I have been told swapping heads on a 5.3 with 243 LS6 heads will increase my combustion ratio. But after some checking, i noticed that the 243 heads have a larger combustion chamber. Do i have the wrong information or what, i am confused. Please help.

 

Yeah, 5.3 heads have a 61cc chamber v. 65cc for a 243 head. So unless you mill them down, you'll loose compression. 243s do flow a lot better though.

 

how much should be milled and is that within tolerance for the intake manifold mounting.

 

Go with a thinner head gasket. Stock gaskets are .055, if you go with a .040 you'll be pretty close to your desired CR.

If you want to mill them, you'll need to shave off .055" to retain your compression. And yes, your intake will still bolt up, but you might want to shave that down a bit for best results.

 

would it just make more sense to p/p 706 heads

 

Do you only want to raise your compression? How much do you want to spend? Which cam do you have or plan on getting?

 

we put my TT3000 in this weekend and the pacesetter LT's are in the box. i had planned on going with the TR220 with 112 LSA. Hearing about the flow on the 243 heads i thought i would go with them. if they need to be milled for the cc to match i might as well mill to bump the CR. just a thought and i was curious why the new 5.3's are making better numbers with those heads.

 

If it's within your budget, ported/milled LS6 heads are a better choice. You can do a moderate mill, .030" and a thinner head gasket, .040". That'll bump your CR a bit, better quench, and better flowing heads. Ported 5.3 heads will also net you nice gains though.

 

I do not know exactly how much milled = how many cc's are removed, but I can tell you that the porting that WCCH did to my 5.3 heads increased the chamber size to 65cc, same as a stock 243. Then milling them down .045 brought the chambers to 58.8cc. .045 was the max that Richard recommended milling them to avoid intake fitment problems, although a few here have milled them slightly more. I haven't heard of anyone ever shaving one of the stock intakes to help it fit with milled heads. Keep in mind that milling heads also reduces piston to valve clearance, although a .045 mill with even a .045 head gasket should be fine with a TR220.

Unported LS6 heads milled down to 58.8cc chambers with a 3.910 bore x .045 thick gasket on a stock 5.3 shortblock will net you ~10.0:1 compression.

 

I get 10.3 to one on that setup you mentioned, unless the dish on the 5.3 piston is greater than the 7cc on a 6.0 piston.