Here is what I did on the R16 engine I am currently rebuilding. The head had
been shaved too many times so the pistons hit the head with no head gasket
(the normal test for fit). I cleaned the head and pistons and put a thin film
of oil on the pistons. Laid the head on the engine with no head gasket and
turned the crank till the pistons just touched the head. Pulled the head and
looked to see where the oil transferred. Using the sandpaper flap wheel in the
die grinder I removed a small amount of material from the head where it
touched. Cleaned things up and put the head back on the engine and checked
where it
touched again. After about 50 to 100 times the head perfectly matched the
pistons (and I added 3" to my arms). This restored the squish band area to
where is was supposed to be. When I measured the combustion chamber volumes
they
were within 1%, Lucky considering the shaving that had been done on this
head. If they had not been so close I would have removed material from the
pocket
area to even them up. The final compression ratio ended up being just a
little over the 9-1 stock ratio, about 9.3 as I recall.
A proper squish band engine setup is very efficient. It allows higher CR and
better performance that more exotic things like the original hemi head. (the
current "Hemi" is a registered trademark and has nothing to do with the
combustion chamber shape. My bet is that it is a squish band engine.)
Enough rambling for now
keith williams
In a message dated 4/8/2006 12:06:00 A.M. Pacific Standard Time,
drlsmith@telus.net writes:
Good information for anyone building, or considering building, a performance
engine.
Daryl
'66 R/G 1800cc Hot Rod :-)
----- Original Message -----
From: Jim
To: megasquirt-tuning@msefi.com
Sent: Friday, April 07, 2006 7:43 AM
Subject: How to tune accel param
A "quench" or "squish" area is any area where a flat area on the
piston crown comes into very close proximity to a matching flat area
of the cylinder head .
This area , in order to have the desired effect , must be VERY tight when
the piston is at TDC , one rule of thumb used goes like this :
When remachining your engine block you want to "deck" the block
(I think you may call it skimming ??) this is when you put the block in a
holding fixture which uses an expandable steel pipe that runs through all
of the Main Bearing bores and then machining the head gasket surfaces to
insure that they are square and equidistant from the main bearing bores .
Before performing this operation the pistons and connecting rods and
crankshaft are carefully measured so that the amount of material to be
removed from the head gasket surface can be calculated .
The end result is that the piston crowns will be perfectly flush with the
head gasket surface at TDC .
This results in the ONLY clearance between the piston and cylinder head
being the compressed thickness of the head gasket .
This is usually about .040 inches or 1mm , this is just approaching the
ideal clearance , ideal would be about half of that measurement but would
result in too much risk of the piston contacting the head at high rpm .
Head gasket manufacturers sometimes provide this compressed thickness
measurement right on the package , if they don't thier website or a phone
call will get you the specs. .
Another note of interest , sometimes "Steel Shim" head gaskets are
availible for your engine , the compressed thickness of these gaskets are
about half that of "composition" style gaskets , this may allow you to
reduce your quench clearance without machine work .
Here is a very good article I found with a quick Google search ,
it generally understates the advantages that I have seen .
http://chevyhiperformance.com/techarticles/94138/
And while it is referencing a Chevy V-8 , the principles apply to all
engines
.
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