Hi Ted. I agree with everything you say. And that's the way you really want
determine how to machine the head.
I think how this got started is that somebody asked how the amount to shave
off could be calculated given only the design spec numbers. For example using
bore and stroke you can calculate the cylinder volume. Using cylinder volume
and compression ratio you can calculate V2.
So, what year is your daughter's TR6 and how much did you shave off the head
to get CR of 9.5?
- Hugh
Ted Schumacher <tedtsimx@q1.net> wrote:
hugh and list. with all the discussion about compression ratio and head
thickness, we need to have an understanding of compresson ratio. compresson
ratio is the comparison of the cylinder volume to the total volume of the
cylinder and combustion chamber (including the volume of the head gasket).
the
formula for this is V1 + V2 divided by V2 = compression ratio. V1 is the
volume
of the cylinder with the piston at bottom dead center. V2 is the volume of
space
above the piston at top dead center (the combustion chamber plus the head
gasket) for example if V1 is 450 cc's and V2 is 50cc's - 450 +50 = 500
divided
by 50 = 10:1 compression ratio. all that you need is a measuring tube graded
in
cc's. with the piston at the bottom of the cylinder, fill the cylinder with
liquid - we use acetone because it's easy to clean up and the residue
evaporates
quickly. record this as V1. then invert the head and prop it on a bench so
it
is level. a small level can be used to check it. fill the combustion chamber
with acetone. record this number as V2. (add about 3 cc's for the head gasket
volume). now all concerns about head thickness, shape of combustion
chamber,
slope of chamber walls, etc have been eliminated. if you want to see the
results
of milling the head, mark the combustion chamber wall at amount you want to
mill. let's say .060". so you measure down the wall of the chamber .060" and
make a mark. fill the combustion chamber to this mark and record the volume.
plug that number into the formula as V2 and see what the compression ratio
became. this is the only accurate method. it will be off a minimal amount
because of using surface tension as the limiting factor. when we do this, we
use
a plastic cap that covers the cylinder or chamber so it can be filled
exactly.
don't worry about the difference. it might amount to a few hundredths of a
point
and won't be noticeable. my daughter's tr6 is at 9.42. we do our engines
normally
to 9.5. the .080 difference could probably be measured on a dyno but not by
"seat of the pants". ted
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