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I have a set of GT40's (F3ZE-AA) that I'm putting on a stock 93 Mustang GT 5.0. They were measured at 65cc combustion chambers. According to what I've read, these are cast flat top pistons in 1993 (see pics, not sure how to tell). Also, installing a XE274HR cam. My question is, is it worth milling the heads a little to gain free horsepower? Right now, I'm about 9.2:1 on compression. I'm hestitant to increase compression due to the cast pistons. How much horsepower could I realistically gain by milling a little off the heads and going up to around 10:1 compression? Also, using a Zex 75/125 shot, ocassionally. Another reason I'm hesitant to increase compression. Thoughts?
 

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Not flat tops. Have valve reliefs in them. Flat tops are just that, flat. Hard to say how much horsepower you can pick up. Theres other things to worry about like quench distance, piston to valve clearance, etc. when you mill the heads. Just because their hypereutectic doesn't make them more prone to breaking on a healthy engine, they just dont handle handle detonation as well because their material make up essentially makes them very hard.

Dont really think youre going to be hitting 10:1 from decking the heads. I only have 10.2:1 with essentially the same pistons but a 53cc chamber. If the heads aren't flat, mill them. If not, not really worth it in my opinion. Oh and my pistons are pretty much .000 across the board for piston to deck height. I would measure yours.
 

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Rule of thumb is, for every .004" you change it by 1CC. A simple surfacing to make the mating surface straight and clean is usually only that much unless there's reasons, (scratches, rust, etc) to cut more. But when doing anything past .010" you start creating other issues. especially if it's a torque down only rocker arm system. You effectively make the pushrods longer and take up more preload. Depending on valve size and valve/piston clearance already, you could be getting very close.
Cast pistons, as stated, detonation is their biggest enemy. 429CJ had cast pistons and it was 11.3-1 comp. The amount of HP gain by increasing the comp to 10-1 would be appx 1.5% over what you have now. Cam profile and heads ports are where the real HP/TQ is.
 
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