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 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. Apart from very early Fiat 124 Sport 124AC and 125BC heads which I am really not sure about, no units of this type incl the supercharged Lancia Volumex had anything other than cast iron guides, they were only used on turbo models incl 16v and then only on the ex side, perhaps for 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 ex seats went over to a softer alloy on the Integrale, 8v and 16v, Tipo, Coupe and others - the exact composition I am not sure of but it was certainly magnetic and thus probably a ferrous, nickel copper alloy. These seat types I estimate about 250 Hv or less and have fantastic heat transfer properties. But as for using very hard nickel/chrome/steel alloy ex seats, I am not a fan of it, Peugeot do it but I have seen them with terrible valve seat/valve distortion on unleaded fuel. Ex valves on all TCs 8v and 18v went over to stellite coated contact faces with the introduction of the 132 series in 1973. 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 as I understand them, 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 of the ex valve and seat 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 condts.
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 materials. They also give a softer landing for the valve on closure, and have a high degree of embedability for exhaust carbon particles that can otherwise cause extreme impact damage over the long term. The contact stress betw valve and seat is quite low in most applications, certainly a fraction of the proof stress of, for example, the super-high strength si/cu/ni insert material 'Trojan' from Columbia metals, 780 MPa and 200 HB.
Since there is a far lower tendency anyway for the seat to overheat, hardness per se is not a principal issue with non-ferrous ex seat inserts in unleaded applications. Alloys in this range have typically lower frictional properties than ferrous ones, usually around 0.45-0.5, a factor which encourages valve rotation and cleaning.
1 post • Page 1 of 1
- Site Admin
- Posts: 5031
- Joined: June 18th, 2006, 9:31 am
- Location: Lincoln, UK
- These ex valves catastrophically burned by a combustion event but the seats (and GC guides) were barely affected by the flame that passed through (literally right through) the valves. The head is Fiat 16v with the later seats mentioned above.
- burned ex valves cyl 3.jpg (112.34 KiB) Viewed 10015 times
Users browsing this forum: No registered users and 4 guests