> Would anyone mind backing up a couple of steps just to
> explain how to determine the compression ratio? I know what
> it is, but do you fill the cylinder with liquid at BDC and
> measure how much come out at TDC?
Ok, so you know compression ratio is (swept volume + unswept volume) /
(unswept volume).
Generally, I compute swept volume as (stroke * bore * bore * .7854). Your
method would work, but it's easier to measure bore and stroke than volume
(and usually it's safe enough using advertised stroke instead of having to
measure it). Be sure to keep your units straight; easiest IMO is to convert
all the linear measurements to centimeters before multiplying, so the
results come out in cubic centimeters.
Unswept volume is the tricky one, usually involving messy liquids and so on.
There are 3 pieces to add together : volume of the combustion chamber in the
head, volume enclosed by the head gasket, and a correction for any
difference between the piston surface and the block surface at TDC (aka deck
clearance). On some engines you also need to account for the shape of the
top of the piston, but most TRactor motor pistons are flat so no need to
worry about that here.
Here's a nice diagram I Googled just now, for deck clearance (which may be
negative but for most TRactor motors should be close to zero) :
http://www.ford-trucks.com/article/idx/14/102/article/Determining_Compressio
n_Ratio.html
and a nice set of instructions on how to "cc" the combustion chamber :
http://www.ford-trucks.com/article/idx/14/102/article/Determining_Compressio
n_Ratio.html
Head gasket volume is gasket height * bore * bore * .7854 ... you can assume
the stock head gasket crushes to about .060" or about 0.15 cm; so for 8.7cm
liners, it's only about 9cc.
In case anyone is wondering where .7854 came from, remember that the formula
for the area of a circle is 3.141592 (aka Pi) times the radius squared. But
in the formulas above, I've used the diameter, which is twice the radius.
So, .7854 is an approximation of Pi/4. Feel free to use whatever your
calculator gives you for Pi/4 ... on this one, it's actually
0.78539816339744830961566084581988 but .7854 is a lot easier to type and is
close enough.
All clear as mud ?
Randall
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