Valve Train

It’s all about the rpms. If our engines turn 7,500 rpm we would love to get to 8,000. If we run 8,000 down the back straight we want to get to 9,000 rpm. Every additional 1,000-rpm may be worth an extra 20 or more horsepower on a highly modified performance motor.  

Spin an engine faster and it will, in most cases, make more power. The standard example is F1. Today’s F1 engines make roughly the same amount of power as they did 10 years ago – at 10,000 rpm. The difference is that today they go to 18,000 rpm. The additional horsepower comes from increased rpms. Increased rpms come from stabilizing the valve train.

Most of the vintage cars in the paddock use pushrods and rocker arms in the valve train. In the United States we have a huge industry focused on making overhead valve engines with pushrods perform better. Everything NASCAR does is dedicated to this technology. Can an MGA benefit from what they’re doing in NASCAR? Of course.  Your MGA has the same basic valve train design as the Toyota engine found in NASCAR. That means whatever works on that engine will work on your engine.

The valve train starts at the camshaft lobe and ends at the head of the valve. This means the lifter, the pushrod, the rocker arm, the valve spring and finally the valve. I’m only going to deal with three items here: The pushrod, the rocker arm and the valve spring.

It’s really easy to describe the perfect valve train. It would have finite stiffness and zero mass. Oh, it would never wear out either. While that obviously isn’t possible it does describe our goal. We want the perfect combination of weight and strength. The perennial question is always “Can the mass be reduced without reducing the stiffness of the overall system?” 

The Pushrod: The pushrod runs from the lifter to the rocker arm and it needs to be as strong as possible while at the same time being as light as possible. Pushrods can flex quite a bit at higher engine speeds, much like a pole vaulter’s pole. As the lifter rises on the cam lobe and pushes the pushrod up against the rocker arm the pushrod deflects. How much it deflects depends on the load (valve spring pressure) and the stiffness of the pushrod.

A few years back a 5/16-inch diameter pushrod was considered large. Now everyone is using 7/16-inch diameter pushrods. Some of the larger engines are even using 1/2-inch pushrods. The limiting factor in any engine is the space available in the cylinder head. If you have room for a larger diameter pushrod you should do so. Don’t be concerned about the increased weight of the larger pushrods. The pushrod is on the slow moving side of the valve train. Any loss from increased weight will be made up in increased valve train stability.

Alan Bechtloff of Crane Cams would add that any performance loss that might come from the added weight would be made up with the increased stability of the system. The pushrod is one situation where strength (or stability) is more important than weight. He would add that this statement is true with solid lifters though. If you’re running hydraulic lifters in your race engine then pushrod weight is more of a factor. 

The really interesting thing is that there are very few problems with pushrods flexing under pressure. The problem occurs when the pushrod un-flexes. As one engine builder said “The valve train just goes stupid.”

Tool steel is a popular choice for single-piece pushrods.  The most common steel is a mil-spec 4130-chromoly seamless tubing. The core hardness of this steel is in the mid-50’s on the C-scale. The case hardness is normally 60-62 Rc.

Keith Dorton of Automotive Specialist, Inc. in Concord, NC says he typically uses 7/16˝ diameter pushrods with tapered ends and .165˝ wall thickness. If he can he goes with thick wall 3/8˝ diameter pushrods to minimize pushrod flex.

The reason for all of this is because of the desire for increased rpm, which in turn means greater spring loads. Increased valve spring pressures that you need pushrods with a substantial wall. Right now the standard wall thickness is .100. Most racing applications though use a .120 to .180-wall thickness. Engine builders want the pushrod wall as thick as possible while still being able to move an adequate amount of oil through the center of the pushrod.

This pushrod also has to run very reliably in spherical sockets at the rocker arm and the lifter. The conditions for lubrication in both cases simply aren’t that great. Because the pushrod is constantly moving relative to its pivot it is difficult to generate a satisfactory oil film that can reliably separate the components under all operating conditions.

If you run high rpms you should look for tapered pushrods. The taper helps to dampen harmonics in the valve train. Not only will you be able to increase your rpms but also you’ll increase the life of the valve springs.

 The Rocker Arm: The rocker arm is actually a lever that reverses the direction of force. As the camshaft turns the lobes push up on the lifters, which in turn push up on the pushrods. The valves though need to be pushed down. This happens by putting rocker arms on top of the pushrods. Now when a pushrod goes up it encounters the rocker arm. As it pushes up on one side of the rocker arm the other side of the arm pushes down and the valve opens.

The rocker arm's pivot point can be modified. Moving the fulcrum point in rocker arm changes how far each end moves in relation to the other. A 1:1 rocker arm ratio, for example, means the valve goes down a half inch for every half-inch the pushrod moves up. If we move the fulcrum point we can have a rocker arm with a 2:1 ratio. That would mean the valve goes down 2 inches for every inch the pushrod moves up. This is called the rocker arm ratio.

Since the amount the valve opens is dependent on the ratio of the rocker arm you can change the valve opening by simply changing the rocker arm ratio. The downside is that there will be a little more wear on the valve guide, and I do mean little. If most people wear out valve guides at 200,000 miles, this may wear the guides out at 150,000 miles. If you’re a racer it’s not even worth considering.

Rocker arms with a higher lift ratio can add horsepower with little or no loss in low rpm torque, idle quality or vacuum. By opening and closing the valves at a faster rate the engine is able to flow more air for the same number of degrees of valve duration. Changing rocker arms is almost the same as changing the camshaft. You can get an improvement in horsepower without the major hassle of changing the camshaft.

