Valve Seat Recession Explained

[Guest head_tech]

Valve Seat Recession

Exhaust valve seat recession (VSR) is a complex process affected by many factors of engine design and use. Amongst these however, high engine speeds and high engine temperatures are major factors in making an engine susceptible to VSR. VSR is a phenomenon experienced only by poppet valve four-stroke petrol engines.

Valve Seat Recession Explained

Valve seat recession (VSR) is the name given to a process of accelerated valve seat wear. The mechanism by which this occurs is as follows:

1. The very high temperatures at which it operates (typically between 600 and 800 degrees Celsius) causes the exhaust valve to weld locally to its seat. When the valve is then opened a small particle of the iron seat is pulled away. This particle is converted into iron oxide in the hot exhaust gas.

2. As the valve then closes again these particles are imbedded in the valve face.

3. With these hard lumps of iron oxide imbedded in it, the exhaust valve becomes, in effect, a grinding wheel. Thus any rotation of the valve will cause it to grind away the seat. Valves tend to rotate under the action of the valve springs and by engine vibration.

Tetra ethyl lead, dangerous if absorbed through our skin when in pure form, is a form of lead previously used extensively, and highly diluted, in gasoline. This compound has TWO major effects. First, in sufficiently high percentage, it can raise the octane value a fair amount, very important in WWII aircraft engines, especially those that were supercharged; some were both supercharged AND turbocharged. The original purpose for using tetraethyl lead in gasoline was specifically for raising octane. Some of those aircraft engines required 145 octane gasoline. Usage for the purpose of raising octane was carried forward for decades for cars, because it was cheaper, even when the lead was sold to the refineries expensively by the then Ethyl Corporation, than using only refining methods to increase octane. Many decades ago, premium (higher octane) gasoline's were simply called "Ethyl". Usually those gasoline's had MORE TEL (Tetra Ethyl Lead).

The 'seat' and 'valve' protection effects were something noticed later on, after WWII [i think]. Tetra ethyl lead leaves a microscopic layer of lead on the valve and seat; some folks call this cushioning. It was generally originally accepted that this effect, a 'lubricating', if you will, prevents microscopic welding of seat and valve, or at least reduced metal to metal microscopic contact, sort of how oil works.

Lead is, at very high temperatures, a LUBRICANT, AND A TERRIFIC CONDUCTOR OF HEAT. The conjecture was that a small amount of lead makes the seat and valve much less likely to weld at microscopic scattered points, and that the valve head heat is more readily transferred to the seat. Lead was thought to generally totally prevent the welding.

Advice on the precise fuel requirements of a particular vehicle should be obtained from its manufacturers, Where leaded fuel is recommended purely for its anti-knock or octane properties the following options exist:

a) Adjust (retard) ignition timing to permit use of standard unleaded (95-octane) fuel.

Use super-unleaded (98-octane) fuel.

c) Use lead replacement petrol (LRP, expected to be 97 octane. Further information on LRP later in this leaflet).

d) Modify engine to a lower compression ratio to permit use of standard unleaded (95-octane) fuel.

Where leaded fuel is recommended to protect against valve seat recession (VSR) the following options exist:

a) Fit hardened exhaust valve seat inserts. This is the approach used on modern engines designed to run on unleaded petrol and is totally effective. However it can be expensive and is not possible on all engines.

Use a commercially available lead-replacement additive in unleaded petrol.

c) Use lead replacement petrol (LRP).

What is the problem?

Lead additive in petrol prevented wear (recession) of the exhaust valve seat under the combined effects of heat (due to combustion) and valve closure (hammering on the seat). Lead acted by inhibiting impact welding of the valve on to the valve seat. Such welding, even on a microscopic scale, eventually leads to a significant loss of soft metal from the valve seat in cast iron heads, and allows the valve to sink further and further into the head.

The withdrawal of lead from fuel removes the protection your engine has enjoyed for all its life.

What exacerbates the problem?

a) Cast iron heads without hard seat inserts

Overheating

c) Sustained engine speeds above 3000 RPM

What can I do to reduce or prevent valve seat recession?

There are several options relating to choice of fuel and engine modifications. Your particular choice will depend on pattern of car usage and the state of your finances.

Fuel Options

Use plain unleaded fuel

Probably OK for low mileage drivers who take it easy, and whose cooling system is in good shape. Problems may not arise for some years.

Use LRP

Lead replacement petrol (LRP) contains alternative additives (potassium based) to combat valve seat wear. The efficacy of these additives is unquestionably inferior to lead, but they will offer limited protection (read the disclaimers!). They are widely available but moderately expensive. The view is that branded additives used with unleaded fuel will provide a higher level of protection, but this is unproven since LRP was not submitted for comparative testing.

Use Unleaded Fuel with Branded Additives

Do-it-yourself liquid additives are available. The Federation of British Historic Vehicle Clubs carried out engine tests in association with MIRA and, as a result, gave its seal of approval to several brands including Superblend 2000, Nitrox, Redline, Castrol Valvemaster, and Millers VSP all of which performed acceptably well. These additives have different chemistries and the advice is not to mix them; choose one and stick with it. Rumour has it that Superblend 2000 performed best under test, and has the advantage that its chemistry is compatible with LRP in case you run out.

Note that some of these additives are also available with an octane booster component.

Engine and Other Modifications

Install Fuel Catalyst Pellets

So-called fuel catalyst pellets (often tin based) can be installed in the fuel tank or in-line feeding the carbs. Rely on them at your peril. They’re inert and won’t do harm, but neither will they provide any protection.

Install Hardened Valve Seat Inserts

Specialist engine machine shops can machine out your cast iron valve seats and press fit hardened valve seat inserts which will resist wear and enable the use of unleaded fuel without additives. Only the exhaust valve seats need to be replaced. Cost is moderate and performance should be reliable if the job is done properly. If it isn’t and the inserts drop out, you’ll probably need a new head and an engine rebuild.

Install an Alloy Head

The ultimate modification. Aluminium alloy heads come ready fitted with special valves and seats. You can run unleaded fuel indefinitely, obtain better engine performance due to the improved head design and carry a lot less weight! However, they’re expensive