Lower Valvetrain Design
Words: Larry Shepard
This article was supposed to be on the typical V-8 engine’s valvetrain, but I ran into a small problem – too many part numbers. The large number of parts in this topic would make the story twice as big as space allows so I decided to cut the topic in half and discuss the lower half first. In reality, you should always look at the whole or complete valvetrain. I will define the lower valvetrain as the camshaft, tappets and pushrods. In the past few columns I have discussed the design aspects of the cylinder blocks, cylinder heads, intake manifolds and the rotating assembly. These bigger parts have a large affect on the valvetrain design. The cylinder heads would be considered a high-profile topic and probably the most popular performance part. The camshaft is probably the second most popular part but tends to be selected toward the end of the engine planning phase or sometimes at the end. In some cases, the cam is used to fix miss-matched selections in other areas. Note: camshafts are designed using computers and large mathematical equations that vary the cam profile’s lift, duration, velocities, accelerations and many other aspects. I’ll leave those aspects to the cam designers and focus on the problems related to the engine installation itself.
There are probably more manufacturers involved in making parts for the engine’s valvetrain than any other area, and cam manufacturers are very common, with each manufacturer offering many, many different designs. Specific cam selection is a function of all of the parts selected for use in the engine, block, heads, intake etc. There are engine-specific books like the Gen II HEMI® book from HP Books – HP Books #1525, or the A Engine book HP Books #1405 or the 6BBL book HP Books #1528 which covers both the small block and big block. These books are available from bookstores or Mancini Racing – (800) 843-2821 or www.manciniracing.com. These types of books give you some general guidelines that can help you select the basic cam specs—lift, duration, overlap, centerline—based on your specific engine parts.
Many aspects of the valvetrain, and specifically the camshaft, are dictated by the other parts of the engine which I have already discussed in previous articles. The block has to hold the camshaft but with hi-load racing setups there are racing blocks that use larger than standard cam bearing journals—up to 60 mm. The blocks also hold the tappets but racing requirements sometimes dictate that the tappet angles change – the A-engine standard is 59-degrees, while the B/RB/HEMI tappets are at 45-degrees. The blocks must be made with the tappet angle. They can’t be converted once machined. The cams are designed based on the tappet angle, so if the block’s tappet angle is changed, the cams have to change also.
The cylinder head has a large affect on the upper valvetrain but the port’s air flow is the main concern for the camshaft. If the ports are large and flow at high valve lifts, then you will want a high lift cam. If you use a dual-plane intake for street use, then you should consider a hydraulic street cam. Displacement, and compression ratio are also concerns along with valve-to-piston clearance, which I’ll discuss in the Upper Valvetrain Design article, next issue.
The basic application for the engine is important to the valvetrain. There tend to be three main levels: street, race and serious race/Pro/hi-dollar. For street applications you generally want to use a hydraulic cam with moderate lift and mild duration and overlap. A good choice for the 383/440 engines (B & RB) is the 284-degree, .484 lift hydraulic P5007697AC or the .509" lift version P5007698AC. A Gen II 426-style HEMI hydraulic is P4529315AC (.484" lift). For a dual-purpose A-engine, consider a small mechanical like P4120653AC – 284-degree, .528" lift and ground on 112 centerline. The Gen III HEMI engines are unique from other HEMIs and the 5.7L, .500" lift, 259-degree cam, designed for the non-MDS tappets, is a good performance starting point.
There are many, many options for the cam lift, duration, overlap and centerline specifications but there are some other issues with the cam itself. The first item is large cam journals for use with large cam bearings (roller designs typically). These cam bearings can be up to 60 mm in diameter. This is a race-only situation and is perhaps more common on 426 HEMI blocks, Gen II, but is also done on A-engine race blocks. The second race aspect is the tappet angle. This is most commonly done on A-engine race blocks where it is changed from the standard 59-degrees to the race-only angle of 48-degrees.
On the B/RB/426 HEMI big block engines, the cam-drive (chain & sprockets) is attached to the nose of the cam with 1-bolt or 3-bolts. The 1-bolt method tends to be the older design, or the base-production unit, while the 3-bolt design tends to be the newer, HP design – 1970 and newer, 426 HEMI and 440-6BBL. Note: 426 HEMI cams fit into the B/RB blocks but do not operate the valves properly. The reverse is also true. Only use wedge cams in wedge blocks etc.
