I have two questions for anyone in the know:

1-why does flow suffer if you mill a head
2-If you need 64cc's on the 205's, how many thous. need to be milled off the head? I assume they come with 66cc's originally.

 

.006" for every cc has to be removed.

 

Most places that stock them have 66 and 62cc on the shelf. I think the lesser flow with milled heads is simply a matter of dynamics. IIRC the reduction in flow is pretty small.

...and like the other guy said, close to 0.006" milling per cc.

 

When you mill, you cut out some of the ramp that leads to the valve seat. Low and mid-lift flow suffers because of this although high lift flow is less affected.

 

Insignificant amount of flow loss (if any) at the mill amount you have in question....

Essentialy the chamber is the completion of the "marraige" of valve job and port design (all three work in conjunction with one another) and helps shape the cone of air coming off the valve (at very high speed)....changing that shape can change the way it effects the cone causing typical flow losses in an already highly optimized port/chamber configuration. Sometimes a mill can help flow depending on the cylinder head but the more optimized the design is to start, the greater the chances a mill will hurt it.

Tony M.

 

Good post Tony. Thanks for the clear explanation. I now have a better understanding myself of the whole milling process.

 

Ok, Tony, what is optimal for the AFR heads?

 

Optimal is not milling at all and getting the desired static CR with the correct piston dome (or dish) volume but that isn't a reality more times than not.

Milling up to .030 usually doesnt hurt things much.....Over .040 will start to dip into the flow curve more....best to get a different piston if you really need alot more than that (and are trying to build a VERY optimized package). Sometimes the availability of shelf components is weak (especially if your building a combination that isn't very common)....thats where a few more dollars spent for custom pistons can certainly pay off.

Tony

 

So the smallest heads would be the 225cc port 62cc chambers, followed by the 205cc port 66cc chambers? .030 would take the 205cc 61cc, which is more than the common 59cc. Is that correct?

 

I would say low 60's is the most common mill on a 205....high 50's on some of the more aggressively cammed combinations with notched pistons. Whats also important to clarify here is the bump in compression is worth more than the offset flowloss but what we have been discussing here is whats optimal.....optimal is the least amount of milling to meet your desired compression target....but even if you need a more severe mill (.040 or greater), its still a good thing to do assuming you have enough octane fuel to handle the higher compression.

Mamo OUT