It's worth restating that all this hinges greatly on how hard the engine gets used. Even a plain cast iron valve seat might last perfectly well if you never use the car hard. A supposedly "unleaded suitable" insert might wear out very quickly in race use.
So in summary there is really no such thing as "suitable for unleaded". There are just a range of materials which will have longer or shorter service lifetimes. Only with plain cast iron can one really say that it is unsuitable for unleaded in anything other than very gentle usage. Copyright David Baker and Puma Race Engines
Can I Use Unleaded In My Engine?
All engines made after 1989 can run unleaded with no reduction in service life. For pre 1989 engines we can summarize all of the above into two rules - one for cast iron heads and one for aluminium.
Cast Iron - A cast iron head without exhaust inserts is not suitable for unleaded except for very gentle use.
Aluminium - Any modern aluminium head (let's say post 1975) will have inserts that are able to run on unleaded petrol, even when the OE manufacturer says it is unsuitable. What they mean by "unsuitable" is that they are unwilling to guarantee the same 200,000 mile or so service life that they would expect from a post 1989 engine. They do not mean that the engine will suddenly and immediately fail just because you run it on unleaded petrol. In practice it will have no impact on the average car owner at all given that very few people keep their cars that long. If you have an aluminium head from a major manufacturer (Ford, VW, Peugeot, Vauxhall, Fiat, BMW, Mercedes etc) then stop worrying and just run it on unleaded. The worst that can happen is that eventually, in 100,000 miles or so, the valve seats will have pitted enough to need recutting but by then the chances are that the rest of the engine will be worn out too. There is no way the inserts will just burn out though in the same way as can happen to plain cast iron.
Valves
All valves have to be made from very tough steels anyway just to resist the temperatures and stresses inside the engine. The same steels are used whether the engine is designed to run unleaded or leaded fuel so in effect it is not an issue. Nearly all modern petrol engines use the same types of steel for valves with small variations in the alloying mix. Inlet valves are generally made from a single forging of EN52B or similar. Exhaust valves have to cope with much higher temperatures on the valve head so they need a higher grade material there. Exhaust valves use EN52B for the stem part of the valve which is then friction welded to the much tougher 21/4N steel for the valve head. Why use two types of steel in one valve? Simply because 21/4N is more expensive and as it is only needed for the head of the exhaust valve there is a small cost saving in using EN52B for the stem. The saving in material cost outweighs the cost of the friction welding operation. It might be minor but when you are making millions of valves it all adds up. Copyright David Baker and Puma Race Engines
The simplest test the home mechanic can use to tell which type of steel a valve is made from is with a magnet. EN52B is magnetic and 21/4N is not. If you take a magnet and an exhaust valve it is quite easy to find where the friction welded join between the two types of steel lies as the magnet will grip the stem until the point at which the material changes to 21/4N. The only important issue for use with unleaded is that the head of the exhaust valve be non magnetic 21/4N and as it is certain to be that anyway on a modern engine you don't need to change the valves for use with unleaded. If in doubt just apply the magnet test. If the valve is all non magnetic then it is one piece 21/4N but that is of no extra benefit really in a road engine. The valve stem doesn't get hot enough for it to matter which of the two types of steel it is made from.
Valve Guides
Valve guide material is not an issue with regards to unleaded petrol. You do not need bronze guides with an unleaded conversion. In fact many post 1989 engines still use cast iron guides by choice.
What Do I need Then With An Unleaded Conversion?
With a cast iron head you just need steel seat inserts fitted on the exhaust valve side.
With an aluminium head you don't need an unleaded conversion in the first place. The chances are that even if you take an aluminium head for a "conversion", absolutely nothing will get done to it except maybe the seats will get recut. If the seats finally burned out several years and many thousands of miles later, do you really think you'd have any chance of proving that the inserts didn't get changed for better ones? To do that you'd have to have the inserts machined out again, sent to a lab for chemical analysis and compared with the same analysis for known OE inserts. You'd be way past any possible guarantee period even if you bothered doing all that. So hands up anyone who thinks they can now tell me just what the incentive is for those companies that advertise unleaded conversions on aluminium heads that never needed a conversion anyway to actually waste time and money doing anything constructive with your head when they can charge you the same for doing nothing. Copyright David Baker and Puma Race Engines
Ignition Timing
The first issue with unleaded is the one of physical wear of the valve and seat. The second one is whether the engine will detonate - otherwise referred to as "pinking", "pinging" or "knock" depending on where you come from. Unleaded fuel usually has a lower octane rating than leaded and also burns differently. It might be necessary to use a bit less ignition advance with it but on many cars the standard setting works fine. There's a simple rule - if the engine pinks, usually heard at low rpm and high throttle openings, then retard the ignition timing a couple of degrees. If not then leave it alone. Pinking sounds like a light metallic rattle, a bit like a nail being shaken about in a tin can. In small doses it does little harm but in severe cases it can damage pistons and cylinder heads.
Thats it. May be the end of discussion or a whole new discussion!