Originally Posted by
Sierrasideways
The new pistons are flat top and 4MM DOWN from the bore top. I was told this is
the same as a normal full size piston at the bore top with valve cut-outs
machined into them. Essentially the flat top of the piston is the same height as
the BOTTOM of a valve cut-out so there shouldn't be an issue. This is the
argument FOR.
How big do they think the valve cutouts are?!?!
This is one I built myself, the cutouts are big enough for a BD16+ and once CC'd this worked out at 7.7:1.
(Which is the lowest I go with a YB ever.)
Note the pistons protrude ABOVE the block as standard to give the 8.0:1 CR and when decompressing I keep that piston height as its important to the engines efficiency that the mixture is squeezed inwards as the piston climbs to TDC.
(That's one of the reasons that spark plug is in the middle... 
).
Would you think it's a good idea to do a compression test just to verify correct
compression before setting up the boost on the rolling road..?
Compression test will not tell you the Compression "Ratio"
To do that it needs to be assembled and filled with fluid, like this.
Once you know how many CC's of fluid the combustion chamber holds, you can work out the CC from Bore and stroke.
That was your engine builders job and you should be given all this info with your engine really.
(We supply all that data plus a CD of pictures from the build)
Or am I just clutching at straws because if I find out the compression is not
correct for my chip, 8:1 compression
The compression ratio is
definitely not right for your chip mate, but its safely low instead of dangerously high.
The only long term worry is EGT (Due to lack of spark lead)as opposed to detonation so just get it live mapped and then you can stop worrying about it.
