Mxsmanic wrote:
Paul kgyy writes:

Probably the temperature. At 25" MP and 2520 rpm, I'd guess you would
be running about 78% power under standard temperature conditions
(15C?). But at -1C the denser air is giving you additional power
output so you're probably closer to 80%.

OK. That sounds like enough to account for the difference in speed
(?).

The power chart in your POH should tell you how to adjust the manifold
pressure for nonstandard temperature conditions.