Head milling

[bob westphal]

In a reply to another thread I said:

Alright, here is the question no one has asked. The milling chart shows the amount needed to mill off as .050 to achieve a 7.5 ratio on an engine with a stock 7.0 ratio. Does that mean I need to mill off .150 to get 8.5 or is the amount variable due to shape of the chamber?

----------------------------------------------------------------------------Greg G. said:

Careful. In a conversation I had with George A, he said the anything more than .090 would require premium gas. I wouldn't think that a CR of 8.25 to 1 would require Premium, as most of today's cars are in that range and run on regular. That said, I would thing that .090 or less would put in the 8+ range.

The other questio is which engine, and how much space there is in the cylinder when the piston is at TDC. I noticed quite a difference between the 218 where the piston stoped with about 3/32 left and the 230 which was very nearly flush. So the same milleing on the head would yield more CR on a 230 than on a 218. Also if you have bored the cylinders and added oversized pistons, these will yield more squeeze also. So it more thna howmuch to take off the head that contributes to CR. Remember you can deck the block also.

So .020 off the block and .050 off the head, is the same as .070 off the head

as far a volume left in the combustion chamber.

This is good info but still doesn't answer my question. The chart shows

that on a 218 you need to take off.050 to get a .5 increase in c/r ratio. Does that mean you need to take off .100 to get 1.0 increase in c/r and so on for more c/r?

[Guest Nile Limbaugh]

I don't believe that the increase in comp. ration is linear due to the shape of the combustion chamber. In other words, twice the removal will probably yield more than twice the compression increase. The question of needing premium fuel probably has to do also with the shape of the chamber as opposed to the volume. Since side valve engines lose a percentage of combustion pressure in the valve clearance chamber it could be that the flame front runs into itself earlier than it does in an overhead valve engine. Damn, now I've confused myself!!

[Tim Frank]

I agree with Nile, it is a non-linear relationship.

Here is a link to check if you are interested.

http://www.bgsoflex.com/cr.html

[Normspeed]

Isn't there a point where too much milling gets the valves too close to the head, unless the heads have been relieved? Also seems like milling off a lot of head would restrict the channels from vaves to pistons, or would that be too little to notice?

[Guest Nile Limbaugh]

The problem is that if too much is milled off, the valves will actually strike the head when they open. And that makes a real mess!