Originally Posted by oldbogie
A low compression, air cooled, valve sticking airplane motor and your surprised by a burnt valve?
Never said I was surprized by that, happens all the time.
Lead in fuel was found to prevent seat erosion. But, it also contributes to sticking valves by both gumming up the lower part of the guide and stem as well as by contaminating the oil which gets into the upper guide. Add that to typical hotter valve temps of an air-cooled engine vis-a-vis a liquid cooled with a little tight guide clearance and you've got a recipe for a sticking valve. Then add to that an airplane motor, like a boat motor, works a lot harder than the typical car motor. Take off power, everything its got till you’re a up a ways, climb power, almost everything its got till you get to altitude which might be a while, cruise power what 60-70 percent of everything's its got till you throttle back for descent. If you operated the typical car engine that way it would be junk and a couple months.
We understand all that. But given the higher temps, one would think the lead would be scavenged better.
Is this low lead av gas or something else? And I guess with such a low compression motor why lead at all?
Running 100LL. Which contains more lead than old leaded premium car gas.
The only reason for running it in the 8.5:1 engines is that the STC to allow car gas, specifies that it must contain exactly 0% alcohol, and must be tested for it (simple to do), But that's not readilly available just anywhere any more.
Apparently lead is the only approved antiknock compound approved by th' manufacturer.
What prompted this thread, was the fact that a friend carried a set of 289 ford heads in to be worked. And the guy at the shop told him that If he didn't install hardened seats, or run lead additive in the fuel, he could not warrenty against burned valves within 10k-12k miles.
Airplane engines running mid grade unleaded, (alcohol free) have none of these problems, like sticky valves, fouled plugs, grey goo in the oil, sticky grey stuff on the underneath, etc.
Most all of them have hardened seats of one kind or other.
With a bore and stroke of 5.125"(bore) and 4.375" (stroke) and a ratio of 8.5:1 the actual compression using an automotive typ compression tester, is pretty high, (plugs pulled, turning with starter, 3 rounds) about 145-150.
I would expect it to be higher when idling at ~600 rpm.
Anyway, why would someone reccomend using lead additive, to prevent burned valves, when lead deposits, cause the sticky/burned valve trouble.