Anyone KNOW what is the most lift I can put on GT40P heads (even with spring swap)

And don't forget that a .550 lift may get you into rocker bolt problems, unless you're going to have studs and guide plates put in. Might check with the 5.0 Stang crowd to see if that sized cam creates rocker problems with pedestal rockers.

 

Ken is correct about P head's flow, it levels off at about .500" lift unless you do some porting. They were designed for flow velocity, not flow volume. As suggested earlier, you can call Comp and get a recommendation. Switching to a roller cam can give you more area under the curve without increasing lift drastically due to the more aggresive lobe ramps. And you don't have to be as fearful of buying a used roller cam as you do a flat tappet cam. I just bought a used Comp XE266HR-12 off one of the Mustang forums for $120 shipped, Summit sells them for $250. Thing is steel billet and looks brand new. Shop around.

 

I have ported a "little" on the exhaust side. Where would I need to port to increase the flow a bit?

 

The exhaust is the main place. If you haven't already, get rid of the "crater" in the roof without enlarging the port opening. Don't mess up the hourglass shape just above the exhaust valve. Smooth the corners of the valve guides without cutting in too far, there's a water jacket in there. Smooth the valve bowls on the intake and exhaust sides. On the intake ports only smooth out the walls where the pushrod tunnels pinch the runners. Check that the intake manifold ports match the head's intake ports.

 

Try these links for info. I posted another but I cant find it now.
http://users.erols.com/srweiss/tablehdc.htm
http://www.stangpro.com/html/articles/gt40p_files/gt40p1.htm
http://www.tmossporting.com/tabid/3517/Default.aspx
http://forums.stangnet.com/showthread.php?t=413332

 

That's the cam I have, and it is NOT billet according to the Comp Cams tech line. Cast iron dizzy gear!

 

just buy a set up aluminum heads that will support 650 lift. buy a cam that is in the high 59s or low 60s and be done with guessing..


IMHO stock heads are not meant to go over 500 without work.. you might spend the same cash than if you bought new aluminum heads.
you already have the rockers to put on the new heads.. and then you need springs, and you can get a same and lifter and spring kit...

 

That is exactly what I have done on the exhaust side.

I have not messed with the intake, but if I take the heads off for this project, I might hit them and gasket-match them.

I was told to leave the intake side rough and figured if I did any work on them, they would smooth out, so I left them alone. And knowing that the exhaust is the weakest side, I figured I would hit it and leave the intake.