Posted by Bob Fetters on March 11, 1999 at 05:41:14:
In Reply to: Re: More avgas lead questions... posted by R.Davis on March 10, 1999 at 19:43:41:
True about the current method of testing. Actually the standard engine had adjustable compression and was adjusted to a "no knock" condition on Octane and that value assigned the number 100. But all that discussion begs the original question which was about the supposed problem of burning valves with avgas - was it because of the tetra ethyl lead leaving deposits on the valves or causing a different burn rate. Albeit poorly stated, I was pointing out the TAG cup flash point of avgas was lower than autogas. Due to the increased amounts of benzine and other hydrocarbons with less branching the burn rate is slightly slower. The more likley cause of any increase in heat to the valves is that the vapor pressure is lower, therefor there is less cooling due to vaporization of fuel. All of this has probably no noticeable impact on an engine designed prior to unleaded fuel. If I had 100 LL available on site, I would blend it with 85 (new way to figure octane) pump gas so I had some tetra ethyl lead to extend valve life yet avoid rapid lead build up incured at extended operations at low rpm rich fuel conditions. My sons drag car is hardly ever at idle. The engine operates up to 7200 rpm and gets fresh bearings and rings each year. There is no adverse impact from 100 LL on the valves or seats.