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| Valve Seats & unleaded fuel............ https://slantsix.org/forum/viewtopic.php?t=13416 |
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| Author: | emsvitil [ Tue Jun 21, 2005 2:44 am ] |
| Post subject: | Valve Seats & unleaded fuel............ |
Found this while searching around........... from http://www.sbintl.com/techart2.htm (second article down) About valve seats...... *************** The growth in the valve seat insert market can be traced back to the early 1970s when the switch to unleaded fuel took place. Most of the engines in use or that were in core or inventory storage had to have replacement seats inserted in the exhaust side to prevent valve seat recession that occurred when the engines were run on unleaded gas. Many people think that lead was a lubricant and somehow prevented wear. In fact, the lead caused a chemical reaction with the cast iron of the cylinder head and the stainless steel valve, forming oxides and halides that locally hardened the wear surfaces. This local hardening is what actually helped to prevent seat recession. During the changeover period, it was not uncommon for a vehicle that had been initially run on leaded fuel to be switched to unleaded. The initial use of leaded fuel had created the local hardening required and the switch to unleaded created no problems. However, if these same heads were then reconditioned, the machine shop would machine away the protective layers and seat recession would occur very rapidly, sometimes in as little as 3,000 miles. The OEMs used an induction hardening technique to locally harden the valve seat areas. This process was supposed to produce a hardness depth of around .070", but in many cases it was found to not be deep enough to allow for re-machining during head rebuilding. These early unleaded fuel heads also needed to have exhaust seat inserts fitted to them when they were rebuilt. **************** Which seems to indicate that unless you rebuild your head, you wont have any problems with valve seat recession, or need any lead substitute...... What's interesting is that I've never heard this mentioned before.... |
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| Author: | SlantSixDan [ Tue Jun 21, 2005 6:42 am ] |
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I'm calling "bullcrap". I'm not a chemist, but this does not match anything I've ever understood about how Lead worked. The article's correct that it was never a "lubricant", but a local hardening catalyst? Huh-uh, don't think so. Here is a less imaginative and, I believe, much more realistic explanation of the phenomena at work: 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 such "pulled" metal on it creates a nice grinding wheel effect on the seat. 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 on the poor valve and depriving it of any prayer of a chance to cool while it's on the seat. The blowtorch effect rapidly 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 mainly on how the car is driven and used (Towing, drag racing or pedal-on-the-floor hauling will heat up the valves--driving down the highway at a constant 70 won't, and neither will hopping from traffic light to traffic light in the city or running down to the local grocery for a carton of ice cream.) Other factors in the margin of safety include the size of the exhaust valve, its material, and the efficiency of valve seat cooling in that particular engine design. 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 you really have to abuse the engine before things heat up to the danger point. Very little Lead is required to prevent the localised welding and "taffy pull apart" effect that leads to the abrasive surface which, through incidental or positive rotation of the valve, eventually grinds-down an unhardened seat. 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 (which introduction was brought about by legislation) , there was generally only one grade of unleaded available, and the octane was *quite* low--less than that of leaded regular. We all know that when you use a fuel of insufficient octane, your engine pings (detonation, pinking, pinging, spark knock--call it what you will.) This creates tremendous heat in the combustion chamber--certainly enough heat 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 (see above). And so the myth was born that old cars' engines "WILL DIE" if run on unleaded. These days we have wider availability of high-octane unleaded fuels, which obviate the insufficient-octane cause of valve heating and subsequent localised welding. if you have an old car that is a low-stress application , used in daily-driver service, then you need have no qualms about using whichever octane grade of lead-free fuel your car runs well on and drive it for a Loooooooonnnnnng time with nary a valve or seat problem. Many US-based 6 and 8 cylinder engines fall into this category in normal daily driving service. (On the other hand, many don't. Don't assume. The amount of valve cooling built into the cylinder head design is the primary factor, and I've seen or heard of plenty of pre-'73 Chrysler V8s quickly developing exhaust valve problems on unleaded). The way to eliminate even the possibility of valve heating causing localised welding and subsequent seat recession is to install hardened 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. I do agree with the quoted article that the factory's induction hardening is frequently not sufficiently deep to last through more than one regrind. Of course, 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, then address the problem. "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. It also causes hard red deposits on your spark plugs that will cause you to need to replace them more often. Other additives are simply octane boosters of varying effectiveness and varying side effects. |
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| Author: | Dennis Weaver [ Tue Jun 21, 2005 7:39 am ] |
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Quote: ...localised welding...
Written like a true, dyed in the wool Canadiun wannabe!   ...seriously, enjoyed the article, sounds plausible to me, and forgive me for not even being an armchair metalurgist, but would the hardness level of the metal really make it any more or less resistant to "localized welding"? D/W |
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| Author: | SlantSixDan [ Tue Jun 21, 2005 9:40 am ] |
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Quote: would the hardness level of the metal really make it any more or less resistant to "localized welding"?
Yes.
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| Author: | Ice Titan [ Wed Jun 22, 2005 3:51 am ] |
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When metal is formed, the method of cooling will largely determine the amorphous or crystaline structure of the molecules. The more compact and ordered material will require greater ammounts of heat to change shape due to the chemically stable state of the molecules. Something about this seems a bit off. Anybody know what it is? |
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| Author: | Doctor Dodge [ Wed Jun 22, 2005 10:21 pm ] |
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Quote: Very little Lead is required to prevent the localised welding and...
Every time I go to the track I alway buy a couple of gallons of leaded 112 and mix it into the tank. It's nice that I get a little lead onto the valves but what's fun is that smell of race gas exhaust that you get from the car for a while afterwards.   The not so fun part is that the 112 costs $4.50 a gallon.  DD |
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| Author: | slantvaliant [ Thu Jun 23, 2005 6:32 am ] |
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Amorphous metals ("metallic glass") are a relatively new - and expensive - technology. The solid metal our old cars are made of is crystalline. |
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