The camshaft is used to drive the oil pump and the distributor. This drive gear is located at the front of the cam in the B/RB/426 HEMI and at the rear on the A-engine and Magnum® engines. Typically this distributor drive gear is made of cast iron. When a mechanical roller cam is used, or any billet cam or hydraulic roller cam (aftermarket), then an aluminum-bronze gear should be used (like P3690874 – A-engine, assembled on HD shaft). Note: an aluminum-bronze gear is a yellow or gold color.
The camshaft is driven off the crank at the front of the engine by a chain and sprockets system. The typical production engine uses a silent chain and nylon-tooth cam-sprocket, while the HP engines use a double-roller chain and cast iron sprockets. The chain and sprockets should be changed as a system. The 1-bolt double-roller chain system for the B/RB/426 HEMI is P5249268, and the double-roller chain set for the A-engine is P5249267. The 3-bolt B/RB/426 HEMI double roller cam drive is available from Mancini Racing, listed above. Once the cam is selected, one of the most important aspects of the cam for proper performance is its installation relative to the crankshaft. This relationship is called the installation centerline. This process starts with lining-up-the-dots which gets it within a tooth, which is around 20-degrees. To fine tune it and achieve an accuracy of 1 to 2 degrees, the B/RB/426 HEMI uses an offset bushing – kit P5249711. The offset bushing goes over the locating dowel pin in the end of the cam and fits into the enlarged hole in the cam sprocket (modified standard part). The A-engine uses keys to locate the cam relative to the crank. Offset keys are available from Mancini Racing – listed on page 41.
The timing chain and sprockets on the front of the engine is covered by a front cover that bolts to the block. The B/RB/426 HEMI front cover is made of stamped steel and commonly chrome plated (painted in production). The A-engine front cover, P5249930AB, is made of cast aluminum.
The tappets should be matched to the cam and typically are changed with the cam. Once a cam and tappets are run together they should be kept together and the tappets should be kept in order. Therefore a new cam means new tappets. A hydraulic cam uses hydraulic tappets – A-engine, P3614321; B/RB, P4006767. Mechanical cams use mechanical tappets, also called flat tappets, P2843177 – both A & B. Mechanical roller cams require mechanical roller tappets, which tend to come assembled as a set – two (2) tappets and a guide bar. The A & B-426 HEMI blocks are not set-up for the production-style hydraulic roller tappets so if a hydraulic roller cam is desired for these engines you should get the cam and tappet system from an aftermarket manufacturer like Comp Cams, www.compcams.com. Note: the Magnum engines (5.2L and 5.9L V-8s from 1992 through 2003) oil the valve gear through the pushrods so they must have a tappet that feeds oil to the center of the pushrod. Caution: mechanical cams require zinc in the engine oil, so use oils that advertise high levels of zinc or zinc compounds such as racing oils or Shell Rotella.
The pushrods are pretty straight forward. A hydraulic cam uses hydraulic pushrods and a mechanical cam uses mechanical pushrods. Typical hydraulic pushrods tend to be round on both ends while mechanical pushrods have a cup on the rocker arm (upper) end. If you plan on using adjustable rocker arms on a hydraulic cam, then you will need a mechanical style pushrod. The only production engines that used adjustable rockers with a hydraulic cam were the 1970–71 426 HEMI and the 1970 340-6BBL. There are standard lengths for these pushrods and the aftermarket will make custom lengths. Note: the 426 HEMI exhaust pushrods are longer than the intake pushrods and the mechanical pushrods are longer than the hydraulic pushrods: 10.81" intake, 11.58" exhaust, for the mechanicals; and the hydraulic intake pushrod is 10.64". Hydraulic and mechanical pushrods are available from aftermarket manufacturers like Comp Cams or distributors like Mancini Racing. Trick tip: looking at the whole valve gear, the pushrod length rarely causes a problem but it can be used to solve many problems originated in other areas especially with hydraulic cams and stamped rocker arms. To do this, you need adjustable length pushrods which are available from Smith Bros., www.pushrods.net and Mancini Racing, listed on page 41.
More Mopar® lower valvetrain parts—cams, tappets and pushrods—are being added to Mopar’s lineup. For additional information, refer to the latest Mopar Performance catalog or visit www.mopar.com. Space limits my discussion; if you have specific questions, please call the Mopar Direct Connection Tech Line at 1-888-528-HEMI (4364).
