I fixed the thread title to IO390.
The Pipistrel Panthera started with the IO390 too but Lyco told them they will never certify it for 91UL.
Silvaire wrote:
A boxer engine has fully balanced primary and secondary vibrations (i.e the opposing the pistons balance each other well) so I think the main source of vibration in a four cylinder boxer is torque ripple, which is higher with larger and fewer cylinders.
Yes, but if you think the Lyc 360 is a shaker compared with the TCM 360, then the 390 will shake 20% more. It’s a simple fact. Not that a 360 shakes all that much. Maybe TCM is phasing out all their smaller engines and focus on Titan and diesels ?
LeSving wrote:
if you think the Lyc 360 is a shaker compared with the TCM 360, then the 390 will shake 20% more. It’s a simple fact.
It’s not always so simple. Unlike vibration sourced from residual mass imbalance, at the same rpm and power, torque ripple for an O-360 and O-390 is the same regardless of engine displacement. Vibration is thereby somewhat ‘manageable’ with the bigger displacement engine, and depends on the plane and service. An example would be a friend’s O-360 powered plane being so clean aerodynamicaly that it’s much smoother in cruise at the same engine rpm and airspeed than my O-320 powered plane. He’s running much lower manifold pressure, and so has less torque ripple if we fly together.
Of course i know that the IO-390 is not the “360”. But it is based on the 360 and its reliability should be similar. AFAIK it’s a 360 that uses the 580’s cylinders.
Silvaire wrote:
It’s not always so simple
No, but 20% more power must mean 20% more vibration (larger amplitude). At least 20% more given all other things equal, which is more or less what the 390 is vs the 360. The only thing that has changed is a larger bore (seemingly the only thing).
20% is not much vibration vise, but it is more.
Vibration doesn’t work like that.
mh wrote:
Vibration doesn’t work like that
In a balanced flat four, the vibrations are mainly from torque pulses. 20% larger pulse means 20% larger vibrational amplitude (everything else being equal).
Larger prop, heavier prob, heavier shaft etc, will dampen this a bit, but then everything else is not equal 
LeSving wrote:
20% larger pulse means 20% larger vibrational amplitude (everything else being equal).
Nope. But read for yourself: https://www.amazon.de/Vibration-Fundamentals-Practice-published-September/dp/B017R3183U
It’s the “same” engine, mh. Nothing has changed except more power. More power can only come from more force acting on the piston. For the 390 this is due to larger bore. Force = pressure * area.
Take a look here:
http://www.epi-eng.com/piston_engine_technology/torsional_excitation_from_piston_engines.htm
LeSving you’re being so stubborn it defies belief. It seems that you try to “win” any discussion by being the “last man standing” by just re-iterating your nonsense over and over and over and…
Real pilots having flown behind real IO-390s have reported that it runs quite smooth. If I may quote one from another forum:
I’ve flown behind a factory new Lyc. 390XP in an RV 7 that I helped build with a friend. The 390XP is the smoothest air cooled recip. engine I’ve ever experienced, more so than the Cont. 550, IMHP and I’ve had several of those. The custom Whirlwind prop may have had something to do with it.
