How an IO550N can turn a C206 slower?
 

Hey folks!
A friend owns a U206E which had the IO-550F installed. He claims his plane was capable to reach 130KIAS with that engine. Last week he decided to install an 0 SNEW IO-550N in exchange of the F, but it turned out with the plane flying barely around 105KIAS. Compressions are high, the prop is a scimitar Hartzell 82", and it has already completed the break-in but nothing changed. Any thoughts?

Magneto timing? How does the fuel flow compare? (Did he forget to put the wheel pants back on?)

Is he comparing the cruise speeds at the same RPM?
Is this the same prop as on the F model?
Was there any work done on the airspeed indicating system or RPM indicating system when the engine was swapped?

What's the altitude (specifically density altitude) it's indicating 105?

What particular STC did your friend use for the IO-550 conversion(s)? I would contact the STC holder and check with them if something is amiss. Apart from the density altitude aspect, there could be other major differences between the installations that could leave the first setup optimised for cruise and the second setup optimised for other aspects.

130 vs 105 represents huge power difference since the airspeed is proportional to the cube of the power. In this case it would represent a 47% power loss.

As others have said consider -
Conditions
ASI
Propeller
Engine
Airframe

I would start with checking the accuracy of the rpm indicator with a hand held tach and the accuracy of the MP guage by checking it for correct field baro. Also If it has a new prop check that the blade angles are set correctly

It is unlikely that an IO550N engine would make a Cessna 206 slower. In fact, the IO550N is a more powerful engine than the standard engine used in the Cessna 206. It is possible that other modifications made to the aircraft or changes in operating procedures could result in slower performance, but the engine upgrade itself would not be the cause.

Can you ask him to go flying again, but note the KTAS? This will always be a better comparison than IAS...