MILLING

[Anonymous]

David mentioned in his small6 head swap sticky that while milling a head raises compression it also improves flow slightly by lowering the valves a little deeper into the chamber. This makes sense to me. My question is if this is true,(and I don't doubt it) then why would you want to mill the block to zero before milling the head? It seems to me to make more sense to mill the deck just enough to clean it up and make it flat, then mill as much as possible from the head in order to lower the valves even more. I realize this is probably just splitting hairs but these engines need all the help they can get in the breathing dept. Any thoughts?

 

[MustangSix]

The reason for decking the block is twofold. First, it straightens the block surface. Just like a head, the block may deform a bit over the years from all the heating/cooling cycles it goes thru. Decking gives you a nice flat surface to bolt your nice flat head to.

Second, it brings the flat outer part of the combustion chamber in close proximity to the piston. This is known a qunch area. By squeezing the mixture, it cools it and foces it into the recessed part of the chamber. This also creates turbulence. The overall effect is to enhance the mixture of fuel and air, increase knock resistance, and improve efficiency. When you open quench are much more than .060", it begins to get ineffective.

An example of a good compromise between unshrouding the valves and still retaining quench is in the Chevy W-motors, the 348-409. The vavles were flush with the head surface and the chambers were in the cylinders themselves. Jaguar V-12's also had a similar design prior to the Mays Fireball high swirl head.

 

[Anonymous]

AAAAHHH! I knew that. I really did. I totally spaced out .(lack of caffeine) No matter how much you mill the head ,that quench area is going to stay pretty much the same unless you mill the deck.

Sometimes you just have to smack me upside the head