Van's Air Force
The definitive Van's Aircraft support community! Buying, building or flying an RV? Join our exclusive family of mentors and enthusiasts!
Science behind 100LL during break in
- Thread starter mechmike
- Start date
Back in the day, they used relatively weak and unhardened alloys for the valve face and seat. They got away with this because the lead in the fuel, which is a lubricant, coated the parts and protected them. So, for break in, you needed a tank or two of leaded fuel to coat the parts and then needed to ocassionally use leaded to re-coat. The auto industry quickly moved to better alloys or hardening to survive without the lead, but am sure aviation did not follow suit for a long time. Don't know when lycoming changed things and presume they now use modern materials, but can't say that with any confidence beyond their current approval of unleaded fuels.
So, fopr a modern engine, there should be no need for leaded fuel ever. The lead propvides no other benefits beyond above along with its octane enhancing benefit.
Last edited:
I did my break-in with unleaded - UL91. If I were you, I'd talk to your engine builder to see what they recommend. If you follow their recommendations and there is a problem, even if unrelated, the conversation with them will be easier.
aviation.totalenergies.com
AVGAS UL91 aviation fuel – TotalEnergies Aviation
Learn more about this unleaded aviation gasoline designed for microlight aircraft, its characteristics and how to identify it
johnpaul44
Well Known Member
Having attended Lycoming's engine build school I was told that the main reason is the extra octane of 100LL can help during engine breakin incase timing is set wrong or the cylinder overheats. You have more detonation margin vs 91UL especially if cylinder temps go crazy.
