Sorry everybody, it looks like my posting was a bit confusing and
started a long string of responses. I didn't mean to imply that the
way to increase compression ratio is by decking the block rather than
the head. This would obviously be a lot more complicated and expensive
that simply pulling the head to have it milled, plus I would think you
wouldn't get much compression increase before pistons started coming in
contact with the head, as others have noted. My main question was that
since mathematically, taking material off the head or the block has the
same net effect on compression ratio, is there a reason to do one
versus the other. I found the answer in Vance's email below - the
"squish" factor. This was something I wasn't familiar with (more
evidence of why this email list is such a great resource!). This
squish factor helps us all understand why Kas recommends decking the
block to zero clearance - this allows us to maximize compression ratio
without getting rid of head material which we want to keep in order to
maximize squish. If further compression increases are desired (which I
would think they would be, since I doubt you can get much compression
increase simply by decking the block to zero clearance), it is at this
point that the owner should start taking material off the head.
Thanks everyone, and sorry to have caused any confusion!
Tim Holbrook
1971 TR6
--- "Navarrette, Vance" <vance.navarrette@intel.com> wrote:
> Steve:
>
> The difference is small but important.
>
> Both raise the compression ratio by decreasing the size of the
> combustion chamber. However, decking the block does something that
> milling the head does not; it decreases the clearance between the
> top of the piston and the head. This is only important where there is
> head deck over the piston as opposed to open combustion chamber.
> When the piston approaches TDC, the air/fuel charge between
> the piston top and the head deck "squishes" out into the open
> combustion
> chamber. This increases the turbulence in the combustion chamber
> prior
> to and during combustion to give a cleaner, faster burn with more
> power.
> The TR6 combustion chamber opening in the head is not round; it
> is flat on one side. This means that where the head deck covers the
> piston top, there is a squish area. Decking exploits this area,
> milling
> the head does not.
>
> Vance
>
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