I have heard that shaving heads decreases combustion chamber efficiency. Is there a way of knowing how much is too much? Where do you draw the line on a conventional head build assuming that point is before approaching the IV seat? What would be the result assuming there is a point where it becomes detrimental?

For example i am i am looking to take a set of BBC heads down from 119cc to 110cc, 0.005+ deck, -18cc domes to be flycut, .030 gasket. Doing my best to shoot for 11:1 static. Cam specs will be 242/258 .625/.605 112LSA 108ICL

I would much rather take the heads down further and flycut a set of flat tops.

How much is too much?

 

Anyone?

 

too much is when you hit the seat angle on your intake... compression is way more important

 

There is a point to where you begin to decrease combustion chamber efficiency. Taking away too much of the chamber just to acheive 11:1 is counter productive. What your gaining in power from the higher CR your taking away IF you have too small of a chamber.

With the piston .005 in the hole and a .030 head gasket your quench should be .035 which is optimal for the burn. I wouldnt take more than 8cc if it were me.

 

Am I missing something hear? Are you guys talking about some other form of "efficiency" that I dont know about? The reason I ask is this. Milling or shaving a head INCREASES chamber efficiency by decreasing chamber surface area and thermal conductivity not to mention burn time. Granted, small amounts in the .020 to .030 range could hardly be deemed a significant increase in efficiency but an increase none the less.

"I would much rather take the heads down further and flycut a set of flat tops."

This is a proper statement! My milling heads and increasing chamber efficiency then milling off the top of the dome or cutting valve reliefs to offset the volume is correct!

The ONLY time you have to small of a chamber is when you have to run a large dished piston or to high compression for the fuel being used. Then the engine combination needs altered unless sanctioning body rules demand it. Dish pistons do not necessarily decrease chamber efficiency but they can if done improperly or become to large . Any more than 10cc in dish and its to large.

 

excellent post above. My 243 heads are milled over .100, chambers are 48cc. It flat out works! Now that i have the larger bore I can open up the walls around the valves and get the valve outside of the combustion chamber at full lift, that really helps the flow being able to hang the valve out in teh open like that

 

How much can you mill 241s before you run in to Intake fitment issues?

 

no idea, it doesnt concern me I just milled my intake to fit

 

well it does me, thats why I'm asking.

 

ICE textbooks show that increasing CR increases thermodynamic efficiency, just as RM said above.