Unleaded Fuel in TC Engines
Cooling system (rewritten & partly due to formula error)
This article appeared in the Mirafiori forum June 16 and was in response to various ideas on the effects of unleaded gas in the dohc engine. The text is by Guy and is used with his permission
UNLEADED FUEL IN FIAT TC ENGINES
Early TCs (up to but not incl Integrale 8v and Delta HF Turbo 1600 ie version) and similar engines of that cost and generation all had cast iron valve inserts, certainly heat treated (induction hardened is my best guess) and certainly top quality alloy (probably silicon moly) hardness in the 400 Hv range, in other words hard but not that hard. Easily scored with a cheap file. There may have been some variance in the metal type among those vehicles but 400 Hv (380 HB) is the very hardest I have seen and moreover I have not seen any great difference in hardness betw inlet and ex ones. It takes quite a lot of work to get CI up to 400Hv, and no-one could fault Fiat on the longevity of their valve seat combination, certainly - in those days, as far as leaded fuel was concerned.
The 130TC, Beta, 131,132, etc are typical. No units of this type incl Vx had anything other than cast iron guides, these were only used on turbo models incl 16v and then only on the ex side, for obvious cost reasons. The Vx and the turbo units and very possibly the odd US n/a spec models (I have only seen one such head and it may well have originated from Spider VX) and sodium-cooled ex valves.
Later heads ie the reversed port started with the horrid nickel steel alloy (Delta HF Turbo 1600, ie version) and then went over to a soft alloy with copper in it on the Integrale, 8v and 16v, Tipo and others - the exact composition I am not sure of but it was certainly highly magnetic and thus probably a steel copper alloy, with maybe bits of nickel in it. These seat types feel much softer to cut than the cast iron ones above, I estimate about 250 Hv or less, have not taken any out to test, will do one day if it ever becomes a pressing issue.
Ex valves on all TCs 8v and 18v went over to stellite coated contact faces with the introduction of the old132 2 liter. This was long before Euro-wide adoption of unleaded fuel was even envisaged - although the necessary technology was well understood in the USA. This was just done for longevity, and even used in Fiat's advertising. It does give protection against damage in the shorter term - on unleaded fuel - but it eventually wears away, speaking as one who has of course refaced enough valves to know.
The design and conversion issues regarding use of unleaded fuel are as follows:
1. Lead or equivalents in petrol acts a lubricant and also a separation medium; the reduced friction betw valve and seat encouraging partial rotation (shimmying)or full rotation of the ex valve (depending on the collet design) and contact face cleaning, thus minimum of build-up of corrosive combustion deposits and good heat transfer. It also tends to reduce pairing and welding under infl of heat and pressure, such as occurs normally betw like matls under those conditions.
2. Removal of the lead tends to cause fouling and carbon impact damage, impaired heat transfer - leading to chronic overheating and consequent softening of valve and seat, sometimes with attendant cracking. These phenomena are aggravated by welding and pairing, corrosion and oxidation. The ultimate end result of running with an unsuitable valve-seat combination is valve recession, a very extreme condition where the valve becomes so distorted it is actually pulled into the throat.
3. The principal factor insofar as leaded/unleaded is valve head temperature; even in a sodium-cooled valve typically the contact face still passes 70% or more of the heat transfer from combustion process, only the balance going down the stem. The amount removed to the coolant by the stem varies according to the thermal properties of the guide, cast iron is a terrible medium for heat transfer, high copper alloys are about as good as you can get. Suffice to say, the better the guide, and valve-guide fit, the less the thermal load on the seat.
4. Steel alloys (containing nickel and chrome esp.) can be designed to have superior hardness, tensile strength, creep and stress-relaxation properties to cast irons, and are thus an obvious and suitable low-cost choice for exhaust valve seats. Whilst they have very poor heat transfer properties, they are less prone to pairing and welding, distortion under press/temp in the lead free situation. But, they offer no great advantage insofar as the longevity of the ex valve itself is concerned. They are best used with bronze/copper alloy guides on high- performance engines if unleaded is being used, though manufacturers tend to reserve these for turbocharged applications. The higher hardness of steel based inserts aids frictional properties.
5. Selected copper-based alloys, eg silicon/nickel/aluminium bronze, copper beryllium, in varying states, have outstanding thermal conductivity and thermal diffusivity, very high corrosion resistance, and do not exhibit any tendency at all to pair/weld with any valve matls.