When pushing your race engine...
When pushing your race engine to its limits, it is easy to float the valves.
While making our usual rounds of race shops, we came across a problem that plagues many racers and unnecessarily costs them lots of money-and we thought we'd share the solution with you. While performing a rebuild of a Late Model-style Chevy race engine, cylinder head specialist Kevin Troutman of KT Engine Development saw the telltale signs of classic valve float. The racer either didn't notice or just didn't bother to mention the problem to KT Engines, and the results were costly. But the good news is that valve float can be avoided.
Valve float occurs when the valvesprings are incapable of holding the valvetrain against the camshaft lobe after peak lift. This happens when either the weight of the combined valvetrain components or the rpm speed of the engine creates so much inertia that the spring is no longer able to control the valve. The most common response to valve float is to increase the strength of the spring so that it can better control valve motion. But stronger springs generally weigh more and cause their own problems. Achieving the optimum strength-to-weight ratio is a delicate balancing act for every engine builder.
The damage to this valve stem...
The damage to this valve stem tip is an obvious sign of valve float, which is caused when the spring slams the valve back into the rocker.
The most efficient and dependable race engines are able to hit the sweet spot in the triangle created between strength of the valvespring, weight of the valvetrain components (lifters, pushrods, rocker arms, valves, retainers, locks, and springs), and the engine's peak rpm levels. In order to maintain good valve control at higher rpm levels, many engine builders have begun reducing valvetrain weight (and therefore the overall valvetrain mass) by using beehive-shaped springs.
Comp Cams is leading the development of high-performance beehive springs, and the results have been good so far. The beehive-shaped spring has several advantages when used correctly. First, its conical shape makes it smaller at the top. This reduces mass in the area of the springs that moves the most every time the valve is opened. Second, the smaller size means that a smaller retainer is also used, reducing mass there as well. Third, the beehive spring is a single-spring design by necessity, and while it cannot be made with as much spring pressure as a more conventional, double-nested design, the simpler design makes it much lighter. Beehive springs are also used in GM's LS1 and LS2 engine designs, making it legal where rule books require "stock-type components only."
KT Engine's cylinder head...
KT Engine's cylinder head specialist Kevin Troutman pulls all the valves to inspect for damage.
The lighter overall weight means that a smart engine builder has more options available for his build, such as using less spring pressure to get the same results. He can also maintain spring pressure to increase engine rpm before valve float occurs. Or he can experiment with even more radical camshaft designs that weren't possible with conventional springs. The one thing that must be remembered, however, is that while beehive springs can help offset the effects of valve float, it is still possible to float a valve when the right conditions are reached.
The Clues During teardown, Troutman says he first noticed the signs of valve float as soon as the rocker arms were pulled to reveal the tips of the valve stems. Because the intake valve is larger than the exhaust valve, and therefore heavier, it almost always enters a float condition first. Whenever the valve floats, the "floor" of the system, or the closing ramp of the cam lobe, falls out from underneath the system. This creates gaps along the line of components between the cam and the valve stem. When the spring is finally able to overcome the inertia of the moving valve and begin closing it, its movement of the valve is uncontrolled. This movement can be very harsh as the spring slams the valve closed. Because of this, one area of high wear is the valve tip, which is repeatedly smashed into the pad or roller tip of the rocker arm.
The cause for the float becomes...
The cause for the float becomes quite evident when the valvesprings are checked.
A damaged valve tip will most often appear as though part of the metal has flaked off. When you have this condition, it is sometimes possible to grind a few thousandths of material off the end of the valve stem, but this is rarely worth it. Grinding down the end of the valve stem, even if only a few thousandths of an inch, changes the valvetrain angles, which can lead to other problems. Also, even if you grind off the portion that is visibly damaged, there is no easy way to tell if the remainder of the valve hasn't been compromised. It's far better to throw the valve away and install a replacement.
The action of the valve being slammed closed is also quite hard on the seats-both at the valve and the combustion chamber. In the worst-case scenario, the valve is smashed against the seat of the combustion chamber so hard that the valve "tulips" or becomes warped. It's easy to tell when this happens, even if you cannot see it, because the valve won't hold a seal against the seat. In less extreme cases, the seat cut into the valve will be pounded until it becomes concave. This was the situation found on our engine. The seat in the combustion chamber may show signs of damage by being wider than originally cut. On a typical three-angle valve job, the 45-degree cut where the valve actually seats against the chamber is usually only 0.040 inch wide. The intake seats in the combustion chamber on this engine were nearly 0.100 inch wide. Particular care will also be required when inspecting the lifters and camshaft after the short-block is torn down.