Sorry but this rambles a bit...
Well all this has got me thinking that my CR might not be as high as I
thought. I measured
the pistons height out of the block myself when reassembling the engine. The
rest of the
info was given to me by my machinist who cc'd the head after he completed the
flow work
and such. I gave him the clearance of the piston from the block and the
thickness of a
compressed gasket and he told me that the CR would be about 10.7:1. Like I
said, I
measured the piston clearance so I remember that one real well and the
resulting
conversation between the machinist and myself. I'll have to measure the head,
check my
invoice from the machinist and get back to you all. I'm not sure if the head
had been
shaved or not previously (my original engine is in the corner of the garage,
the hot rod
engine is from assembled pieces parts) but the head number and the total
current
thickness should get me the info I need. A lower CR won't hurt my feelings as
I was
originally wanting @9.5:1 but my machinist had other ideas (his primary
business is high
end race motors for circle track cars) and am now wondering if he was pulling
my leg on
the CR.
Shawn J. Loseke
1972 TR6
Fort Collins, CO
http://www.loseke.net/shawn
>Getting back to my question about your compression. I tried to understand
how 10.7:1
CR could be achieved primarily through proud pistons and am perplexed, to say
the
least.
>
>Using Goodparts compression tech article as a basis (right or wrong), it
states that
removing .010" from the block decreases volume by 1.10 cc, while removing the
same
.010" from the head decreases volume by 1.03cc. Not a huge difference.
>
>According to the chart based on zero deck clearance, with an earlier 8.5:1
head there
needs to be approximately an 11.7 cc decrease in volume to reach 10.7:1
compression
with a +.040 overbore (51.5 cc to 39.8 cc). If your pistons stand proud .015"
that would
result in a volume decrease of 1.65 cc. This leaves about 10 cc of volume
still to be
removed. If the balance were to come from the head that translates into about
.097"
removed from the head. That seems to be a decent amount of material and still
leads
back to my belief that head milling is a much larger contributor to a
compression increase
than decking can ever provide.
>
>Is there something about your motor that voids any of this? Maybe my
calculations
overlooked something? Larger Mazda pistons? Or...?
>
>Thanks.
>
>Greg Dito
>CD6250L
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