Changing the rocker arms in your C5 or C6 Corvette may not deliver the dramatic results that a cam swap can, but it's a lot less work. And the power increases you'll experience make the change a good value for the money.
First, take a second to think about what a rocker arm does: It reverses the direction of force. As the camshaft turns, its lobes push up on the lifters, which in turn push up on the pushrods. This is all well and good, but the valves 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 on the valve.
The rocker arm's pivot point can be modified. Moving the fulcrum point 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. By moving the fulcrum, you could have a rocker arm with a 2:1 ratio, so the valve goes down 2 inches for every inch the pushrod moves up. General Motors usually uses rocker arms with a 1.7 ratio, so when the pushrod goes up 1 inch, the valve should open 1.70 inches.
Here's where we're going with this: One of the ways to produce more power in an engine is to open the valve further, which is often accomplished by increasing the camshaft's lift. But the height of the valve opening is also dependent on the ratio of the rocker arm. So you can change the valve opening height by simply changing the rocker arm ratio. Actually, there are three things you're doing with a rocker arm change: increasing the valve lift, creating a faster ramp angle so the valve opens faster, and increasing the duration--the length of time the valve is open. Sound familiar? These are the things that generally happen when you change the camshaft.
Before all of you start writing letters, I do know you can do a lot more with a camshaft change than you can with a rocker arm swap. You can also spend a lot more money and time with a camshaft change. We're talking about value here.
The stock Corvette LS engines use a 1.7-ratio rocker arm. The most common change is to a 1.8 ratio. That's not a huge change, but it could be worth almost 30 hp. In most cases it will be worth around 20, but it could easily be worth more if you've done anything else to your engine.
If this is so easy, why didn't the engineers at Chevrolet do it? Because they didn't need to do it. When GM engineers develop an engine, they can change camshafts any time they want. Why would you screw around with rocker arm ratios when you can design any sort of camshaft? Racers generally don't worry about rocker arm ratios either, because they, too, can easily swap camshafts.
So what's the downside? 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. Chances are your Corvette will be down the road before you even have to think about anything like this.
Things to Consider
Rocker arms come in all different varieties and prices. Generally you're going to get what you pay for. Danny Kellermeyer at D.J. Racing has tried them all. Some of them never even got completely installed before he threw them away.
The stock LS rocker arm is a really good component with only one flaw--its sintered steel fulcrum pivot. SLP figured this out and made rocker arms using the stock GM design with a much higher quality steel for the trunion. That, and their 1.8 ratio, makes them a good choice at a reasonable price.
Harland Sharp also changes the stock trunion to tool steel, like SLP. What these rockers may lack in sexy appearance is more than covered by their value. Remember, Harland Sharp has been around since front-engine dragsters. It's a small company that produces quality products.
Always check for the necessary...
Always check for the necessary clearance between the cylinder head casting and the rear of the rocker arm (arrow). D.J. Racing has found that some companies simply don't leave the necessary clearance. Make sure you check this area while you can still get your money back.
Shaft-mounted rocker arms...
Shaft-mounted rocker arms are a little more stable than rockers that are simply mounted on a stud. Most people consider them superior, but they do involve additional cost and installation time.
This shim is 0.001-inch thick....
This shim is 0.001-inch thick. If you tighten the rocker arm nut down using this shim, then you'll have the virtual zero that's called for. This is much easier than some of the other techniques I've seen. You can thank D.J. Racing for this little tip.