I'm building a mild gen4 5.3 with cam and boltons. Problem is I don't have the original 799/243 heads that the motor came with How much power if any will it lose by bolting on a set of gen3 5.3 heads? I have a set of 706 heads milled .010" which should raise the compression ratio up quite a bit.

I really don't want to spend $300-500 on. A set of 243 heads for this motor unless it is a big difference in power between the 2 type of heads

 

243 heads have larger valves in them than the 706 heads . And the 706 had is small cc ( more compression )than 243 . So if you milled them .010 already then your 706 are perfect. There really isn't enough gain ( if any now since you milled them) to justify 300 to 500 bucks to waste .

 

My thing is if you have a gen4 5.3l then don't those have flattop pistons?

If so you would be bumpin the compression a good amount. My guess is around 10.5:1 compression.

I would think you would leave 10-15hp on the table going with the 706 heads but im more use what you got as you can change them later.

Also get some arp bolts/studs to put the heads on to make it easier later on if you get other heads.