info on need for lead , danger of avgas etc


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Posted by George in NW Michigan on Saturday, October 22, 2005 at 6:41PM :

In Reply to: Is lead additive necessary? posted by Kevin in Rehoboth [24.34.38.98] on Saturday, October 22, 2005 at 8:19AM :

Our flathead engines don't need leaded gas because of their low compression, rather they don't need it because they were always built with hardened valve seats. So no, you do not need leaded gas. If anything, it is harmful.

Note that aviation gas has high levels of lead. This can cause real problems in our engines. Not because the octane level is high but rather because the lead oxide deposits left behind can build up on the value seats and exhaust gases leaking under the value can cause them to burn.

Also, when the lead burns, it leaves behind lead oxide. This when in contact with water (which is a product of combustion in the engine) forms acids. This causes things like exhaust systems to rot out from the inside.

Note that high octane gas, no matter how made, does not mean it burns hotter, or with more energy or gives more power etc. It only means it has a greater resistance to knocking. Since spark knock is not a problem for us, avgas is useless at best, and a problem at worst if the deposits crap up your engine, eat up the exhaust system, polute the oil etc.

Some asked about how lead raised the octane level. First, the "lead" we talk about is actually a compound called tetraethyl lead. This and many other compounds, even ethanol, some ethers etc, will raise the octane level of gas. The octane level can also be increased by certain refining methods. Since it was cheaper to add lead than do the more complicated refining, that is what used to happen.



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