Let’s take a hypothetical example. Going from a stock 1.5 ratio to a 1.6 ratio rocker arm the net at-the-valve lift is increased approximately 7%. This means a camshaft with a .320-inch lobe lift will open the valve .480-inch with a 1.5 ratio rocker arm. Now if you switch to a 1.6 ratio it will open .512-inch. Duration will also increase but only nominally because when the speed of the valve accelerates the engine thinks it has 2°-4° more camshaft duration.

When it comes to rocker arms stiffer is always better. In some applications you can increase the stiffness by using chrome moly steel. With chrome moly you can have thinner cross sections because this particular steel has superior strength density. A common aftermarket alternative is aluminum. Just make sure the aluminum rocker arm is machined from a billet. We’re starting to see some cast rocker arms from offshore suppliers.

An interesting thing about rocker arms is that total mass is not all that critical. It’s the moment of inertia that counts. Moment of inertia refers to the rocker arm’s resistance to rotation. The higher this number is the greater the valve spring pressure needs to be. Manufacturers lower the moment of inertia by reducing the weight on both ends of the rocker arms. The center mass isn’t that critical since it doesn’t figure into the calculation. This means you want the ends of the rocker arm as light as possible. Low weight is especially important on the valve side of the rocker arm.

Since stiffness is critical it’s important to look at the way your rocker arms are mounted to the cylinder heard. Stud mounted rockers have evolved over the years. In the early years the rocker arm was a very basic ball-and-socket mounted stamped steel piece of steel. The first modification was to change the mount to a roller bearing and a transverse mounting axle. Then people began using larger screw-in studs.

The next step in rocker arm technology was the shaft-mounted rocker. Rather than being mounted on a stud, a horizontal shaft works as the fulcrum. This increased the mounting stiffness significantly and hence valve train stability. These shaft-mounted systems are for the really high-end performance applications. You can spend over $3,000 on these systems. A cost-effective alternative is to use a stud girdle to reduce deflection with stud mounted rockers.

The Valve Spring: Valve springs have two main functions. First, the spring closes the valve.The second job of the spring is to keep the lifter in constant contact with the cam lobe as the lifter begins its rapid decent down the backside of the cam lobe.

This valve closing really isn’t the easiest thing to control. An old, weak spring will allow the valve to close too quickly. Your valve then bounces off the valve seat. It's also the spring's job to push the lifter against the camshaft lobe when the valve is closing and keep the lifter from literally flying off the nose of the lobe as the valve opens.

Until the late 1970s valve springs were made of hard-drawn wire of high carbon steel having a tensile strength of about 1,700 MPa. Then oil-tempered wires of 1,900 MPa grade began to be used. Nowadays, oil-tempered wires of 2,100~2,200 MPa grade are commonly used.

The best springs are made from “super clean wire” that is a high-grade alloy with almost no inclusions or imperfections. When the wire is formed, it is rolled in such a way that any inclusions in its microstructure are pushed to the center of the wire. The center experiences the least stress so the overall strength and durability of the wire is enhanced. The wire is then scanned with an electrical eddy current to reveal any hidden imperfections before it is made into a valve spring. 



All of this technology dates back to a small company in Michigan know as Performance Springs Inc. Steve Bown and Larry Luchi recognized that most high rpm engine failures are associated with inclusions within the steel used for the valve springs. Other companies knew this but they never instituted the appropriate manufacturing procedures. Bown and Luchi simply took the time and set new quality control standards for the manufacturing of valve springs.

The standard for valve springs has been Japanese Kobe steel but there has been a real revolution in materials recently. VIMVARVAR   (vacuum induction melted twice vacuum arc re-melted) valve spring steel produced in Pennsylvania is currently an alternative to the Kobe steel that’s produced in Japan.

Some spring manufacturers also use special surface finishing procedures to extend spring life. Shot peening has long been used to create compressive residual stresses in the outer layer of the spring wire. Shot peening leaves a matte finish on the springs, while hardening the surface to help the spring handle higher loads and speeds without failing. Nitriding has a similar effect.

By diffusing nitrogen into the surface of the spring, the surface is made harder and stronger. Polishing is another technique that can eliminate small surface imperfections and extend spring life. Springs can also be cryogenically treated to improve their metallurgy and longevity.

There are two ways to make a valve spring selection. The easiest way is to simply stay with what has always worked for you. The alternative is to find a manufacturer who has invested time and money to run both dynamic and durability testing.  A typical testing procedure is to run 2-3 days of dynamic testing on the Spintron.

The Spintron® was created in 1993 by Bob Fox to test the pushrods made by his company, Trend Performance. The Spintron® is basically an engine block with only a dummy crankshaft and the valve train components—camshaft, pushrods, rocker arms, valves, and springs. The engine doesn't run but is cycled by a 60-hp electric motor connected to the crankshaft.

With an array of sensors and high-speed video cameras the Spintron reveals formerly invisible details, like how an undamped spring continues to oscillate after the valve closes. In the case of valve springs the machine is used for the analysis of spring dynamics at any given rpm. These machines are capable of turning the valve train up to as much as 20,000 rpm. This might be followed up with 3 to 5 days of durability testing.

The essence of valve train dynamics is to take control of the variables. We now have the tools to analyze the system and we have a much greater appreciation of the quality control that has to take place in the manufacturing facility. It’s also critical to the process that the customer is willing to pay for all of this. Technology is never cheap. Then again most of us don’t want to go back to 6,000-rpm motors. Actually the real question is how many of us want to run our vintage engines at 10,000 rpm? Actually a 12,000 rpm small block would be a whole lot of fun.