Martin S. Posted January 15 Report Posted January 15 (edited) Evening gents, Is there anything that argues against operating lean of peak during engine break-in (first 25h after overhaul), provided the engine is run at 75% power or higher and the CHTs remain reasonable - say, below 380 °F? Many thanks for your thoughts, Martin Edited January 15 by Martin S.
KSMooniac Posted January 15 Report Posted January 15 Advanced Pilot Seminar and GAMI guidance says that is the optimal way to do it. The goal during break-in is to run the highest intra-cylinder pressures you can AND keep the CHTs reasonably cool... running sufficiently LOP does both without extremely high peak pressures that occur while ROP. The wildcard is if your engine will run LOP well on the initial flights. My IO-360 did not as I had an induction leak from an O-ring in the sump getting snagged on assembly, and stock injectors that needed to be swapped to balance the fuel flows. After fixing those, it ran great LOP and I continued the break-in. (10 years ago now) Coincidentally I'm about to fly some break-in hours after getting a couple of exhaust valve guides replaced. I know my injectors were optimized and I hope my induction system is tight now after reassembly. I plan to do a big mixture pull (BMP) to ~9 GPH at full throttle and 2500' to see if it will run smoothly LOP, and if so, then enrichen a bit to get to 10.7 GPH and run that for break-in. If I have an induction leak that prevents LOP running, then I'll continue ROP for the rest of the first flight and monitor the offending cylinder(s) to make sure they don't get too lean with a leak. Lycoming of course will not say this is good, but the data from a real test stand says otherwise. 1
Fly Boomer Posted January 15 Report Posted January 15 4 minutes ago, KSMooniac said: I plan to do a big mixture pull (BMP) to ~9 GPH at full throttle and 2500' to see if it will run smoothly LOP, and if so, then enrichen a bit to get to 10.7 GPH and run that for break-in. Is your everyday technique for LOP to do the BMP at WOT, or is that only for break-in?
Jackk Posted January 15 Report Posted January 15 Don’t think lyc says anything, but pretty sure continental says ROP, I’d probably at least break the thing is ROP, it’s not going to cost you that much more in gas. https://www.airmarkoverhaul.com/wp-content/uploads/2019/08/EngineBreakIn-3.pdf
EricJ Posted January 15 Report Posted January 15 I just did "break-in" training for an organization I fly with, and my motor is currently at overhaul, so I've been reading their break-in procedures as well as Lycoming's. They all just talk about maintaining 75% or 65% power and keeping CHTs from getting too high. They all also say operate per the POH, and only my overhauler's procedure says anything specific about mixture, and it just says operate per the POH.
Hank Posted January 19 Report Posted January 19 On 1/15/2026 at 3:46 PM, EricJ said: ... my overhauler's procedure ... Whatever that document says is all that matters.
KSMooniac Posted January 19 Report Posted January 19 On 1/15/2026 at 2:58 PM, Fly Boomer said: Is your everyday technique for LOP to do the BMP at WOT, or is that only for break-in? If I'm down low/high-power, that is what I do generally. If I'm up above 7k or higher, sometimes I do BMP and other times I lean slowly with the JPI lean find active since I can't hurt anything at 65% or below with the mixture. However, since I added dual Electroair, the timing advance at altitude will drive the CHT up during a slow lean, so I've been doing the BMP more and more and then approach peak from the lean side now and then to make sure I'm suitably far LOP. This is standard guidance from APS/GAMI for turbo or TN leaning where they will cruise at 87% power but well LOP. I flew this weekend after replacing valve guides and rings on two jugs, so I had to do break-in flights at low altitude/high-power. Fortunately I did not have any induction leaks and flew most of it LOP at ~80% power with cool CHTs. First flight on Saturday had OAT <20°F so that was perfect...lots of engine power from dense air and cold air to keep the temps down.
Fly Boomer Posted January 19 Report Posted January 19 14 minutes ago, KSMooniac said: This is standard guidance from APS/GAMI for turbo or TN leaning where they will cruise at 87% power but well LOP. Thanks. BTW I just watched the Mike Busch/George Braley interview on YouTube. Pretty fun listening to George talk about being courted by Cirrus. The Columbia 400 was eating their lunch because they were turbocharged, and Cirrus was not. George flew up to Duluth to show them his prototype. Test pilot jumps in, and George takes his Cirrus up to 30,000 feet with no cooling problems. Later that day a roomful of Cirrus engineers explained to George why that was impossible, because they hadn't had any success. 1
KSMooniac Posted January 19 Report Posted January 19 I wish Mooney would've contracted with GAMI for a FWF-package on the Acclaim instead of the fake turbo-normalized setup they advertised. It likely would not have saved the production line, but it would have been one heckuva final evolution of the M20 family! Imagine running an Acclaim with 270 hp continuous at cruise on 17 GPH!
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