of the body (this is known as leak down), allowing the plunger to slowly drop into the lifter body. The net result is the elimination of any clearance in the valvetrain.

If the valve lifter was solid, clearance would have to be set in the valvetrain. As the components increase in temperature, this clearance would gradually diminish. This is why engine with solid lifters have the telltale clicking sound. This is the sound of the rocker arm contacting the valve tip. When the oil in a hydraulic lifter becomes contaminated with dirt, the plunger can seize within the lifter body, unable to take up the clearance. The result is the clicking sound associated with solid lifters.

In years gone by, the bottom of the hydraulic lifter was almost flat, but slightly convex. This contour allowed the lifter to rotate during operation, resulting in an even wear pattern. In late model engines, however, the need to reduce friction led to the use of a roller on the bottom of the lifter. Previous to this change, roller designs had been limited to use in race engines.

Finally, most OHV engines have some type of mechanical device to keep the lifters in place (refer to Figure 2). The 8.0L uses an aligning yoke around a pair of tappets and a yoke retaining spider to secure the yokes. A similar set-up is used in the minivan V6s. The new HEMI engine uses a tappet retainer for every four lifters.

Pushrod Service Problems

Pushrods, for the most part, present service problems on a very infrequent basis. Some problems, though, can occur. The most common is probably blockage of the hollow tube. This results from very dirty engine oil. This dirt can accumulate in the tube, limiting or blocking the flow of oil to the upper valve train.

The other service problem that is encountered is the result of over revving the engine. Such abuse can actually bend the pushrod. To determine if a pushrod has been bent, roll it on a smooth, flat surface. If the pushrod is damaged, it will wobble as it rolls.

Hydraulic Lifter Service Problems

Most often, the problem encountered with hydraulic lifters is noise, but that noise can be caused by a few different conditions. One of those conditions is air in the engine oil. Check the engine oil level. The oil level should never be above the MAX mark on dipstick, or below the MIN mark. Either of these two conditions can be the cause of air in the engine oil that results in noisy lifter operation.

If oil level is above the MAX mark on dipstick, it is possible for the connecting rods to dip into the oil while engine is running, causing foam in the oil. This foam (essentially air) is fed to the hydraulic lifters by the oil pump. This air causes the lifters to become soft, allowing the valves to seat noisily.

Low oil level can allow the oil pump to take in air, which when fed to the lifters causes the same noise problem as foam. Any leaks on intake side of the oil pump, through which air can be drawn, will create the same lifter noise. Check the lubrication system from the intake strainer to the oil pump cover, including the relief valve retainer cap.

When lifter noise is due to aeration, it may be intermittent or constant, and usually more than one lifter will be noisy. When oil level and leaks have been corrected, the engine should be operated at fast idle to allow all of the air inside of the lifters to be bled out.

Valve lifter noise ranges from light noise to a heavy click. A light noise is usually caused by excessive leak-down around the spring-loaded plunger, or by the plunger partially sticking in the lifter body. If these are the causes, replace the lifter. A heavy click is caused either by a lifter check valve not seating, or by foreign particles wedged between the plunger and the lifter body causing the plunger to stick in the down position. This heavy click will be accompanied by excessive clearance between the valve stem and rocker arm as valve closes. In either case, the lifter assembly should be removed for inspection.

After a hydraulic lifter has been cleaned and inspected, it should be tested for a specified leak down rate to ensure zero lash operation. This is done using a leak down tester (see Figure 3). Using a special hydraulic lifter test oil, the lifter is submerged in the oil until it is filled. The air is pumped out; the weighted arm then places a force on the ram, causing the lifter to leak down. Using a stopwatch and the scale on the tester, the leak down rate is determined. Those lifters that have an acceptable leak down rate can be re-installed in the engine. Those that fail the test are discarded.