Randy:
Decking is the process of shaving metal off the top of the
engine block, where the cylinder head is mated to the block. There are
two reasons you might do this: one is to make sure the deck is flat
which insures good clamping of the head gasket. The other is to increase
power.
If you shave the head, the combustion chamber gets smaller,
raising the compression and increasing power. BUT, if you deck the
block, not only does the compression chamber get smaller, you increase
"squish" which also increases power. So you get MORE power by decking
the block that you do by shaving the head.
Normally pistons are recessed into the block by up to 0.025",
depending on the engine. For our TR6s, when the engine left the factory
the pistons are recessed 0.017" when they are at top dead center. If I
were to shave the head 0.017 I might get the compression up by 0.2 from
say 8.5:1 to 8.7:1 for example. That might be good for 1 additional HP.
If I deck the block by 0.017", I get 3 HP even though the
compression is up by the same 0.2 points. Why? Because decking, unlike
shaving the head, moves the head closer to the top of the piston.
Remember that the combustion chamber does not cover the entire top of
the piston. It is actually smaller than the cylinder bore. So as the
piston approaches TDC, some of the air/fuel is squeezed between the top
of the piston and the face of the head. This is "squished", and like
squeezing a grape seed between your fingers it squirts out at high speed
into the combustion chamber.
This high speed "squirt" of gas causes turbulence in the gases,
which promotes better mixing and hence better combustion. So you get
more power than by raising the compression alone.
Here is a drawing of "squish" in a motor cycle engine, which
shows what I am talking about:
http://www.muller.net/mullermachine/docs/squish1.html
Cheers,
Vance
-----Original Message-----
From: 6pack-bounces+vance.navarrette=intel.com@autox.team.net
[mailto:6pack-bounces+vance.navarrette=intel.com@autox.team.net
<mailto:6pack-bounces+vance.navarrette=intel.com@autox.team.net> ] On
Behalf Of j randolph sandner
Sent: Friday, February 22, 2008 7:07 AM
To: v.navarrette@comcast.net
Cc: tr6; 6pack
Subject: [6pack] decking
morning vance,
to the uninitiated in engine machining, what is 'decking'?
TIA
randy
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