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From: Tim Tilton
>Well, first off I have to admit that my spreadsheet which I use to
calculate all of my >expenses to date had an error. The correct price of the
pistons, piston rings, piston >pins, Tru-Arcs, and shipping was $380. I
forgot to add in the $100 deposit!
$380 is still in the right ballpark if the pistons are any good.
Were only talking a hundred over OEM stuff here.
>As I mentioned previously, the head I am using has been milled and the
combustion >chamber is smaller than normal. I want a compression ratio of
approximately 10.5 to >1. To achieve this ratio, I calculated the following:
>I calculated my combustion chamber to be 33.4 cc's; used a .020" deck
height; >estimated the head gasket thickness to be 0.038" compressed to
0.025"; and used >a piston dome displacement of -6.5 cc's. This combined
with a 3.25" stroke and >3.2402" bore gives me a compression ratio of 10.55
to 1.
>The -6.5 cc's answers Jarrid's question or comment regarding whether or not
this >piston has a dish. Yes, it does...I would have liked to run a flat
top, but I just couldn't. >If I can decipher the order form correctly, I
believe the piston pins are 0.94", slightly >larger than the stock 0.9377"
I'm not follinging you on the piston, if its a dish, there is a sperical
"dent"
in the top of the piston, if its a dome, the piston top will be rounded and
not flat.
If its a dished setup, the 6.5cc should be a positive number adding to the
head gasket volume, deck loss volume and chamber volume.
If its domed, you will add everything, then subtract out the dome volume.
I will assume you do have and mean to say you have dished pistons.
If I understand that you have set the pistons to stand pround of the deck by
.020, you will have a volume loss of 2.68cc. Giving a net volume of...
33.4 chamber
6.5 dish
3.6 gasket
-2.68 deck loss
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40.92cc
giving a net CR of 10.54/1, which is as you say.
Does my description sound accurate as to what you have?
If so, here is a potential issue, the pistons standing proud the block
by .020, and a compressed gasket thickness of .025, leaves .005 squish
clearance, assuming that the bearings are and allways will be tight.
The issue here is thar con rods stretch with RPM and heat, sometimes
up to .020 inch. This will cause the pistons to clobber the head at
higher RPMs.
Anyone have other thoughts on this?
>I will use a Kent Cams Scatter Pattern 286SP Rally cam and Crane dual valve
>springs with the following specifications:
Tells us the specs and order info on the springs.
We want to know the unloaded len and loaded len/force for each spring.
Weight info gets you extra credit as 1/2 the mass of the spring becomes
part of the inertial component for modeling valve float.
>I am NOT an engineer. I hope there is not a fatal flaw in this calculation.
If you find a >fatal flaw, please be kind.
Sounds like your doing fine, I think you may need to take some material
off the tops of the pistons though.
Good luck, and keep us abreast.
Jarrid
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