Would you gain any performance by swapping a set of 2004 GTO LS1 heads to a 1999 Silverado 5.3 ? I've got a set and was wondering if it would be a good idea.
Thanks

 

It would probably be a trade off. The 5.7 heads should flow better, but you would loose some compression, unless you milled them.

 

What white1 said. Also check this thread from LS1Tech, great info on compression vs chamber size.. https://ls1tech.com/forums/showthread.php?t=111207
There is also a condition referred to as "valve shielding" that can cause flow problems around the intake valve with the smaller bore size on the trucks ( 3.78" vs 3.90" car ).

 

You'd probably see no gain at all without other mods ie: cam, FI...etc