curious what head yall think would be best for a 408 build with about 10.5 cr. would the 317 head be ok ported and milled or the 243 heads milled down. this 6.0 build is all new to me so any help is really appreciated. it will be a nitrous motor.

 

408 with .040 head gasket and -4cc pistons with 71cc 317 heads will give you 10.8:1 compression. same set up with 64cc 243 heads will be 11.9:1, with 243 heads milled .030 (59cc), compression ratio would be 12.4:1

 

The factory GM castings will hold up to a decent amount of N2O abuse. I'm constantly amazed.......... That being said, your choice of which head should be based on what combustion chamber volume you need to start with. The 243 heads start with 65cc and the 317's start with 71cc. There is usually a cost difference in the price of the head cores. That may help make your decision if you can get a piston for either chamber size. As far as flow is concerned, both heads have the same power potential in stock trim. Even ported with big valves they'll achieve similar flow capacity.
Comparing the 243 with the 317 the biggest difference in your situation is cost for cores. I personally would have no problem running the 317's on a 408ci engine.

Good luck,

Richard

 

How did you get 10.8:1 compression? Using the values you gave I came up with 11.2:1


Bore 4.03 (inches)
Stroke 4.00 (inches)
Rod Length 6.125 (inches)
Cyl. Head Vol. 71. (cc)
Deck Height -0.008 (inches)
Head Gasket Bore 4.03 (inches)
Head Gasket Thickness 0.040 (inches)
Piston to Cyl. Wall Clearance 0.003 (inches)
Top Ring Land Height 0.2 (inches)
Piston Dome Vol. -4 (cc)


Compression Ratio:
11.231 : 1

Displacement:
408.2(ci), 6.7 (liters)

Bore / Stroke Ratio:
1.008 : 1

Rod / Stroke Ratio:
1.531 : 1

The reason Im asking is that Im building a 408 and been trying to decide on CR. I enter in the values you said to see how accurate the compression ratio software Im using is. Sorry to highjack the thread

 

I currently run 317s on my 427. I ran the same heads on my 408 for 3.5 years. I made huge horsepower with the heads and ran pretty decent boost.

Mine were TEA stage III c&c, but as a head casting goes, nothing at all wrong with them.

As Richard already said

 

Deck height is usually 8 thousandths out of the hole or .008, both my stroker motors were that way.

 

Bingo. Old Chevy was in the hole, but starting with gen III, they are out of the hole by default.

 

I understand that. After going through some of my books on LS motors and some looking on LS1tech. Gen 3 and 4 blocks are negative deck height blocks so the -0.008 would be the correct value. Over on tech for example the LS1 spec would be -0.007