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The Muzio Gambit is -0.8 by Stockfish depth 22. However, if the f7-pawn is moved to the g-file, the evaluation falls to -3.5. How is that so? Both positions seem fine for White.

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[Title "Muzio Gambit"]

1. e4 e5 2. f4 exf4 3. Nf3 g5 4. Bc4 g4 5. O-O!
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    This is not very clear at all what you're asking. What is the exact position you're referring to where moving a black pawn from f7 to g7 drastically changes the evaluation? If the position is what I'm suspecting, then the answer is tied to the fact that a pawn on g7 can be used to guard f6 and to discourage checks from h5 in some lines, but I'm not sure at all that this is the position you're actually asking about.
    – Scounged
    Sep 13, 2020 at 0:58
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    To figure out why one position differs significantly from another, play out the engine's top suggestion(s) for how White should play in the first position. Then try doing the same thing in the second position, and see why this no longer works for White. This is a very useful analysis technique with engines.
    – Inertial Ignorance
    Sep 13, 2020 at 7:40

3 Answers 3

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As for "both positions seem fine for White": well, Black has a significant material advantage after 5...gxf3, so White has to find some compensation. If you don't have a concrete continuation which demonstrates that compensation, I don't think there's any reason to think the engine evaluation is confused.

In both positions, a sensible continuation is 5... gxf3; 6 Qxf3 Qf6; 7 d3. Now in the original position Black has doubled f-pawns and a bad pawn structure; in particular the pawn on f4 can't be supported effectively; when it falls Black will have two isolated pawns. Black stands better but has some weakness that White can try to exploit. In the position with the pawn moved from f7 to g7, Black could play 7... g5 immediately, or alternatively 7... Bc5+ and then 8... g5. Now Black's structure is more solid and White doesn't have anything like enough to justify the material disadvantage.

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One of Black's biggest problems in the diagram position is that White will get in Qxf3, Qxf4, followed by Qxf7+. With the f7-pawn on g7, Black can shut this plan down with ...g5. It's true putting the f7-pawn on g7 exposes Black's king to Qh5+, but it's not a big deal: Black can meet Qxf3 with ...Qf6, then Qh5+ with ...Kd8 or simply ...g6, and White has no followup. Meanwhile, Black is up a full piece.

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In the Muzio proper, White doesn't have a threat other than 6. Qxf4 (which doesn't mean so much, because that's what the c1-bishop wants). Shift the f7-pawn to g7, now White's threatening Qf3-h5+, and that's hard to deal with.

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    the engine evaluation states that Black is winning when the f7-pawn is on g7.
    – user24344
    Sep 13, 2020 at 4:48
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    Ha! Huh, I dunno. Black's structure is lots sturdier... Oh, I reckon with the pawn on g7, Black has ...g7-g5 in store (again). If White can't break down f4, his chances are much less, So if Black can manage the h5-e8 diagonal, he might shut White down on the file.
    – friscodelrosario
    Sep 13, 2020 at 4:54
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    I'm extremely confused: What 6.Qxf4? There's no way for a queen to move to f4 in the game given in the question. Also, by the comment it seems that OP thinks this answer is wrong - but why is it accepted then?
    – JiK
    Sep 13, 2020 at 12:08

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