Valve Seat Recession on Modern Cylinder Heads


INTENSE Racing strives to continually improve our processes and our products. We vow to bring you the best products for the 3800 that we can engineer and build at competitive pricing. Our research and design does not stop when we introduce products for sale; rather we constantly look for ways to improve them. One such improvement is that we can now offer the same high-flowing INTENSE Stage 3 heads but do not require a lead substitute additive in order to avoid Valve Seat Recession. Through extensive work on the flow bench, we've been able to improve the flow characteristics of our already high-flowing exhaust ports and runners enough that we can maintain the same exhaust flow we had with our previous 1.60" valves while using slightly smaller 1.57" exhaust valves.

How did Valve Seat Recession come about?

Most people realize that back in the days when we ran leaded fuel in our cars, the lead was great at improving combustion and raising octane. This high-octane fuel is what enabled cars to run much higher compression ratios than cars do today. Another property of that fuel that contributed to the health of the older motors was that the lead oxides that were formed during combustion would settle on the valve seats and act as a cushion protecting the seats from erosion.

Once people realized the harmful effects of lead on the environment however, pressure caused leaded fuel to be phased out. In response to this, car manufacturers lowered compression ratios and started using hardened valve seats that did not require the lubricating properties of lead.

That's fine for newer cars, but those that owned cars that were originally designed to run on leaded fuel (i.e. cars that used non-hardened exhaust valve seats) needed to take certain precautions to prevent valve seat recession. Car enthusiasts soon learned that unleaded fuel required the use of hardened valve seats on the exhaust valves, and that without some precautions, their cars would exhibit valve seat recession.

Exactly what is Valve Seat Recession and what causes it?

Simply, VSR is the erosion of the valve seat causing the valve to slowly sink deeper and deeper into its seat. Without the protective lead coating or hardened inserts on the exhaust valve seats, the intense heat (650C or 1200F) and the constant hammering effect of the valves opening and closing causes iron deposits from the valve seat to become micro-welded to the valve edge as it closes. When the valve opens again, some small amount of metal from the seat will be ripped from that seat. Left alone, this continual tearing away of metal particles will result in the exhaust valve digging a deeper and deeper hole for itself into the head. Eventually, the engine will break down and require to be overhauled. VSR is only a problem for the exhaust valve seats since they run at much higher temperatures than the intake valves.

If my heads exhibit VSR how fast can my valve seats erode if I ignore the issue?

Without hardened seats or some lubricant in the fuel, they can erode quickly under the right conditions! The two worst contributors to this problem are prolonged high engine speeds, and high exhaust gas temperatures. If you baby your engine, it's likely that you may never experience the problem. But since you're reading this at the INTENSE website, and if you're anything like us, then you probably like to push your car hard fairly frequently, creating the right conditions for VSR. To give you a better idea of just how fast this can occur, independent testing has shown that for some types of engines, exhaust valve seats can recede by as much as 0.020" when run continuously under both high load and high RPMs for 48 hours.

How come my stock heads never had this issue?

The valve seats on stock heads as they come from the factory are induction hardened. This is a process where the valve seat surface is case hardened by applying a heated coil to the seat for some time. The result is a hardened surface in the seat area that is resistant to VSR. Most experts believe that typically, the depth of the induction hardened material runs approximately 0.030" deep. Fortunately for us, we've recently discovered that the General has done a little better on our heads. When one decides that they will fit oversized exhaust valves in their heads, one must be careful because some this hardened area will have to be cut. If the valve is large enough and the cut is deep enough, this hardened material may be completely removed.

The stock exhaust valves on our heads are 1.52" in diameter. Over the past year and a half we have machined dozens of Series II heads and have collected much empirical data. We've discovered that by the time we machined the exhaust seat enough to accommodate a 1.60" valve the hardened seat area was usually (but not always) gone. We also learned that if we only cut the exhaust seats to accommodate a 1.57" exhaust valve, there would be a sufficient amount of hardened material left so that a lead substitute additive is not required.

My friend who also has an L67 has oversized valves in his heads. How come he doesn't have this problem?

Well, there's one of a few possibilities. It's possible that he also uses 1.57" or smaller exhaust valves. Or it's possible that he may suffer from VSR, but he just doesn't know it yet. We feel that on cars with 1.59" or larger exhaust valves, driven daily on unleaded fuel the problem will eventually show itself, whether his machinist warned him about this or not.

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