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Triumph Valve Seat Recession and Unleaded Fuel

by B. John Mitchell, 15 March 1996

The problem of valve seats in internal combustion gasoline engines became apparent with the gradual phasing out of leaded fuel in the late 60s and early seventies. Lead in the fuel accomplished more than increasing the knock value of a fuel. When the auto companies began testing "virgin" engines with unleaded fuel, valve seat failure was encountered within 10 to 20 hours during a wide open throttle 4000 rpm durability test. Lead in the fuel provided exhaust valve seat lubrication and excellent heat transfer from the hot exhaust valve to the seat in the head than to the coolant media.

Several methods were tried to fix the problem, including seat inserts, valve rotators, and cooled valves, either sodium filled or a "heat pipe" design. I ran all types of designs including the "heat pipe" and also with a jet of coolant (water) sprayed directly on the valve. It became apparent that keeping the valve cool was a good solution but costly. The hardened valve seats became a cost effective compromise and was adapted by most if not all of the manufacturers.

With the effect of unleaded fuel under control, the next step was to incorporate a catalytic converter to reduce the level of CO and HC. A catalyst reacts very quickly to an increase in unburned fuel causing a catastrophic catalyst failure within microseconds. Thus it was apparent that the then widely used breaker point ignition systems in use would allow the engine to operate, although poorly, and cause a catalytic catalyst failure. This began the expedited development of a breakers or electronic ignition distributor, referred to as a "walk home" design since if the distributor began to fail, the engine would stop preventing any costly catalyst damage. With the advent of relatively inexpensive on board computers, electronic fuel injection was introduced as well as non rotating ignition systems all receiving inputs from sensors located in strategic necessary location sensing temperatures, fuel mixtures and exhaust makeup.

First, there never was a "requirement" for valve seat treatment, it was up to the particular automobile manufacturer. Further, I mentioned valve seat failure (valve seat recession) on a "virgin" engine. If the same test was applied to an engine that had been run with leaded fuel, the valve seats did not fail. There is a sufficient quantity of residual lead in the system affording protection and very few engines are ever subjected to the durability type test that produces a valve failure. In the case of your Triumph, you should encounter no problem using unleaded fuel. In the test that I mentioned, valve seat recession measured 0.000150 to 0.000180 of an inch in 40 hours of wide open throttle, 4000 rpm operation. Incidentally, valve seat inserts are not a good solution because of the expansion differences and the tendency for the seats to loosen - staking the seats only aggravated the situation in the long run. Hardening the seats was a good compromise (the depth of hardening is only .030 or so). Enough said!

B. John Mitchell, 15 March 1996

(Mr. Mitchell retired from GM in 1975 after 37 years. He was involved with engine development during his tenure. One of his more well known engines was the Buick aluminum V-8 designed in the early 1960s and still in production today as the Rover 3.5 family. He was also involved in the development of the current Buick 3.8L engine.)



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