8.9 How serious is valve seat recession on older vehicles?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
8.9 How serious is valve seat recession on older vehicles?
The amount of exhaust valve seat recession is very dependent on the
load on the engine. There have been several major studies on valve
seat recession, and they conclude that most damage occurs under
high-speed, high-power conditions. Engine load is not a primary factor
in valve seat wear for moderate operating conditions, and low to
medium speed engines under moderate loads do not suffer rapid
recession, as has been demonstrated on fuels such as CNG and
LPG. Under severe conditions, damage occurs rapidly, however there are
significant cylinder-to-cylinder variations on the same engine. A 1970
engine operated at 70 mph conditions exhibited an average 1.5mm of
seat recession in 12,000km. The difference between cylinders has been
attributed to different rates of valve rotation, and experiments have
confirmed that more rotation does increase the recession rate [29].
The mechanism of valve seat wear is a mixture of two major
mechanisms. Iron oxide from the combustion chamber surfaces adheres to
the valve face and becomes embedded. These hard particles then allow
the valve act as a grinding wheel and cut into the valve seat
[115]. The significance of valve seat recession is that should it
occur to the extent that the valve does not seat, serious engine
damage can result from the localised hot spot.
There are a range of additives, usually based on potassium, sodium or
phosphorus that can be added to the gasoline to combat valve seat recession.
As phosphorus has adverse effects on exhaust catalysts, it is seldom used.
The best long term solution is to induction harden the seats or install
inserts, usually when the head is removed for other work, however additives
are routinely and successfully used during transition periods.
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