Again: Unleaded or leaded fuel for mid 70's /6?
I have a 1974 225 slant six under the hood. I've found out that hardened valve seats were inducted in 1972.
1972 for V8 engines;
1973 for Slant-6 engines (link is to Chrysler publication on the subject).
Does that mean all engines from 1972 and up can be operated with unleaded fuel? And how is it with 95 octane fuel instead of 87 octane? 95 fuel is lowest and standard here in central europe. Do I need any additives or am I fine running 95 octane without any additives?
In the US and Canada since about 1975, the published octane ratings are AKI, antiknock index, which is the average of the numbers resulting from two different test procedures. One is called the RON (research octane number); the other is MON (motor octane number). The RON is higher than the MON for any given gasoline.
In most of the rest of the world, published octane ratings are RON. So any given gasoline will have a higher octane number outside than inside upper North America. The RON is usually about four to five higher than the AKI, so the gasolines typically available at sea level in North America (87 regular; 89-90 mid-test, and 92-93 high-test) equate to approximately 91 regular; 94-95 mid-test, and 96-98 high-test elsewhere in the world.
With an unhardened exhaust valve and seat, the valve and seat can micro-weld to each other if they get hot enough. Lead halides act as a buffer (a physical cushion/barrier) to prevent this happening. The important thing is that exhaust valve and seat recession only takes place when the valve gets hot enough to undergo localised welding. Then, when the valve opens next, the metal pulls apart like taffy. This roughens the meeting surfaces, and they become quite abrasive. The pounding/turning of a valve with this kind of "pulled" metal on it creates a nice grinding wheel effect on the seat, which is the mechanism behind valve seat recession. In addition, the roughened surfaces no longer seal against each other properly, which eventually allows still-burning combustion gases to flow through the closed valve, causing a blowtorch effect and depriving the valve of its chance to cool while it's on the seat. The blowtorch effect deteriorates the seal further, snowballing the seat recession.
The main thing to remember is that this bad stuff cannot happen If the valve never reaches the crucial temperature. Whether the valve reaches the crucial temperature depends on how the engine is configured (how big is the exhaust valve, how much head metal surrounds it for heat transfer when it's closed, how well is that metal's temperature managed by coolant flow) and how the car is driven and used—towing, drag racing or pedal-on-the-floor hauling will heat up the valves much hotter than driving around town or on the motorway at more or less constant normal speeds.
The Slant-6 has ample exhaust valve seat cooling and stout valve material, and the valve itself is small enough relative to the combustion chamber area that even with a pre-'73 head, without the hardened seats, you'd really have to abuse the engine before things heat up to the danger point.
Very little lead was required to prevent the localised welding effect that leads to seat recession. The majority of the lead was in the fuel as an octane booster, that's all. It was widely used because it was a very cheap and very effective octane booster.
When unleaded fuels were first widely introduced, there was generally only one grade of unleaded available, and the octane was sometimes less than that of leaded regular. We all know that when you use a fuel of insufficient octane, your engine pings. This creates tremendous heat in the combustion chamber, enough to push the exhaust valves to the crucial
temperature.
Because for quite a while only unleaded fuel of subregular octane was available, plenty of people experienced these effects from using unleaded. While many of those engines that suffered under this low-octane unleaded really did need the lead (high load and/or high-RPM engines), the bulk of the failures were due to the low octane increasing combustion chamber temperatures. And so the myth was born that old cars' engines will inevitably die if run on unleaded.
These days we have wide availability of high-octane unleaded fuels, so the insufficient-octane cause of valve heating and subsequent localised welding is no longer a thing.
if you have an old car that is a low-stress application , used in daily-driver service, then just use whichever octane grade of unleaded fuel your car runs well on and drive it for a loooooooonnnnnng time with no valve or seat problem.
The factory's induction hardening was just fine for the original service life of the engine, plus a valve grind/head rebuild or two, but the hardness is about 0.040" (1 mm) deep, so eventually it gets ground through.
The way to eliminate even the possibility of valve heating causing localised welding and subsequent seat recession is to install hard exhaust valve seat inserts and exhaust valves of upgraded material. This is utterly standard practice in the rebuilding of cylinder heads, and has been for years. It is foolhardy to rebuild a "leaded" cylinder head without these upgrades, which are not expensive.
But there's no reason to tear into the engine solely to install hard seats. There is no collateral damage from seat recession. Drive and enjoy until a problem develops (if it ever does), then address the problem.
You do not need additives, and won't benefit from them. "lead substitutes" are largely useless. Some of them use a sodium salt and claim to duplicate the buffer effect of lead, but can't substantiate the claim. Some of them use MMT (methylcyclopentadienyl manganese tricarbonyl), which is highly toxic, is of highly questionable benefit in buffering exhaust valves, and tends to foul spark plugs. Other additives are simply octane boosters of varying effectiveness
and varying side effects.
