difference between a decompression
plate and a solid copper head gasket – is
very little. These two items are essentially the
same (i.e. a metal version of the original head gasket), although the solid
copper head gasket has extra locating holes machined for Ferriday Sealing Rings.
A solid copper head gasket is needed when
a conventional head gasket is not strong enough for the amount of power a
modified engine can produce.
plate is used -
As well as a conventional head gasket to lower the compression ratio of a
turbocharged engine or turbo/supercharged conversion
(a 1.5mm thick decompression plate will lower the C.R. of an RS Turbo CVH to
7.4:1 without compromising piston strength). Decomp plates for Zetec & ZVH turbo conversions are available, and for any other turbo conversion. Available in a variety of thicknesses, we can help calculate your compression ratio
and supply exhaust manifold adaptor plates (e.g. RS Turbo manifold onto Zetec
To restore the piston to valve geometry or compression ratio of a heavily skimmed head
(especially useful for rare multi-valve heads, Sierra Cosworths etc).
A decomp plate simply becomes an extension of the block or head, and
allows the head gasket to do its job in the normal manner. Ferriday Sealing
Rings are not needed with decomp plates.
When using a Decompression Plate - only one head gasket must be used. All that
is needed to seal the plate is a thin smear of high temperature non-setting
sealant across the two metal surfaces only.
Let the conventional head gasket do its job in
exactly the same way, as if the spacer plate wasn’t there. Don’t
forget that with OHC engines it may be necessary to use a vernier pulley to
allow correct cam timing settings. It is not
necessary to anneal a decomp plate.
Aluminium-alloy or Copper? – The majority of engines now have aluminium cylinder
heads, so the decompression plate can also be made from aluminium alloy.
The plate should be placed next to the aluminium
head, so effectively becoming an extension of the alloy combustion chamber.
Customers often express concern as to whether an aluminium decompression
plate is strong enough - both your pistons & cylinder head are made of aluminium
alloy and perform their separate tasks without problems during normal engine
running. An alloy decomp plate will also perform
well under normal conditions. If an overheat
occurs (localized or general) then all these components are in danger of
destruction, but it is quite conceivable that the plate may fail before the
pistons or head. This should be regarded as a
good thing as it is cheaper & easier to replace than a set of pistons or a
cylinder head! Another advantage with aluminium
is that the end product is some 20% cheaper than copper.
As far as the decomp plate is concerned there is no limit to the amount of boost
you can run, but pinking/pre-ignition, engine management limitations, piston
strength, and head gasket strength will still be the limiting factors.
here for a list of some decomp plates currently held on file.
Please e-mail or phone if you'd like any help in calculating
compression ratios. I've written a program to get an immediate list of new
compression ratios for different plate thicknesses. There are plenty of
engines already on file, but for anything new all I need is the
bore, stroke, current compression ratio, and the compression ratio that you'd
like to achieve. A complete table of options can then be e-mailed as an
If you're undertaking a turbo engine build and are struggling
to get the compression ratio down to an acceptable level, or are unhappy with
your combustion chamber design, you may consider having
the combustion chambers CNC machined. More details on
CNC Combustion Chambers page.
ZVH Engines. The "ZVH" engine is a
Ford Zetec bottom end (1.8 or 2.0 litre) with a
Ford CVH head fitted. To
fit the head to the block there are 3 problems to overcome - 1)
Compression ratio. 2) The redundant oil return passages in the Zetec block that
need to be blanked off. 3) There is a waterway in the Zetec block that sits very
close to an oil gallery in the CVH head.
A copper decompression plate will tackle all of these 3 problems in one go
(assuming this is a turbo engine), and will allow easy fitment of block
to head. If you have already sorted out issues 2 & 3, then a (cheaper) alloy
plate can be used.