Posted by Lmack on September 30, 2012 at 17:37:22 from (188.8.131.52):
In Reply to: Lead Replacement posted by WI_JJ on September 30, 2012 at 15:36:55:
Tetra-Methyl-Lead was added to gasoline as an antiknock compound and is a flame retardant. It tends to prevent preignition spark knock in high compression engines. Gasoline with no additives to prevent spark knock will ignite in a hot cylinder via compression alone hence the knock since the ignition often occurs too early in the compression cycle. Lead is still used in older piston driven aircraft (and NASCAR) with the approval of the government because these engines have very high compression and other additives can't prevent spark knock. Tractor gasoline engines never came close to having the compression ratios needing high octane gasoline. Older automotive engines do not do well on unleaded gasoline until rebuilt with a different cam to change the valve timing and the compression ratio had to be reduced. Since tractor engines were made to run on almost anything knocking is not a problem. If you needed more antiknock compound in your gasoline for your tractor you would know by the sound of the engine running. Otherwise the lead does absolutely nothing to help your tractor engine. Direct timed fuel injection also will eliminate the need for antiknock compounds so that modern gasoline has fewer antiknock additives. It would be intuitive to assume that gasoline without a flame retardant would perform better than otherwise since it would burn more completely.
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